Usefulness or otherwise of 20x fixed eyepiece (Nikon ED50 related) (4 Viewers)

Steve, a diffraction limited 50mm optic can resolve details as small as the Rayleigh limit subject to the atmospheric conditions, details far smaller than can be seen with the naked eye. The mag limits represent the powers needed to make those fine details visible to us weak-eyed humans.

In practical terms, the math says detail as small as ~0.5mm in size should be "seen" at 20x in the ED50 at a distance 40m. Think bird feather details. But the microscopic dust mites living on those feathers might always be "invisible" no matter how much mag you apply since they are smaller in angular size at 40m distance than the Rayleigh limit of the 50mm objective.

I think 80x would be way too high given that the scope would be considered diffraction limited at just ~50x. Making a capable SP erecting prism is the main hurdle to a better than diffraction limited spotter. More than 50x magnification could be useful for splitting double stars but I suspect the airy disk of any point source would be a blurry blob way before 80x in the ED50.

Most of the things we look at during the day could be considered extended objects anyway where contrast (AND brightness) is key to resolving detail. Adding a small lens hood to the front of the ED50 can improve that performance aspect.

Rick,

I agree with you that a person with higher acuity needs less magnification to resolve the smallest details in a scope image, but I disagree with the particular values you’re assigning to eyesight acuity and telescope resolution.

To begin with, there’s some longstanding confusion about the arcsecond value for eyesight acuity because eyesight measurements from eye charts are traditionally expressed in lines rather than line PAIRS. Telescope resolution, measured the same way, is based on line pairs, so the traditional figure of 60” given for 20/20 vision should be doubled to 120” for comparison to telescope resolution. The Rayleigh criterion (138/D) is based on resolving point sources (double stars), rather than line pairs. Point sources are harder to resolve than line pairs, so for line pair charts a value closer to the Dawes criterion (116/D) is more accurate for a diffraction limited telescope. If you substitute those numbers (120” eyesight acuity and 116/D telescope resolution) then a magnification of around 52x instead of 22x is needed by a person with 20/20 vision (120”) to see the smallest line pairs resolved by a diffraction limited 50mm scope (2.32”), 120”/2.32”= 52.

You can determine how much magnification a person with better than 20/20 eyesight needs by just substituting a different arcsecond value for eyesight. For instance, someone with extremely exceptional 20/10 eyesight can resolve 60” line pairs rather than 120”, so that person should need only 26x to see the resolution limit of a 50mm scope (60”/2.32”=26). These numbers are, strictly speaking, only good for resolution of high contrast line pairs on a well illuminated chart. The details we might want to see on birds, under field conditions, are likely to be more difficult and so may need somewhat higher magnification than that. I keep in mind the telescope maker Jean Texereau‘s rule of thumb for the magnification “sweet spot” for detecting the smallest low contrast planetary details in a high quality scope. That’s a magnification 20% above the aperture of the telescope in millimeters. Experience led him to conclude that for magnifications below that value the low contrast details are too small to be seen well and above it the details lose too much brightness and contrast to be seen well. That sweet spot may be a little high for birding scopes; for them a magnification equal to the aperture in millimeters seems about right to me.

Henry

Rick I was just using what Henry said he used on a Pentax PIF 10x50 binocular 8x10X equals 80X for lines pairs resolution.

So I guess the little Nikon ED50 is able to use at least 40x .

henry link said:

I keep in mind the telescope maker Jean Texereau‘s rule of thumb for the magnification “sweet spot” for detecting the smallest low contrast planetary details in a high quality scope. That’s a magnification 20% above the aperture of the telescope in millimeters. Experience led him to conclude that for magnifications below that value the low contrast details are too small to be seen well and above it the details lose too much brightness and contrast to be seen well. That sweet spot may be a little high for birding scopes; for them a magnification equal to the aperture in millimeters seems about right to me.

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That's a 1mm exit pupil then, about the smallest that can be used relatively easily in the field, provided you use a good tripod, it isn't too windy and the atmospheric conditions allow the use of high magnifications. I totally agree with you here. I've got 20/20 vision, and I definitely see more details on a bird at 60x with my EDIII than at lower magnifications.

By the way, that's the reason why I'm so unhappy about the latest crop of zoom eyepieces. 25-50x on an 80mm scope is a bad joke IMO, and I personally suspect some manufacturers use the wide angle of their latest zooms as an excuse to limit the magnification, making it easier to hide manufacturing flaws. Even the 60x of the Kowa eyepiece for the 883/884 doesn't do that scope justice.

Hermann

My thoughts are if a spotter is not useable to 1x per millimeter of lens diameter in the right conditions it is not a good sample. So far I have had two Nikon 60mm Fieldscopes with the one I have now an ED version and both work up to 60x.

I agree with Herman about 25-50 etc.

22X sounds a bit low for the optics the ED50 has,..A good ED50 ,that is...My actual unit was pretty average,..star tested it with the 13-40 at max power and saw a bit of a pattern that didnt look great...SA was not present ,and clear rings ar present in and out of focus, but i can see a bit of astigmatism and was able to see some coma-miscollimation too..the scope performed poorly at anything but 13-16,and 20 X was already a bit frustrating...40X useless in most situations...I think I re-collimated the scope to very good standards,and have been meaning to comment about it in the forum,now seems a good time..
I read someplace that the objective cell was easy to uscrew,so I did loose mine a little.. I didnt remove the cell, just made it slightly loose ..I kept the objective pushed down and could feel that I was able to turn It..My main interest was to know if this miscollimation was inherent to the objective ,or if it was created in some other area of the scope,..so I figured turning the objective around would grant me some information..I did turn it slightly,tight the cell back and tested..no difference..I repeated the operation this time turning the objective 90º...guess what?...I tested again and most of the coma was gone..I did another slight turn and could see no coma...Astigmatism seems to be less evident either...This is very subjective but the visual tests are conclusive...I had a distant target, A label in a heater box,in the roof of a nearby house...Before my experiment , I was not able to discern individual letters at 40X nor I was able to read them at 104X(13x boosted with my 8x binos),but after the turn took place,at 40X I could
see how many letters or numbers were in a sequence and boosted at 104X I WAS ABLE TO CLEARLY read some of the print..I was very happy and gives a bit of info on the scope capabilities..
I dont have the zoom anymore and use the 20X WA ,and I like it..I boost the image to 160X and althoug too much magnification ,IT STILL GRANTS MORE DETAIL THAN 20X,even with the break-up of the image..I think a 2X or 3X doubler would be a great accesory ..I saw the bushnell elite 2.5X(or is it 3X?) going really cheap on ebay for a while..I should look for one of these..
Still have to recognize that 20X gives its best under 25 m and anything beyond..is beyond...but I am enjoying a lot close range woodland observation...The pointing device i use is primordial for me ,to this type of use...I use a red dot finder and gives me amazing,fast accuracy in close range...Here are a couple of pics of the scope with the device..

henry link said:

Rick,

I agree with you that a person with higher acuity needs less magnification to resolve the smallest details in a scope image, but I disagree with the particular values you’re assigning to eyesight acuity and telescope resolution....

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Henry, sorry for the delay but after several attempts at writing rather long-winded counter arguments, I have decided in the end it makes no difference what values we think represent the "facts" in our practical observations. Like I said, it's all fun with numbers for me at least.

BUT in modern optical engineering it is the Rayleigh Limit and normal visual acuity = 1 arc minute that are used in the calculation steps when designing an optical instrument. These are the common accepted engineering values since they are mathematically proven, not just the observed values by a fortunate few with 2 sigma acuity under ideal conditions.

If you know of a optical design software or OEM that considers Dawes, Sparrow or other hyper limits for normal visual acuity when building their instruments, please let us know! For now, I remain pessimistic that the sports optics OEMs build their intruments to approach the razors edge of optical quality. Just no profitable upside to doing it.

Rick,

As it happens I just ran into this article, which I think is the best thing I have seen on this subject.

http://www.cityastronomy.com/rez-mag-contrast.htm

Somehow I had never encountered the Campbell-Robson diagram before.

Notice that the definition of line pair resolution in the article is illustrated as the separation between two lines (“a” in the diagram that compares point sources to line pairs). 60“ for 20/20 vision is correct under that definition. However, the definition of a line pair is often given as the distance between the centers of the light (or dark) line pairs on a chart, which would be “2a” on that diagram and would bring line pair resolution into conformity with point source resolution. The conversion formula that comes with the Edmund 1951 USAF chart, for instance, assumes the second definition.

Henry

I take from this that a 50mm perfect scope could possible take 98x with someone that has 20/20 eyesight and after that it would be empty magnification for spitting double stars. I need to reread that again, but have to get off of here.

Steve,

The smallest resolvable element on the USAF chart is just barely possible for me to see at 50x through my Takahashi 50mm scope. It's difficult to see because it's apparent size is still quite small at 50x. It's much easier to see when its apparent size is enlarged by increasing the magnification to 100x. I don't experience any loss of resolution on the chart by increasing the magnification to 200x, but I can't resolve any smaller elements. Eyesight no longer matters at such high magnifications; the scope resolution has become the limiting factor.

Henry

Henry,

If I understand what I read from that link. Someone with 20/10 super vision could possible see that same thing with about 50x [25x per inch]using your scope give or take a few.


I know I have read that some Pacific Island people could see Venus phases without optics and how we knew was because these people asked why Venus was backwards in a telescope. That is if I remember this right.

Rick,

additional to the information Henry gave I would like to add that, if memory serves me right, according to the latest edition of the Handbook of Optics (Volume III), Chapter 2, there is a maximum resolution power of less than 10" of human vision mentioned. I wouldn't overestimate the knowledge of optic makers in respect of physiology of perception or human vision. In general there are two groups of people working at optical companies: technician and physicist on the one side and business economist on the other. Both groups have, to say the least, only vague and mostly one-dimensional conceivabilities about how human vision really works. A typical example is the idea that the fovea works like a photo sensor where the number of cells, just like the number of pixels, could translated directly into resolution power. Unfortunately, these days apparently there seems to happen not much research into topics like human vision. So much of the results we are dealing here came from WWII and cold war times when the military had an interest about it. I doubt that all of these results are still valid and up-to-date.

There is no absolutely scientific accuracy needed to do this little experiment with a couple of people of normal (no hyperacuity needed!) vision. For this experiment naive users (no birders, hunters or astronomers please, but children and wives use to be unerring ) are best suited because they should be mostly free of prejudices. Give them pair of binoculars of the same magnification and lens diameter with known very good resolution and another one of low resolution. I have done this already several times and find the results pretty amazing. A significant number of people are able to determine within a few seconds, which binocular give the sharper view, that is to say the one with the higher resolution. It could be a very enlightening experience to works with such impartial groups. For instance if you gave them distorsion free binoculars and ask them to pan around. How much of them do see something like a rolling ball? Everybody could give it a try to find out more.

Steve

henry link said:

Rick,

As it happens I just ran into this article, which I think is the best thing I have seen on this subject.

http://www.cityastronomy.com/rez-mag-contrast.htm

Somehow I had never encountered the Campbell-Robson diagram before.
Henry

Click to expand...

Henry,

Once again you exceed any reasonable expectation!
This is wonderful stuff.
Wherer else would we get this kind of info on our hobby?

Yes, I remember Mardi's website many years ago when she was active on Cloudynights. She is obviously an excellent researcher and meticulous in her observations. BUT, like most of us here she is self taught on the subject of optics with no formal education/professional background in the field.

She likes using the Moon for her testing. Her basic premis behind all her research is people wildly overestimate their local seeing conditions and the useful objective size/quality needed in their optics. She argued a 6" achromatic refractor is "good enough" for most amatuers on most nights. Eventually she was "booed" off the forum and stopped posting. No one likes a party pooper!

hinnark said:

Rick,

...I wouldn't overestimate the knowledge of optic makers in respect of physiology of perception or human vision. ...

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Yes, but there is apparently a natural human tendency for us to overestimate our own physical and mental capabilities compared to our fellow man. Statistical studies have shown this true across a wide spectrum of the population. The most amusing result being 80% of any population believes they are smarter than the average!8-P

Frankly, I feel vindicated that most find more magnification useful to see detail as that is EXACTLY what the science predicts when you have less than perfect optics, visual acuity, and lighting/seeing. I myself use the ED50's max 40x quite often. I would be more puzzled if folks were going the other way, saying that the can see all there is to see at 13x!

RJM said:

I would be more puzzled if folks were going the other way, saying that the can see all there is to see at 13x!

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Well, I sure don't, and my vision has consistently tested to be ~20/12. Anyone who owns the 13-40x can do their own test whether they can see any more detail at 27x than 20x and select their fixed power eyepiece accordingly.

--AP

RJM said:

Yes, but there is apparently a natural human tendency for us to overestimate our own physical and mental capabilities compared to our fellow man. Statistical studies have shown this true across a wide spectrum of the population. The most amusing result being 80% of any population believes they are smarter than the average!8-P

Click to expand...

You might be right with that but I cannot see what this has to do with the topic of this thread.

RJM said:

Frankly, I feel vindicated that most find more magnification useful to see detail as that is EXACTLY what the science predicts when you have less than perfect optics, visual acuity, and lighting/seeing. I myself use the ED50's max 40x quite often. I would be more puzzled if folks were going the other way, saying that the can see all there is to see at 13x!

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I still don't understand the logic on this. It might be the case that someone with perfect vision doesn't need the same magnification compared to someone else who doesn't have perfect vision to see the same amount of details. But, given that the scope performs well (mine shows a perfect result in star testing, BTW), why shouldn't someone with perfect vision benefit from the ED50's max 40x as well as someone else without? I think, at least at bright daylight and also at the stars, the only difference is that the first person would see more details at 40x than the other at the same magnification.

Steve

Steve, perhaps you don't know that without sufficient magnification to reveal the Airy Disk (50x+) you are not performing an optically "challenging" star test. Since the ED50 eyepiece choice limits you to 40x I think the best you can do is get an idea of the degree of axial alignment, astigmatisim, over/under correction present.

Look there are only 2 parts to this equation. The Rayleigh Limit is a barrier to the size of detail that can be resolved at ANY magnification. If you believe the ED50 is manufactured to a higher precision than the Rayleigh Limit, well I've got some magic beans to sell you too. If your acuity is better than 1 arc minute then you need less magnification to see this Rayleigh limited detail. If your acuity is worse then you need more to see the SAME detail. That simple.

Like I said in above in post #41, the math says one can see Rayleigh Limit sub-millimeter linear-sized detail at 20x at a distance of 40m with normal vision and a Rayleigh limited optic. Now go try that test.

Alexis, as 2 Sigma acuity outlier you have the same level of "natural talent" as a pro athlete. Maybe there is a second career for you somewhere as an optical test subject!

Thank you for this very useful discussion, and that very interesting article.

Rick,

since I have actually a shortage of time participating here only a few short remarks that could help to understand things a bit better than before. If you could take a look at chapter 18.10 of Rutten and van Venrooij's (page 223-225 in the 5th edition) Telescope Optics you can see why your calculation in #37 isn't correct. As a side note I'd like to mention that it is also not right to use the Rayleigh criterion for terrestrial resolution determination since this is, as well as the Dawes criterion, valid only for point sources of light.

As a rule of thumb one could say (you can reread this or similar in many references, BTW) that for terrestrial use, where contrast plays a very important part for resolution, a maximum magnifiation of M=1.0*Dmm is a realistic value, just as Henry found out in his careful testing (#50). In case of the ED50 thus 50x fits well. For targets like for instance double stars the value for usable maximum magnification is much higher: more than 2*Dmm.

Steve