Field Curvature and Astigmatism

[henry link]

This thread continues a discussion of field curvature and astigmatism that began with a question on another thread about the correct terms to use in describing softness at the edge of the field.

Today I tried an experiment to see if I could crudely measure field curvature and astigmatism in a group of binoculars. This is all very preliminary, so I’m hoping for some feedback and ideas from Alan and Alexis and anyone else who is interesred, maybe our resident Australian optometrist ?

As a target I used some ordinary lined paper from a memo pad. I taped two pieces on the wall, one with the lines horizontal and the other vertical. The horizontal lines were taped directly above the vertical lines. In each binocular I placed the target at 3 o’clock on the field edge and observed it, using only one eye, from about 25’. I used two pairs of 1.25 diopter reading glasses as my very crude measuring tools.

In binoculars with astigmatism the two sets of lines cannot be brought to best focus simultaneously. In the worst cases one set of lines will look focused while the other set will be spread so wide by astigmatism as to become invisible. I focused the horizontal lines without glasses, then used the glasses to change the best focus to the vertical lines. My only choices for correction were 1.25 diopters or 2.5 diopters, if I wore both pairs at the same time. Clearly I am going to need some 1.5, 1.75 and 2 diopter glasses to make this more accurate.

I used a similar technique to measure field curvature. First, without glasses, I found the best, least astigmatic focus at the edge where the two sets of lines were about equally unfocused. Then, without changing the focus I used the glasses (if necessary) to bring the center of the field into focus.

Here are the crude and preliminary estimates. The first number is astigmatism and the second number is field curvature, both in diopters:

Zeiss 8x42 FL - 2.5, <1
Swarovski 8.5x42 EL - 1.5, 1.25
Nikon 8x32 SE - <1, 2
Nikon 8x32 EII - 1.25, 2
Nikon 7x50 Prostar - almost none, <1
Kern 8x30 - 5, <1

I included the Nikon Prostar because I thought it would be unusually good and I thought the Kern would be particularly bad, although I didn’t realize its problem is almost all astigmatism. Keep in mind that the apparent fields of this group vary from 51 degrees for the Prostar to 70 degrees for the EII and the Kern so they are not exactly comparable unless the fields are close ( EL, FL, SE).

I should also mention that I am 58 and don’t have much focus accomodation, but I don’t know how much. I think the field curvature figures should be the amount of field curvature above my ability to accomodate, but I am unsure about how the astigmatism measurement is effected by accomodation. An ophthalmologist told me recently that she had encountered young people with over 12 diopters of accomodation ( I hope I remember that correctly). For someone like that field curvature in binoculars should never even be noticed.

Alan, was your experiment similar to this (without the glasses)?

[richt]

Zeiss 8x42 FL - 2.5, <1
Swarovski 8.5x42 EL - 1.5, 1.25
Nikon 8x32 SE - <1, 2
Nikon 8x32 EII - 1.25, 2
Nikon 7x50 Prostar - almost none, <1
Kern 8x30 - 5, <1

I included the Nikon Prostar because I thought it would be unusually good and I thought the Kern would be particularly bad, although I didn’t realize its problem is almost all astigmatism. Keep in mind that the apparent fields of this group vary from 51 degrees for the Prostar to 70 degrees for the EII and the Kern so they are not exactly comparable unless the fields are close ( EL, FL, SE).

Henry

Whilst i can just about understand the tests and what is being examined i find it particularly interesting that the Nikon SE and E11 are very close in this measurement
Forgive my naivety but does this result actually place the E11 in a better light so to speak due to its greater fov ?

Regards
Rich

[henry link]

Rich,

Using numbers probably makes this look more precise than it really is. 0.5 to 1 diopter differences are probably real, 0.25 maybe not. I need to do this a few times to see how repeatable the results are. It's not quite ready for a peer reviewed journal of binocularology ;-).

I just compared the SE and EII more closely. Moving the target in a little from the edge in the EII to try to match the field width of the SE seems to improve the astigmatism a liittle, maybe to 1 diopter. Looking at the SE again I think its astigmatism is not just less but probably much less than 1 diopter. It's really almost as good as the Prostar even with the wider field.

For anyone who is mystified by these numbers, the very best result would be a low number for each, but I think astigmatism is more important. People with wide focus accomodation probably won't see field curvature, but refocusing the eye doesn't correct astigmatism.

Henry

[richt]

Rich,


I just compared the SE and EII more closely. Moving the target in a little from the edge in the EII to try to match the field width of the SE seems to improve the astigmatism a liittle, maybe to 1 diopter. Looking at the SE again I think its astigmatism is not just less but probably much less than 1 diopter. It's really almost as good as the Prostar even with the wider field.

For anyone who is mystified by these numbers, the very best result would be a low number for each, but I think astigmatism is more important. People with wide focus accomodation probably won't see field curvature, but refocusing the eye doesn't correct astigmatism.

Henry


Hi Henry

Thanks for the reply yes i can see that a low result for each figure would be a good outcome i was just intrigued that the much wider fov E11 seemed close to the SE obviously further investigation probably proves there is a greater differential

binocularology ;-). ?? what a great word this is could be a new entry to the dictionary

Regards
Rich

[jcbouget]

Hello Henry, and thank you for your very interesting measurements, as always.

Now my optogeek comments :

For the naked eye, the accommodation in diopters is 1/d, where d is the minimum distance of accommodation in meters. For example, if you can see clear objects at 40 cm, then your accommodation is : 1 / 0.4 = 2.5 diopters. This is valid only if the eye is perfectly corrected at infinity by glasses or contact lenses.
In my old courses I have a graph showing the decrease of accommodation with the age : for 10 years old children, the accommodation is between 11 diopters and 16 diopters, depending on subjects. From 56, accommodation is stable and between 0.5 diopters and 1.5 diopters.
I am 37, and I can see very distinctly objects as close as 16 cm (with one eye at a time, with effort, and with my glasses). Thus my accommodation is 1 / 0.16 = 6.25 diopters. This is the average value for my age.

I think that in you experiment, trying the whole range of corrections would be necessary to avoid eye accommodation.

I’m very intrigued by the amount of field curvature in the Nikon SE. I have read so many ecstatic reviews about the flat field of the Nikon SE that I am perplexed. It is possible that the focal surface is really flat across most part of the field, and is curved only near the edges. It would be interesting to know the amount of field curvature at 70% from the centre. If the focal surface is a perfect sphere, you should find 1 diopter. Maybe it is less than that, I am curious about that.

It is also possible that people instinctively accommodate when they are looking near the edges. Personally I am rather sensitive to field curvature (despite my 6.25 diopters of accommodation) and I was surprised to see the field curvature in my Zeiss 10x42 FL very near the centre, maybe at 30% or 40% from the centre. My Zeiss 10x40 Classic and my cheap Meade 10x50 are better in this respect, providing a more relaxing view, at least for my taste. For me, field curvature is a real pain for astronomy.

For your measurements, I believe that you can use the focus wheel, instead of additional lenses.
With a binocular, the accommodation in diopters is : G^2 / d, where G is the magnification and d is the distance of the object in meters.
Look at a distant object (1 km ... ) note the position of the focus wheel. Look at a nearby object, note the new position of the focus wheel. (You must always turn the focus wheel in the direction allowing to see more and more distant objects, in order to avoid eye accommodation, but I think you know that). Thanks to the formula above, you know the difference in diopters, and you can infer the amount of rotation for 1 diopter.
For example, let’s say that you have a 8x binocular, and that between infinity and 10 m, you must turn the focus wheel by 30 notches of its surface. Then the difference is 8 x 8 / 10 = 6.4 diopters. So 1 diopter corresponds to 4.7 notches of rotation, and you have a scale to evaluate astigmatism and field curvature just by turning the focus wheel.

Binocularology is the very first word that doesn’t appear in my English-French dictionary and that I am able to understand. I am making progress :king:

Jean-Charles

[henry link]

Jean-Charles,

Thank you very much for this information. Your formulas will be very helpful. A quick check of my accomodation suggests it may be around 1 diopter.

Have you tried to measure the field curvature and astigmatism of the 10x42 FL and 10x40 Classic using your formula for the focusing wheels?

Tomorrow I will try to get a more accurate measurement of the SE field curvature at a few points between the center and edge. I had always assumed that reports of a completely flat field came from people with more accomodation than I have.

Henry

[ceasar]

Henry,
You probably are aware that Al Nagler of Tele Vue
(www.televue.com) is marketing an accessory for use with eyepieces that corrects a viewers astigmatism. It fits over the eyepiece. It is called Dioptrx. There are some thorough reviews of it on the above website. They are worth reading.

Q. Could these adaptors be "adapted" to binoculars?

Bob

[AlanFrench]

Henry,
You probably are aware that Al Nagler of Tele Vue
(www.televue.com) is marketing an accessory for use with eyepieces that corrects a viewers astigmatism. It fits over the eyepiece. It is called Dioptrx. There are some thorough reviews of it on the above website. They are worth reading.

Q. Could these adaptors be "adapted" to binoculars?

Bob

Bob,

The Dioptrix, as you say, corrects for your eye's astigmatism. It will do nothing for the astigmatism at the edge of the field, which is primarily due to the eyepiece. The best solution for people with astigmatism is corrective lenses, which improve vision for all uses.

Looking at mine, I imagine the Dioptrix could be adapted to binoculars, but am not sure it would be worth the trouble. The Dioptrix is most useful for low power, wide field eyepieces with insufficient eye relief to allow viewing the entire field with glasses. It works quite well. At higher powers, with smaller exit pupils, it is less useful, since smaller exit pupils tend to reduce the effects of your eye's astigmatism (as a general rule).

Clear skies, Alan

[AlanFrench]

Jean-Charles and Henry,

A most excellent suggestion on using the focus wheel after "calibrating" it to 1 diopter. This approach might take a bit of care in some cases - I seem to recall that some binoculars have focus wheels with variable "gearing."

I tied a somewhat different approach to produce some numbers, but was hampered by my crude setup and the lack of an assistant. I set up my 7x42s on a tripod and focussed on a target in the center of the field and about 18 feet away. I then placed a target with horizontal lines on one side of the field, and one with vertical lines on the other side of the field. The idea was to move the targets until they were in focus, and then measure the difference between the distance of the targets and the center field target.

I did notice there was an obvious difference between astigmatism close to the edge of the field and right at the field stop.

The test results are likely to vary depending on available light and your eye's pupil, and between testers who have differing pupils under the same lighting.

Clear skies, Alan

[zuiko]

Hi Henry,

Field curvature in binoculars would be noticeable in wider field binoculars ie. lower magnification in general.

At 1000m the field of view of a 10x42 bin is 110m. Now if you construct a right angle triangle the hypotenuse to the flat field is 1001.5 m indicating that there is only a 1.5m difference between the plane of focus at the centre of the field versus the edge of the field (55m radius to each side of centre).

In practice the depth of field at 1000m would encompass the 1.5m around the plane of focus in the centre - so blur due to field curvature at that distance will be negligible. The diopter difference between 1000m and 1001.5 m is miniscule. Field of view in degrees is 3.148 x 2 = 6.3

Lets make the example at 10m focus at the centre of the field. The field of view is 1.1m. At these distances the field curvature at the edge of the view would be a mere 1.5 cm from the focal plane at the centre. Again I believe this distance is within the depth of field of most binoculars.

With a 7x42 binocular that say has a 150m field of view at 1000m - the field curvature figures are doubled from those above. So you have 3m and 3cm rather than 1.5m and 1.5cm. This is more likely to cause noticeable blur at the edge.

Don't know how much that helps but I'd imagine that field curvature measurements would be much less than 0.25 diopters given the small distances involved between centre and edge of view in field curvature.

Regards.

[Pileatus]

Are you folks using a single lens for your study or are you looking through both lenses?

John

[jcbouget]

Henry,

I’ve performed the test today with my binoculars.
Finally, it’s not so easy, the accuracy of the values below is no less than +/-0.25 diopters, and probably more for some values. In fact I don’t have enough patience to make very careful tests.
The results for the edge of the field are imprecise and, in my opinion, meaningless. For the Zeiss Classic I was unable to determine field curvature and astigmatism at the edge. The most interesting numbers are those given for 50% and 70% from the center.

The first number is astigmatism and the second number is field curvature.

At 50% from the center :
Zeiss classic 10x40 : 0 , 0
Meade 10x50 : 0 , 0
Zeiss 10x42 FL : 0 , 0.5

At 70% from the center :
Zeiss classic 10x40 : 1.25 , 1
Meade 10x50 : 1.75 , 0.75
Zeiss 10x42 FL : 2 , 2

At the edge :
Meade 10x50 : 4.25 , 1.25
Zeiss 10x42 FL : 4 , 2

According to these numbers, it is obvious that the Meade has more astigmatism than the Zeiss 10x40 Classic. The Zeiss 10x42 FL has nearly the same amount of astigmatism as the Meade, but also a strong field curvature, even near the center.
Despite my quick test, I am happy to see that my previous feelings are entirely confirmed.

Jean-Charles

[John Russell]

Hi Zuiko,
I don't believe that these purely geometric considerations are the cause of field curvature, rather that an objective depicts a flat field as a curved image. Hence the need for additional correction of astronomical telescopes, when used for photography. Films don't accommodate!

Regards,

John

[AlanFrench]

Hi Zuiko,
I don't believe that these geometric considerations are the cause of field curvature, rather that an objective depicts a flat field as a curved image. Hence the need for additional correction of astronomical telescopes, when used for photography. Films don't accommodate!

Regards,

John

John,

You are quite right. Not only does the objective have a curved focal plane, but so does the eyepiece. And then you put the two together...

Clear skies, Alan

[henry link]

The test results are likely to vary depending on available light and your eye's pupil, and between testers who have differing pupils under the same lighting.

Alan,

Yes, this does need to be taken into account. The first tests I did were under interior light dimmer than daylight, so I was probably close to using the entire objective of the 4mm exit pupil bins. Maybe there would be some improvement in sunlight for all the binoculars, especially the 7x50. I woud expect the 7x50 to do worse in very low light.

I haven't quite figured out how your test set-up works. Maybe your results will make it clearer.

Henry

[John Russell]

Henry, Jean-Charles,
This is a very interesting thread and opens up the possibility of a more objective qualitative assessment of birding optics in the future. As Alan hinted, the eye's perception of sharpness is affected by pupil diameter (depth of field) so it might be advisable to conduct the accommodation and binocular tests under similar lighting conditions.
BTW, as an old dog myself, I am very sensitive to field curvature, an unusual instance of a "disability" leading to increased perception ;-) .

Regards,

John

[henry link]

Jean-Charles,

Thanks for taking your patience to the limit! I agree that field edges need to be nearly free of astigmatism to allow a good enough focus for a field curvature measurement to seem very meaningful. That can be done with the Nikon SE because there isn't much astigmatism at the edge.

I think calibrating the focus wheels is going to try my patience to the limit, especially a fast one like the Zeiss FL. Until I can accomplish that I tried a different method today to measure the field curvature of several so called "flat field" binoculars; the 8x32 SE, 7x50 Prostar and Fujinon 8x30 FMT-SX. Checked against my reading glasses it appears that the diopter scales on the eyepieces of these binoculars are pretty accurate. All three have very low astigmatism even at the field edge, but only the Prostar has low field curvature. I measured the field curvature simply by focusing on a target (a tree top at 100m) at the field edge and at a point in the field I judged to be about 20 degees from the center (about 70% out in the SE and Fuji). Then I focused the same target at the center and noted the change on the diopter scales. Here are the results. The first number is curvature from the edge and the second number is curvature from 20 degrees off axis.

Nikon 8x32 SE 2 , 1
Fujinon 8x30 FMT-SX 2.5 , 1
Nikon 7x50 prostar 0.25 , <0.25

These results and my earlier astigmatism measurements suggest to me that, with the exception of the Prostar which has a unique design, "flat field" eyepieces actually correct astigmatism much better than they correct field curvature.

It surprises me that you identify the off-axis problem in your FL as field curvature. I tried mine again today and found relatively little field curvature and that is hard to detect because it is overwhelmed by astigmatism. Could we be seeing the same thing at 50%, but interpreting it differently? In my 8x42 star points begin to show what looks to me like astigmatism at about 50% and get worse from there to the edge.

Henry

[henry link]

Are you folks using a single lens for your study or are you looking through both lenses?

John

John,

One lens.

Henry

[jcbouget]

Thanks for taking your patience to the limit! I agree that field edges need to be nearly free of astigmatism to allow a good enough focus for a field curvature measurement to seem very meaningful. That can be done with the Nikon SE because there isn't much astigmatism at the edge.
Henry,

To determine field curvature, I have somewhat cheated : I’ve just taken the average value between sagital focus and tangential focus. When there is a pure astigmatism, like in the FL or the Meade, the result is certainly more precise than guessing the best focus.



I think calibrating the focus wheels is going to try my patience to the limit, especially a fast one like the Zeiss FL. Until I can accomplish that I tried a different method today to measure the field curvature of several so called "flat field" binoculars; the 8x32 SE, 7x50 Prostar and Fujinon 8x30 FMT-SX. Checked against my reading glasses it appears that the diopter scales on the eyepieces of these binoculars are pretty accurate.
My Meade 10x50 doesn’t have marks on the focus wheel or on the diopter setting, so I was forced to imagine this method. I was also afraid that touching the diopter setting would alter somewhat the focus, because the eyepiece “bridges” are not extremely rigid.
Otherwise, I agree that the scale on the focus wheels can be used. The “calibration” with the formula is only used to verify its accuracy (the scale in my Zeiss Classic is 15% too narrow).



It surprises me that you identify the off-axis problem in your FL as field curvature. I tried mine again today and found relatively little field curvature and that is hard to detect because it is overwhelmed by astigmatism. Could we be seeing the same thing at 50%, but interpreting it differently? In my 8x42 star points begin to show what looks to me like astigmatism at about 50% and get worse from there to the edge.
Interestingly, the most accurate result in my opinion is the field curvature of the Zeiss FL.
No, we don’t interpret differently the same thing. You do know the difference between field curvature and astigmatism, as I know it. You simlply have the 8x42 FL, and I have the 10x42 FL. Quite different bins. It is possible that the good eye relief of the 10x42 is obtained at the expense of field curvature. The very short focal length is surely another strong constraint for the eyepiece design, especially in the 10x version.
What surprises me is that nobody has noticed the field curvature of the 10x42 FL.
Nobody ? Not really ! Remenber the post #459 by Kimmo Absetz in the long "sweet spot" thread :
http://www.birdforum.net/showthread.php?t=13239&page=19&pp=25

<< In this test, the point at which resolution dropped noticeably (from 100% to under 95% of maximum) was about 12% off center with the FL 10x42, 15% with the FL 8x42 and 25% with SE 10x42. […] The point where resolution had dropped to 70% of maximum came at roughly 36% off center with FL 10x, 48% off center with FL 8x, and 55% off center with SE 10x. Finally, 45% of maximum resolution came at roughly 55% o.c. with FL 10, 60% o.c. with FL 8, and about 85% o.c. with the SE 10. […] In field use, my impression had been that the FL 10x had somewhat better edge performance than the FL 8x, although significantly worse than the SE or, for that matter, the older Victory II 10x. Now, with my own test data at hand, it looks like the 8x is better than the 10x, which surprises me more than a little. >>

--------> Field curvature ! ;)

Jean-Charles

[zuiko]

Hi,

The problem I see for trying to use any of these methods is that your eye is the final measuring instrument.

For an object at 3m I can turn the focussing wheel 2.5 rubber markings on my Leica 10x42 BR and still have a sharp image in view because of both depth of field of the binocular as well as accomodation my eye exerts.

There are 33 rubber marks around the circumference. So I can move the wheel approximately 30 degrees without affecting the sharpness of the image.

Regards.

[henry link]

Zuiko,

Yes, more variables to take into account and good reasons to be cautious about the absolute accuracy of the numbers in these measurements.

What I hope will be left standing after all the objections and caveats is a method for improving the current haphazard and totally subjective way of describing what we see toward the edge of binocular fields. These techniques will probably generate different numbers for the same binoculars from different sets of eyes. I don't think that will be a problem as long as we can understand why that has happened; whether it is differences in test set-up light levels, accomodation or a happy accident of complimentary astigmatisms between the binocular and the eye. I'm particularly interested in being able to distinguish between binoculars with low astigmatism and high field curvature like the Nikon SE and Fujinon FMT-SX and those with high astigmatism and low field curvature like the Zeiss 8x42 FL, since these things may look superficially similar to me, but could be expected to look quite different to others. From my experience the last few days I think one set of eyes, using a consistent test set-up can make repeatable measurements with enough accuracy to evaluate how different binoculars sort out for these characteristics and that would be a step up from the familar exchange; "the edge is perfectly sharp!... No, it isn't, it's terrible!"

[WmCCO-5]

Zuiko,

Yes, more variables to take into account and good reasons to be cautious about the absolute accuracy of the numbers in these measurements.

What I hope will be left standing after all the objections and caveats is a method for improving the current haphazard and totally subjective way of describing what we see toward the edge of binocular fields. These techniques will probably generate different numbers for the same binoculars from different sets of eyes. I don't think that will be a problem as long as we can understand why that has happened; whether it is differences in test set-up light levels, accomodation or a happy accident of complimentary astigmatisms between the binocular and the eye. I'm particularly interested in being able to distinguish between binoculars with low astigmatism and high field curvature like the Nikon SE and Fujinon FMT-SX and those with high astigmatism and low field curvature like the Zeiss 8x42 FL, since these things may look superficially similar to me, but could be expected to look quite different to others. From my experience the last few days I think one set of eyes, using a consistent test set-up can make repeatable measurements with enough accuracy to evaluate how different binoculars sort out for these characteristics and that would be a step up from the familar exchange; "the edge is perfectly sharp!... No, it isn't, it's terrible!"


Sounding more and more like a Zeiss or BVD witch hunt disguised as scientific banter! Obsessing and fuming over the high rankings of the FL ( which I think are pretty much deserved). My SE's and HG's are good but I believe the FL's are better when you total the subjective and objective, (optical performance), columns as Kimmo touched on in his report and as other less qualified observers have related in various glued-together "studies".

I do not believe that people need or want a three ring binder of measured optical performance data, half of which engineers may not understand, to select a good binocular. Who among you did not realize what we had/have in the SE, Trinoivid or EL without any data? I did and I know that folks much more knowledgeable than myself, such as Henry and Otto, did also.

The FL is excellent and I think that most of the published reviews have some merit. Try not to be so obvious in your contempt for evaluators and products.

Regards,

[Pileatus]

Sounding more and more kike a Zeiss or BF witch hunt disguised as scientific banter! Obsessing and fuming over the high rankings of the FL ( which I think are pretty much deserved). My SE's and HG's are good but I believe the FL's are better when you total the subjective and objective, (optical performance), columns as Kimmo touched on in his report and as other less qualified observers have related in various glued-together "studies".

I do not believe that people need or want a three ring binder of measured optical performance data, half of which engineers may not understand, to select a good binocular. Who among you did not realize what we had/have in the SE, Trinoivid or EL without any data? I did and I know that folks much more knowledgeable than myself, such as Henry and Otto, did also.

The FL is excellent and I think that most of the published reviews have some merit. Try not to be so obvious in your contempt for evaluators and products.

Regards,
It's clear to me that no one is out to denigrate a particular brand. If someone said the SE was clear to the very edge I'd almost agree. Say the same about the Nikon LX and, again, I'd nod positively. Neither, however, is perfect. Tell me the Ultravid is sharp to the edges (reviewers have said exactly that) and I'll laugh in your face. So, when I read an FL review that says "clear to the edge" I simply discount the review as a promotional piece. Most observers see the drop off in sharpness in the FL, some more than others. The Trinovids were always criticized for poor edge sharpness, but no one made such a stink about the criticism. It's an observable fact in the Ultravid, the Trinovid, and the FL and many others. The question is does it bother you.

Guess what? You, along with your eyes, may not see a problem with any binocular and you are the final judge. For years, I used a 10X50 B&L that was poorly collimated and 3 millimeters too large for my IPD. My eyes were so good at the time they compensated for all shortcomings. I guess I just didn't know I wasn't seeing the wonderful things I thought I was seeing. That was a bit of humor, in case you missed it.

The FL is a great binocular for a particular subset of eyes. Why is that a surprise to anyone? Take the FL centerfield, stretch it out to SE standards, package it in an Ultravid housing and I'll be the first to buy it. I'd also sell my SE. If you're looking for the best binocular money can buy you better look at the FL because it clearly is among the very best.

I think the effort is to quantify differences in the hope of explaining what people report seeing. There are reasons we prefer what we do and I'd like to know what they are.

John

[Alexis Powell]

Sounding more and more like a Zeiss or BF witch hunt disguised as scientific banter! Obsessing and fuming over the high rankings of the FL...

What? I don't see this at all. I agree with what John said and I applaud Henry's efforts to quantify/distinguish amounts of field curvature and astigmatism in binoculars. His approach seems to be fruitful, and to have much promise with further refinement. Such data won't supercede hands-on testing of the top models when choosing among them, but it will definitely facilitate communication about different models among those of us, who believe it or not, ARE interested in having more measures of optical performance available. It also helps newly addicted optics nuts to learn how to evaluate what they see through different binos, and what properties are most important for them personally. I find it irritating that I have to look for myself to find out which binos have good edge of field performance (that is, that I cannot trust any but a handful of reviews for this info), and that so few of the decent reviews distinguish field curvature from astigmatism. I have the Zeiss 8x32 FL, and find indispensable for its capabilities, but I am greatly disappointed by the amount of astigmatism it has outside its center sweet spot compared to my Leica 8x32 Ultra. Henry's efforts keep us talking about these issues, help increase consumer awareness, and in turn help keep pressure on manufacturers to address these issues in future models. I'm for all those things.

--AP

[zuiko]

I think what needs to kept in mind is that the eye itself has a very sharp dropoff in sharpness off centre.

You can verify this for yourself by looking at one of the words in the sentences above and then by keeping your gaze on that word try to appreciate how the surrounding text becomes increasingly more difficult to see the further one gets from fixation.

I know there is dropoff in the FL and my Ultravid if I examine the peripheries with eccentric gaze but I am always looking through the centres anyhow, and I can't appreciate the edge dropoff when I am using them normally like this.

The only situation in which this issue would matter at all is if the binoculars are fixed on a tripod and one needs to scan the available fixed image for detail. But the majority of people who do bird watching are not going to use their binoculars like that.

Optical design is always a compromise. Increased cost from correcting this is not sensible in my opinion. The cost will be higher price and also higher weight from using larger diameter lenses and stopping down. Existing designs are a compromise, but these are portable and practical optics we are talking about.

Try looking down an operating microscope or top quality telescope and you will get the edge to edge sharpness you desire; but they are hefty pieces of equipment which don't need to be carried for use and cost many many times more.

Regards.

[Alexis Powell]

Zuiko,
My first real binocular was the Nikon 8x40 DCF Classic Eagle, which still remains unsurpassed in my collection (including Nikon 8x32 SE, Nikon 10x42 HG/LX) when it comes to field flatness and lack or astigmatism across its 7 degree field. When I switched to the Zeiss 7x42 Classic, I found maintaining my gaze and object of interest in the center of the FOV to be an uncomfortable limitation. Since then, I've gotten used to using binos in that way, but I still appreciate/prefer being able to dart my eyes around the view when scanning for warblers in treetops or sparrows in brush (hence my love for relatively flat/anastigmatic binos like the SE and EL). I think that I still do a lot of eye-darting, but have learned, when using a bino like the Zeiss Classic, to make constant adjustments to the focus to correct for field curvature as I move my eye around (I am a very active focuser). That's probably why I am so irritated by the astigmatism of the Zeiss FLs and the Bausch & Lomb Elite (previous incarnation, not the most recent version)--there is nothing to be done about it. My Leica 8x32 Ultra/Trinovid has very little astigmatism, MUCH less than does my Leica 8x42 Ultravid, but the minimal eye-relief of the former make these a draw for me when it comes to overall visual comfort.
--AP

[Pileatus]

What? I don't see this at all. I agree with what John said and I applaud Henry's efforts to quantify/distinguish amounts of field curvature and astigmatism in binoculars. His approach seems to be fruitful, and to have much promise with further refinement. Such data won't supercede hands-on testing of the top models when choosing among them, but it will definitely facilitate communication about different models among those of us, who believe it or not, ARE interested in having more measures of optical performance available. It also helps newly addicted optics nuts to learn how to evaluate what they see through different binos, and what properties are most important for them personally. I find it irritating that I have to look for myself to find out which binos have good edge of field performance (that is, that I cannot trust any but a handful of reviews for this info), and that so few of the decent reviews distinguish field curvature from astigmatism. I have the Zeiss 8x32 FL, and find indispensable for its capabilities, but I am greatly disappointed by the amount of astigmatism it has outside its center sweet spot compared to my Leica 8x32 Ultra. Henry's efforts keep us talking about these issues, help increase consumer awareness, and in turn help keep pressure on manufacturers to address these issues in future models. I'm for all those things.

--AP
Alexis,

Yesterday I had the opportunity to sample an FL 8X32, a Zeiss Classis 10X40, and Cabela's 10X42 Euro (SLC look alike). The Euro was a disappointment and not even close to Swarovski SLC quality. The much-praised FL 8X32 was also a disappointment off center (this mirrors your observation) and I just could not get comfortable with it. The Zeiss 10X40 ClassiC was simply a pleasure to use. The diopter was effortless and the image clear, bright, and detailed. As she enjoyed the ClassisC image, my wife commented how stable they were for a 10X. At $749 these are a bargain.

John

[henry link]

Sounding more and more like a Zeiss or BF witch hunt disguised as scientific banter! Obsessing and fuming over the high rankings of the FL ( which I think are pretty much deserved). My SE's and HG's are good but I believe the FL's are better when you total the subjective and objective, (optical performance), columns as Kimmo touched on in his report and as other less qualified observers have related in various glued-together "studies".

I do not believe that people need or want a three ring binder of measured optical performance data, half of which engineers may not understand, to select a good binocular. Who among you did not realize what we had/have in the SE, Trinoivid or EL without any data? I did and I know that folks much more knowledgeable than myself, such as Henry and Otto, did also.

The FL is excellent and I think that most of the published reviews have some merit. Try not to be so obvious in your contempt for evaluators and products.

Regards,

Bill,

Witch hunt? I know most people are not interested in this stuff. I hope that will be apparent after a few sentences and they will move on to something they actually want to read and discuss. I suppose one potential problem with optogeeks lurking around a forum like this (where else are we going to lurk?) is that some novices may feel inhibited about posting reviews or comments for fear of beeing put down. I honestly don't believe Ive seen much of that here and I hope I haven't been guilty of it. Perhaps your comment about "contempt for evaluators" refers to the Cornell reviews or the recent BVD review of the 8x42 FL. Those evaluators speak from bully pulpits and have a real influence on what people buy. I think they should be held to higher standards.

Regards,

Henry

[Pinewood]

Optical engineers must be able to predict much of the qualities in advance of manufacture. Surely, there are proprietary programs to work these things out in advance of making a single component. The first model or bench assembly determines the bugs or overlooked factors.
What Henry seems to be doing is going after the unstated qualities of a binocular. As he writes this is for "optogeeks," who want to know "how and why" a binocular has certain properties, not just how it performs.
For some of us, knowing how a manufacuter arrived at a design compromise or "opitmization," may be fascinating. In the real world, we chose the manufacturer's optimization which best suits our needs.

Happy bird watching,
Arthur Pinewood :egghead:

[AlanFrench]

Bill,

[SNIP]. Those evaluators speak from bully pulpits and have a real influence on what people buy. I think they should be held to higher standards.

Regards,

Henry

Henry,

I agree completely. It is most disturbing when an evaluator writes about optics and, in writing, reveals an ignorance of optics. At least two well known evaluators have said ED stands for Extra-Dense, rather than Extra-low Dispersion. Many readers assume that these folks know what they are talking about.

Clear skies, Alan

[Chris C]

Henry et al.,

Thank you for an excellent and informative string of posts--I've learned much and have enjoy the unbiased (to my reading) evaluations.

For me, all of this discussion begs the question of why? I generally understand the sources of both astigmatism and field curvature, but is there anything unique about each of the various designs that make them prone to either effect. In particular, are there aspects of the FL design that limited Zeiss' ability to provide both stunning contrast and resolution in the sweet spot and a sharp edges? In other words, can we hope for a modified FL in the future that will be the best of all, or are there inherent constraints as in so much else of optical design?

Cheers,
Chris C.

[John Russell]

[QUOTE=John Traynor]

Yesterday I had the opportunity to sample an FL 8X32, a Zeiss Classis 10X40, and Cabela's 10X42 Euro (SLC look alike). The Euro was a disappointment and not even close to Swarovski SLC quality.

John,

Apart from the badging and minor variations in the rubber armouring, the Cabela Euro looks very similar to the Meopta Meostar B1. Specifications and price would seem to tally as well.
Now I haven't seen the 10x42 and don't want to start one of those defiant "Well I've got 'em and think they're great" arguments but I recently bought some 7x42 Meostars as a Christmas present for my wife and have had ample opportunity to compare them to my own Swaro 7x42 SLCs (Swarobright dielectric level). I picked the Swaros after a visit to Photokina having compared them to everything else on the market including Ziess FLs - a very personal choice, of course.
The only other bins with which I would be equally satisfied are the Swaro 8,5x42 ELs - or the Meostars!
The Meostars are somewhat shorter, and lighter, than the 7x42 SLCs but optically they are VERY similar. Both have better edge sharpness than the Zeiss 7x42 FLs, albeit with somewhat less fov, but show more lateral colour than the FLs. However, this is only apparent in backlit conditions, when one wouldn't get a good view of the bird anyway.
My only criticisms of the Meostars are that the eyecups screw in and out too loosely and that the focussing, while smooth and free of backlash, is still rather stiff.
There have been numerous reports that Meopta supply lenses to one or more of the Big 3 and my experience is that their quality and pricing is extremely competitive.

John

[AlanFrench]

Henry et al.,

Thank you for an excellent and informative string of posts--I've learned much and have enjoy the unbiased (to my reading) evaluations.

For me, all of this discussion begs the question of why? I generally understand the sources of both astigmatism and field curvature, but is there anything unique about each of the various designs that make them prone to either effect. In particular, are there aspects of the FL design that limited Zeiss' ability to provide both stunning contrast and resolution in the sweet spot and a sharp edges? In other words, can we hope for a modified FL in the future that will be the best of all, or are there inherent constraints as in so much else of optical design?

Cheers,
Chris C.

Chris,

The difficulty in obtaining the same resolution and sharpness at the edge of the field as in the center is a general problem in optics. There are two main culperts. Moving off-axis means the optical system is asymmetric, which makes it hard to get all the photons to go where they belong in the image plane. Glass disperses light, so different wavelengths take somewhat different paths whenever they travel through glass. Getting all the colors from a particular point in your field of view to wind up in the same point in the image is difficult, especially when you add in the off-axis part.

In our binoculars we have two optical systems. The binocular, with the objectives, prisms, and, perhaps, a focussing element, forms an image. The eyepiece, serves as a magnifier to examine the image. The binocular must cover a view angle, which is the true field, of perhaps 5 to 7 degrees. The eyepiece covers a view angle, which is the apparent field, of perhaps 50 to 67 or so degrees. (You can see which has the harder job.)

Although we can break down our binocular into two main components with differing job descriptions, they are designed to work together (at least we hope so!).

I don't think we will ever see a binocular that doesn't show some loss of resolution and contrast at the edge of the field, especially if we remember that the eye plays a role here too (and has to cover the same view angle the eyepiece handles).

Clear skies, Alan

BTW - somewhere I read that our eye's performance suffers when we look to the side. I'll have to see if I can find that.

[iporali]

I generally understand the sources of both astigmatism and field curvature, but is there anything unique about each of the various designs that make them prone to either effect. In particular, are there aspects of the FL design that limited Zeiss' ability to provide both stunning contrast and resolution in the sweet spot and a sharp edges? In other words, can we hope for a modified FL in the future that will be the best of all, or are there inherent constraints as in so much else of optical design?
Chris,

That is something I have been interested as well, and I would hazard a guess that it is a compromize Zeiss has deliberately chosen - not only in their FL binoculars but also in their FL scopes, which are known to have some edge softness. They call their objective a superachromat, which is optically more complex than an achromat or even apochromat (see http://www.zeiss.de/C12567A8003B58B9/Contents-Frame/2D4B2DCF22C996C3C12569BC005C33AF). If I understand the end of the link text correctly and speculate a couple of steps further ;) my conclusion is that Zeiss uses the fourth lens of the objective as a "compensation element" which may be deliberately asymmetric to compensate imperfections of the other lenses. If they mainly concentrate on center-field colour-correction, would it be possible that astigmatism is the trade-off?

From Henry's and Jean-Charles's contribution it looks like the edge softness in low-powered FLs is mainly astigmatism and in 10x (where the eyepiece is looking at the centerfield) mainly field curvature. This would indicate that by using a flat field eyepiece they should be able produce an "edge-to-edge" sharp 10x42FL, but to achieve this with lower powers it would probably require significant changes in design and manufacturing processes (=> astronomical prices).

Best regards,

Ilkka

[WmCCO-5]

Bill,

Witch hunt? I know most people are not interested in this stuff. I hope that will be apparent after a few sentences and they will move on to something they actually want to read and discuss. I suppose one potential problem with optogeeks lurking around a forum like this (where else are we going to lurk?) is that some novices may feel inhibited about posting reviews or comments for fear of beeing put down. I honestly don't believe Ive seen much of that here and I hope I haven't been guilty of it. Perhaps your comment about "contempt for evaluators" refers to the Cornell reviews or the recent BVD review of the 8x42 FL. Those evaluators speak from bully pulpits and have a real influence on what people buy. I think they should be held to higher standards.

Regards,

Henry


Henry,

Please excuse the late response, my computer keyboard died on the job, yesterday. Solid answer on all counts, to my original post. Further, I agree with you across the board. Good, meaningful data and unbiased comparisons are frustratingly slow/hard to come by.

Keep up the good work!

Regards,

[henry link]

Bill,

Thanks for the reply!


Ikka,

Your guess is as good or better than mine about all this. One thing I've noticed about information based on camera lenses is that it naturally doesn't consider the effects of eyepiece optics since camera lenses don't have anything analogous to an eyepiece. All the optical elements are in front of the focal or film plane. The enlarging lens is the closest thing to a photograghic equivalent of the eyepiece in a binocular. I think most of the transverse or lateral CA in binoculars originates in the eyepiece. I can remove the eyepiece from a binocular and place it on an f/7 APO telescope and still see about the same amount of TCA. I've never seen any telescope eyepiece that was completely free of it, especially toward the edges of wide field eyepieces, so I think we will probably be stuck with it in binoculars for some time.

Zeiss Victoy and FL, Leica Trinovid and Ultravid and Nikon LX/HG all have objectives with the same number of elements (Swarovski SLC and EL have one less). I suppose you can either describe them as 4 element objectives or 3 element objectives with a focusing element. The focusing element appears to me to resemble a Barlow lens. Its a negative element which changes the focus when moved and also can be used to modify the focal length of the objective. I don't know, but I doubt that any of these surfaces are aspherical. The Nikon Prostar, which is a 20 year old design, does appear to have a non-moving aspherical element between the porroprism and the eyepiece just in front of the focal point of the objective and that may partly explain how it manages to have both low field curvature and low astigmatism.

Henry

[John Russell]

[QUOTE=henry link]
The enlarging lens is the closest thing to a photograghic equivalent of the eyepiece in a binocular.

Henry,

Are not photographic enlarging lenses just like any camera lens?
They are just optimised for close-ups like a macro lens and are indeed sometimes used as macro lenses with a bellows attachment on the camera.
Although the focal lengths are usually longer, nomal camera lenses corrected for infinity are perhaps more like reversed eyepieces.

Regards,

John

[henry link]

John.

I'm sure you're right. Sometimes I prop my 50mm camera lens on my telescope to make a 50mm eyepiece. If I place a telescope eyepiece behind the same lens it becomes a telescope objective with a 50mm focal length. What I meant was that the role of the enlarging lens in photography is similar to that of an eyepiece in a telescope. Both are used to magnify the image that forms at the focal plane of the camera lens or the telescope objective.

Henry

[iporali]

Henry,

Many thanks for your comments. This subject is on the limits of my language skills so I hope what I'm saying is what I mean. ;)


Your guess is as good or better than mine about all this.
Thanks - it may be as good as yours, but most probably less correct :t: .


...since camera lenses don't have anything analogous to an eyepiece.
No, but they are analogous to objective, which I was referring to. My "guess" originated from spotting scope objectives, where Leica traditionally is talking about APOs and Zeiss about superachromats - just like they have done with their respective high-end camera lenses. So I figured out that Zeiss might be just applying "their" solution for colour-correction into their FL binoculars.


I don't know, but I doubt that any of these surfaces are aspherical.
I certainly don't know either, but my claim was actually that the Zeiss "compensating lens" would be purposely asymmetrical/astigmatic to somehow correct the measured imperfections of the three other lenses. I have no idea about binocular/objective manufacturing, but I couldn't understand the idea of compensating lens any other way... and it puzzles me how consistently astigmatic the FLs seem to be - almost like it were part of their design.

Best wishes,

Ilkka

[AlanFrench]

In the world of high end telescopes, some binocular eyepiece holders, which split the light into two paths, have a compensating element that corrects for the color added by the beamsplitter/prism assembly. Perhaps the usage in the world of binoculars is to correct for the color added by the prism system, which would be important in a superachromat.

Of course, it is often hard to know where reality ends and marketing hype begins.

Clear skies, Alan

[Chhayanat]

I do not want to start a new thread and this is the closest I could find to what I want to ask.

2. Should a sample of a high quality binocular - e.g. Zeiss FL, Leica Ultravid, Nikon HG/LX, Swarovski - turn out to have noticeable astigmatism, what would be the undesirable visual effects on field use?

3. Is this kind of astigmatism correctable by the repair shop of the manufacturer or must the offending piece be regarded as consigned to the world of lemons in perpetuity?

Chhayanat

[henry link]

Chayanat,

First of all it's important to know whether you are detecting astigmatism at the center of the field or only toward the edge. This thread addressed only the kind of off axis astigmatism that results from eyepiece design. That kind of astigmatism is absent at the field center and grows gradually worse in a symetrical radial pattern as you move your gaze toward the edge. You can distinguish it from field curvature by trying to refocus the image at the edge. If field curvature is the main problem it may be possible to refocus the edge to a fairly sharp image, but if astigmatism is the dominant problem the image will never come to a good focus. Since this form of astigmatism is a design problem present in every specimen it's not really fixable, and every eyepiece design has at least a little of it.

Astigmatism at the field center is probably a manufacturing defect. In a binocular it is likely to be present in only one barrel or at least be worse in one barrel than the other. It might result from a deformed or tilted element in the objective or focusing element. The deformation could come from too much pressure from a tight retaining ring or an optical defect in the glass. If it is severe enough it makes good focus at the center of the field impossible. A star test with the magnification of the binocular boosted to at least 30-40x is the only reliable way I know to be certain that poor sharpness at the center is caused by astigmatism rather than some other defect. I don't think star testing at low magnification is accurate enough. The biggest problem in fixing it is probably figuring out which element is causing it and why.

Henry

[trashbird]

I don't want to start a new link either, so I will put this comment here. People always ask: why can't a binocular manufacturer simply design a binocular that completely overcomes this or that or all optical defects? My question is more specific. Why don't manufacturers completely redesign a new binocular from the ground up rather than than simply modify existing designs, throwing in an ED lens here or an aspherical element there? I understand that cost is probably the biggest reason not to do a complete redesign. But the premier European companies are charging $1500 dollars for their newest models. Granted, many of their designs are indeed new and not just modifications. But the redesigns often don't seem to be very well thought out. Why are the big optical companies not studying what is written on this forum by people who are both amateur optical experts as well as binocular and scope users of the pickiest and most discerning kind?

While not a premier European binocular, the new Swift Audubon 820 is simply the same optical design put into a (supposedly) waterproof body. Customers were not just asking for a waterproof Audubon. They also had for many years criticized the eye relief issue in the older Audubon 804. At the price point of the new Audubon 820, a complete redidgn was probably out of the question, but it just seems like binocular manufacturers don't really listen to their knowledgable customers and what they recommend for improvements.

The European makers (along with Nikon) have been responsible for new optical technologies that overcome many optical abberrations, and these have filtered down to the lower-end Asian manufacturers. But what I haven't seen is the synthesis of all these technologies that should combine to make binoculars as close as possible to being optically perfect and the most user-friendly. A lot of model designations are just marketing gimmicks. The "FL" in the Zeiss FL is simply another name for ED glass, no?

[ksbird/foxranch]

Hi maybe I'm misunderstanding terminology used here on this forum but I am guessing that "flat field" here has something to do with edge sharpness when the center is in focus or perhaps not. I sort of learned the terminology of the Insttitute of Physics as used by someone like Holger Merlitz (www.holgermerlitz.de) when he describes binoculars testing.

If you want a binocular that is totally sharp to the edge then try to test a Fufinon FMTR binocular and that should give you a standard of reference. If you want a binocular with straight lines that stay straight AND sharp right to the edge, then test a KMOZ BPO 7x30 (too bad the color fidelity is sub-par for many Russian binoculars). I sometimes think that some responders are complaining about distortion on the edge of the field sometimes refered to as "coma" by physicists, where the outer view items get elongated or "dispersed" towards the edge of the field, although there are other reasons why the edge view of a binocular can be out of focus compared to the center. Field curvature due to barrel or pincushion distortion is sometimes built into binoculars. The description of a binocular with true angular fidelity by Holger Merlitz explains how difficult it is to "pan" a binocular with no curvature near the edge of the filed of view. If the binocular is always held still then there is no problem, but if the binocular with no angular bending near the edge of the field is panned (except at sea) then it can make you dizzy or nauseous.

This is a great website and I'm just trying to understand the terminology used by the members who make posts. There are bins with none of the distortions being discussed here. But I rarely see them mentioned in discussions about distortions (like Fujinons). I am just trying to get a handle on exactly what is meant by the way things are discussed.


This thread continues a discussion of field curvature and astigmatism that began with a question on another thread about the correct terms to use in describing softness at the edge of the field.

Today I tried an experiment to see if I could crudely measure field curvature and astigmatism in a group of binoculars. This is all very preliminary, so I’m hoping for some feedback and ideas from Alan and Alexis and anyone else who is interesred, maybe our resident Australian optometrist ?

As a target I used some ordinary lined paper from a memo pad. I taped two pieces on the wall, one with the lines horizontal and the other vertical. The horizontal lines were taped directly above the vertical lines. In each binocular I placed the target at 3 o’clock on the field edge and observed it, using only one eye, from about 25’. I used two pairs of 1.25 diopter reading glasses as my very crude measuring tools.

In binoculars with astigmatism the two sets of lines cannot be brought to best focus simultaneously. In the worst cases one set of lines will look focused while the other set will be spread so wide by astigmatism as to become invisible. I focused the horizontal lines without glasses, then used the glasses to change the best focus to the vertical lines. My only choices for correction were 1.25 diopters or 2.5 diopters, if I wore both pairs at the same time. Clearly I am going to need some 1.5, 1.75 and 2 diopter glasses to make this more accurate.

I used a similar technique to measure field curvature. First, without glasses, I found the best, least astigmatic focus at the edge where the two sets of lines were about equally unfocused. Then, without changing the focus I used the glasses (if necessary) to bring the center of the field into focus.

Here are the crude and preliminary estimates. The first number is astigmatism and the second number is field curvature, both in diopters:

Zeiss 8x42 FL - 2.5, <1
Swarovski 8.5x42 EL - 1.5, 1.25
Nikon 8x32 SE - <1, 2
Nikon 8x32 EII - 1.25, 2
Nikon 7x50 Prostar - almost none, <1
Kern 8x30 - 5, <1

I included the Nikon Prostar because I thought it would be unusually good and I thought the Kern would be particularly bad, although I didn’t realize its problem is almost all astigmatism. Keep in mind that the apparent fields of this group vary from 51 degrees for the Prostar to 70 degrees for the EII and the Kern so they are not exactly comparable unless the fields are close ( EL, FL, SE).

I should also mention that I am 58 and don’t have much focus accomodation, but I don’t know how much. I think the field curvature figures should be the amount of field curvature above my ability to accomodate, but I am unsure about how the astigmatism measurement is effected by accomodation. An ophthalmologist told me recently that she had encountered young people with over 12 diopters of accomodation ( I hope I remember that correctly). For someone like that field curvature in binoculars should never even be noticed.

Alan, was your experiment similar to this (without the glasses)?

[henry link]

There is certainly a lot of confusion about the meaning of optical terms. I often see things like spherical aberration confused with field curvature, field curvature confused with distortion, or barrel and pincushion distortions reversed. A glossary of optical terms would be useful to keep us all on the same page. I found this one: http://observers.org/beginner/glossary.html. These non-technical, and sometimes funny, definitions wouldn't be good enough for optical designers, but I think they are mostly sufficient for us users.

I didn't look at every definition, but, as far as they go, I agree with the ones for terms like "spherical aberration","field curvature", "distortion", and "coma". One exception in my experience is that the author accepts at face value the term "field flattener" as an element for correcting field curvature. One thing that surprised me in my experiments at the beginning of this thread was that the binoculars I tested with "field flatteners" (Fujinon 8x30 FMTR-SX, Nikon 8x32 SE, Pentax 10x50 PIF) did not actually have low field curvature. Instead I found very low astigmatism in those designs. I suspect field flattener is just a better sounding marketing term than anastigmat.

FWIW I've never found any binocular that is completely sharp at the edge of the field. I don't think the impression of complete sharpness at the edge ever stands up to the really close scrutiny of examining a resolution chart at the edge. The problems at the field edge appear to me to be mostly a combination of field curvature, astigmatism and lateral chromatic aberration. I don't see much I can identify as coma in star points at the edge of binocular fields. I seldom disagree with Holger Merlitz, and I'm aware that some designers believe a little pincushion distortion is beneficial for panning, but I'm not at all nauseated by panning with a nearly distortion free design like the Nikon 8x32 SE, or even a binocular with slight barrel distortion, like the Fujinon 8x30 FMTR-SX. Only some old binoculars with pronounced barrel distortion cause enough "rolling ball" effect to bother me.

[hinnark]

Henry,

coming back to topic I don´t understand how you can measure astigmatism by diopter. I think this would work only for field curvature.

Steve

[henry link]

Hi Steve,

The idea was to measure the difference in diopters between the point of best focus for vertical lines and the point of best focus for horizontal lines at the same spot (3 o'clock) on the field edge. I assumed that would indicate the spread between tangential and sagittal focus and a wider spread would mean higher astigmatism. I've found this to be difficult to do very precisely because there is really no "good" focus when astigmatism is high, but it does work well enough to separate optics with very low astigmatism at the field edge, which show vertical and horizontal lines almost simultaneously in focus from optics with high astigmatism, which show one set of lines in focus while the other set is so defocussed that the lines completely disappear.

It's much easier to measure field curvature by simply comparing the positions on the diopter scale (if it's accurate) of best focus on an object at the center and best focus on the same object at the edge.

Henry

[hinnark]

Henry,

now I got it. Thank you for explanation.

Have a nice weekend!

Steve