What make and type of binoculars are sharpest at the edge? (1 Viewer)

WmCCO-5 said:

Zeiss 8x42 FL: Big surpise with this one.
Bill

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Not a surprize with me. I live in an area where Zeiss FL's are very hard to get your hands on (a few Conquests around but no FL's). I finally got to look through a pair last weekend when I was down at the Cabela's in Salt Lake. They had one pair, 8x42, on clearance for $1549 marked down from $1649. I asked to look through them fully expecting to see soft edges to the image as I had read so much about. Holding the binoculars still and moving my eyes about the FOV the edge sharpness looked to be every bit the match to my 8x32 SE's. When I focused on small text about 100 yds. away and moved the binoculars so the text was moved to the edge of the field it stayed sharp very close to the edge, again seemed to be a match to the SE's. I thought the image was fantastic. I loved the way the FL's fit my hands. The focus felt a bit sticky when you initially started to turn the knob and overall felt kind of plasticy instead of metallic, but worked well enough. Maybe this was a really cherry pair, but it sure took me by surprize. This was not scientifc testing by any means, but I really value wide FOV and as much sharpness as possible across that whole FOV and these impressed. Believe me, if $1549 had been within my budget I would of walked out with them.

Bill,

I've noticed the same difference between horizontal and vertical edge performance. In fact I've seen it to some extent in every binocular when I've looked for it. I don't think it is a characteristic of any particular binocular's optical quality, but has to do with differences in the way the pupil of the eye aligns with the exit pupil depending on whether we look to the side or up and down. To simplify things try using only the right eye and the 3 and 6-o-clock positions. I find that even if I rotate the binocular 90 degrees I still see worse edge performance at 6-o-clock. My speculation has been that looking to the side cuts off more of the opposite side of the exit pupil than looking down does. That means that more of the 9-o-clock side of the objective is blocked from forming edge astigmatism at 3-o-clock than the top of the objective is blocked when looking down.

I tried testing this idea today by blocking off parts of the objective of the 8x42FL. I found if I masked the top half of the objective the edge performance at 6-o-clock improved slightly but when I blocked off the lower half it improved much more to approximately equal the 3-o-clock position. Then I tried masking the 9-o-clock side of the objective and saw no significant improvement at the 3-o-clock position, but saw some small improvemant if I masked at 3-o-clock. In the end I found that masking ANY side of the objective improves edge performance somewhat. All this tends to suggest to me that the partial blocking of the exit pupil by the eye pupil position has a big effect on what we see when we try to examine the edge of the field, but I'm a long way from understanding just how it works.

The fundamental difference I see between the edge performance of the 8x32SE and the 8x42FL is that field curvature dominates in the SE and astigmatism dominates in the FL. You can focus a star to a pretty good star point at the edge of the SE, but the same star at the edge of the FL cannot be brought to a good focus. Instead it goes through the classic pattern imposed by astigmatism. The star is seen as a vertical line one one side of focus, horizontal on the other side and forms a little cross at "best" focus. BTW the edge performance of the 8x42FL is really the only thing that can be "star tested" at 8X. To see the true diffraction pattern of a centered star requires about 30x (60x is better) if the objective size is 42mm.

Henry

cbushme,

Glad you had an opportunity to try the FL's. Your observations seem to fit those of the vast majority of owners. In many cases, we only hear the negatives/complaints such as rough focusing in Brand X, internal fogging in Brand Y and so on.

The FL's that I have been privileged to own for two years have been excellent in every regard. They were Shot Show demos, (apparently untouched), from Natchez Shooters Supply and were about 1/3 less than the best price that you were quoted at Cabelas. They arrived in new condition with a full warranty. When I checked with Zeiss they had Natchez's parent company listed as a fully authorized dealer and the bins were registered as new. You can google Natchez if you are interested. The banner for the Zeiss products is shown at the bottom of the opening page. I can assure you that they are reputable and honest people and that they encourage returns if the customer is in the least dissatisfied.

Gotta run.


Best,
Bill

henry link said:

... My speculation has been that looking to the side cuts off more of the opposite side of the exit pupil than looking down does. That means that more of the 9-o-clock side of the objective is blocked from forming edge astigmatism at 3-o-clock than the top of the objective is blocked when looking down...

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No one responded to my question/observations on a couple threads in the Zeiss forum w/respect to eye position and the amount of field edge astigmatism in the FL, but my own (perhaps cynical) thoughts on how to explain it have been along these lines. By keeping one's eyes centered while looking to the edge, there is some stopping down of the exit pupil as one looks to the edge, which may reduce astigmnatism. My description of slightly decentering my eyes to better see (by minimizing vignetting) into field edge of binoculars might even be wrong. Perhaps, given pupil movement, my "decentering" is actually maintaining alignment!
--AP

Henry,

Your testing and discussion of our (similar) observations is incredibly interesting. I'm still revisiting your well written synopsis and attempting to take it all onboard. The human eye's relationship with the exit pupil of an optical device is intriguing.

If convenient, I would like to revisit this with you and Alexis a bit later. By-the-way, you are correct, I notice the same or similar image degradation in the distal "arcs/segments" at the top and bottom, (read @ 12 and 6), in my 8x32 BN, but little or none in the SE. In the BN the softness appears to be mild astigmatism rather than field curvature (at 12 and 6). I would rate the subjective/unmeasured areas of image degradation, (astigmatism/FC/other), in my two samples as follows:

Bin / 12:00 o'clock/ 6:00 o'clock/ 3:00 and 9:00 o'clock/

FL / 15-20%/ 25-35%/ None
BN / 5-10%/ 20-30%/ 2-5% (Field Curvature)




Bill

Bill,
I can't seem to duplicate the excellent result you see at 3-9 o'clock in the FL. I agree that there isn't much change at different clock positions in the SE, perhaps because there isn't much astigmatism to be eliminated by masking part of the exit pupil.

I'm sorry I somehow missed Alexis' posts about edge performance and pupil position. Evaluating edge performance is certainly frought with pitfalls. There is really no way to move the edge of a 60 degree field to the center of your vision without looking through the binocular unaturally in one way or another. Perhaps we should call it the Binocular Uncertainty Principle; if you look directly at the field edge to evaluate its performance, you change its performance. I do think the very edge of the field is not so important unless it is so poor that you can detect the problem even with peripheral vision. I would say in normal use direct gaze tends to wander around the center 30 degrees, so the area that establishes the percieved "sweet spot" size is between 10 and 20 degees off-axis. For my eyes a binocular which maintains a semblance of center sharpness out to 15-20 degrees off-axis will probably appear to have a very wide and comfortable sweet spot while a binocular that begins to show deterioration at 10-12 degrees will probably look bad even if its performance at the field edge is OK.

Henry

henry link said:

There is really no way to move the edge of a 60 degree field to the center of your vision without looking through the binocular unaturally in one way or another.

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I'm glad you said that, Henry. I thought it was just me.

henry link said:

Perhaps we should call it the Binocular Uncertainty Principle; if you look directly at the field edge to evaluate its performance, you change its performance.

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Link's Law?

Michael.

henry link said:

... Evaluating edge performance is certainly frought with pitfalls. There is really no way to move the edge of a 60 degree field to the center of your vision without looking through the binocular unaturally in one way or another. Perhaps we should call it the Binocular Uncertainty Principle; if you look directly at the field edge to evaluate its performance, you change its performance. ...

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Acknowledged, but my point/issue has been that there seems to be something different about the eyepiece design of at least the 8x32 FL such that when I look off center in my preferred way (by slightly decentering/shifting the position of the eyecups relative to my glasses), I get a massive increase in astigmatism relative to what I see if I only rotate my eyes (and risk edge of field vignetting/black-out). I know that one is not supposed to do this when looking through binos but the fact is, I can do it with my other binoculars to overall viewing advantage, and without significant ill effect (certainly without increasing astigmatism in the same fashion). This sensitivity of eye placement is what makes the view through the FL seem constrained to me (despite their wide field) and it contributes to the impression of them having a small sweet spot.
--AP

Otto McDiesel said:

I think the SE, followed closely by the LX. Swaroski's (all of them) are very close.

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Ditto on Otto's selections.

henry, thanks a lot for your reply. I didn't realize there is 6 o'clock to 3/9 o'clock until you mentioned it. Yes, i see the same effect. Although it does not bother me at all since I mainly use inner 2/3 of FOV any way, it is quite interesting to see this effect. I also noticed that 8x format edge fuzziness is way better than 10x binoculars. Is field curvature worse for higher power binoculars?

I won't be able to comment on some of the binoculars mentioned above, but I have published a table of sharpness ratings for about 25 different binoculars, all tested to the same standards with data recorded over several years.

Nikon SE 12x50 is significantly sharper across the entire field than the Fujinon 16x70.

Out to 80% of field, the Nikon SE 12x50 is still best BUT then beyond 80% out, The Oberwerk Mariner 10x60 surpasses both the Nikon SE 12x50 and the Fujinon FMT SX 16x70 out to the edge.

Between 70-80% out, the William Optic 7x50 ED begins to surpass all three mentioned above. It is capable of equivalent sharpness at 90% out that can only be seen in the Fujinon and the Mariner at 65% out. The WO 7x50 ED at 90% matches the Nikon SE 12x50 ability at 80%.

The Pentax 10x50 PCF WP at 75% out matches what the Oberwerk Mariiner can see at 85% out.

So for the binoculars above,
inside 70-80% of the fov, the rank would be Nikon SE 12x50, Fujinon 16x70, WO 7x50 ED and Oberwerk Mariner 10x60, then Pentax PCF WP 10x50

outside 80% of the fov, the rank would be WO 7x50 ED, Oberwerk Mariner 10x60, Nikon SE 12x50, then Fujinon 16x70 and Pentax PCF WP 10x50


For giant astonomy binoculars, most of the 15x70s, 20x80s and 25x100s do not equal the sharpness of field of any mentioned above. BUT the Oberwerk binocular telescope, BT100, with interchangable eyepieces, allows the use of well corrected eyepieces. The BT100 with a pair of 20mm TV plossls has an edge sharpness that none of the others above can achieve. At 31x, I was able to watch a 7.1 arcsecond double star drift right off the edge.

You can judge the sharpness of one power binocular to any other power binocular by observing even magnitude double stars. The doubles, and careful notes on field of view position, allow you to determine at what point in the field of view the double is no longer visible as a cleanly separated pair, a very precise measure of sharpness, repeatable from one binocular to another. A variety of doubles separations provides you with a mapped scale of readings across all the fields of view of your various binoculars.

Binoculars of different magnifications will require different separations of doubles pairs. It should be easily recognized that while a 10x binocular might see a 24 arcsecond pair clean, a 12x binocular of equal sharpness must be able to see a 20 arcsecond pair at the same position. Likewise an 8x binocular would need to see a 30 arcsecond pair at the same position to be considdered equivalent to both above. Since you can very easily multiply the separation of the doubles viewed by the magnifiaction of the binocular used, you can tabulate all the readings as apparent separations. These apparent separation readings are then comparable across any magnification of binoculars you would like to compare and allows ranking of sharpness.

edz

Thanks for this summary of your extensive efforts. One thing I'm puzzled about is that although everything is expressed in terms of sharpness over some percentage of the field, it seems to me that the fields would typically differ between binoculars. What have I missed?

Blue skies,
Ed

Well, you haven't missed anything. It's the easiest way to record and report the information. Fields will always differ between binoculars. Reporting information by percent field once again allows you to utilize the information to compare one model to any other.

In my experience, in almost all cases, viewers are most concerned about what percentage of the field they have will be in sharp view. I have found the most sought after value is percent of field, regardless of what that field is.

For example, let me tell you about two binoculars. One is very sharp accross 3.0° of field. The other is equally sharp across only 2.0°. Which binocular has the better sharpness, the one that shows a sharp 3.0° fov or the one that shows only 2.0°?

In the data I've compiled, all the other information necessary about Afov or Tfov is reported in the tables and in the discussions about testing, so the user who wants different information has it readily available. Actually, converting to degrees Afov takes just a matter of seconds of thought.

When recording data, I'm fairly meticulous about positioning. I report data in 5% increments and I would say the accuracy of positioning is half that increment. I do not ever estimate the position of sharpness readings. I place target stars in the fov such that I have other field stars on the field stop. In this way, I can record the data in the field, then later refer to charts to determine exact position of the target stars in the fov by measuring the distance from the target stars to the filed edge stars. All of this results in percentages of field.

Of the two binoculars in the example above, the one that is sharp for 3° is a 10x50 and has a 6.5° Tfov, so only 45% of the Tfov is sharp. The one that is sharp for 2° is a 25x100 and the Tfov is only 2.4°, so the 25x100 binocular is sharp across 80% of the field. The 10x50 should be rejected as a poor binocular. The 25x100 is one of the finest in it's class.

Here's another example. I have two binoculars. One shows a sharp image across about 45° of the Afov. The other is extremely sharp but only across an Afov of 40°. Judging by sharp Afov, which would you prefer?

As it happens the binocular that has a sharp 40° Afov is the Pentax 16x60 (Afov = 45°) and literally shows almost no aberration across more than 90% of the field. The other happens to be a 20x80 with a 3.7° fov, or 74° Afov. Beyond 60% out image quality gets so poor that it ranks as one of the lowest quality binoculars in it's class.

edz

In birding applications I think there is a lack of agreement concerning the tradeoff between sharpness falloff from center to edge, and the AFOV size. Both properties enter into birding tasks, since centering targets for foveal vision is important and detecting peripheral motion is also important. This might be contrasted with protracted off-axis viewing of several targets at once, as might be the case in astronomy. (I don't know, since I'm not a sky observer.) Speaking for myself, in daylight birding the vividness of color gradation is the single most important visual quality. Since this can only be optimized near the center with correction of axial CA, to the extent this is accomplished the rest of the field takes on secondary importance. The extreme edge of a very wide apparent field has almost no importance to me so long as it supports motion detection when I'm viewing near the center.

Just my opinions, of course.

Blue skies,
Ed