Exit pupil mysteries - Spherical Aberration? - too short eyecup phenomenon (1 Viewer)

[Tobias Mennle]

Eyecups should be 10mm longer to keep me in exit pupil, but then image vignettes...

So I have 6 alpha 8x binos for a couple of weeks testing. 5 are very good without major issues, it´s more a matter of taste and money which one to prefer.

I had a really puzzling phenomenon with one though. And that´s probably the sharpest and the one with best colour. No matter if I watch objects in 15 or 4m distance, I have more eyestrain than with the others (collimation and diopter is perfect) and I search for focus quite a bit, it´s almost sharp but missing that last bit of real sharpness. If I put my eyes further away, in about 10mm in front of the eyecup rim the image snaps into precise focus and becomes really crazy sharp. But the image is then vignetting.

I projected the exit pupil on black cardboard, and surprise: It is 10mm in front of the eyecup in its longest position. The image is definitely vignetting in that position. On the other hand, the manufacturer´s specification for eye relief fits with my observations. That is, the eyecup is simply 10mm too short. Plus, too strong vignetting for my taste, maybe from tight baffling?

Any ideas or similar observations?

 

[OPTIC_NUT]

It's hard to place the issue...

Some binoculars have the 'blackout' issue, where there is one tight spot the
eyes have to be in, or the image develops a big black spot. There is spherical
aberration, but that's just a consequence of the rays converging on the image
too steeply. Forcing an eyepiece for absolute maximum performance can do that.
Obviously, at your extra-sharp position there is no aberration.

Then, some binoculars have a super-sharp 'extra 3D' effect at one special spot.
That often comes with collimation being off.

What you're describing doesn't neatly fit either, but you can learn more
by backing the eyes away. As you move the eyes away, to maybe a few inches back,
can you form a nice stable image, so you are just 'approaching the keyhole' and seeing
parts clear? This is a comfortable design, where the rays are substantially parallel
going into the final image. Does it black out or get sensitive? ...design is too forced.
further out, do you see double, or different images? This is collimation trouble.

So...I'm thinking, moving the binoculars away slowly might magnify the problem and
tell you what it is. Or...give more clues.

 

[Tobias Mennle]

Then, some binoculars have a super-sharp 'extra 3D' effect at one special spot.
That often comes with collimation being off.

That´s interesting, I have one bino like this in my cupboard and always wondered how can it be supersharp extremely 3D and decollimated at the same time...

 

[LPT]

"collimation and diopter is perfect" - How do you know the collimation is perfect?

 

[WJC]

So I have 6 alpha 8x binos for a couple of weeks testing. 5 are very good without major issues, it´s more a matter of taste and money which one to prefer.

I had a really puzzling phenomenon with one though. And that´s probably the sharpest and the one with best colour. No matter if I watch objects in 15 or 4m distance, I have more eyestrain than with the others (collimation and diopter is perfect) and I search for focus quite a bit, it´s almost sharp but missing that last bit of real sharpness. If I put my eyes further away, in about 10mm in front of the eyecup rim the image snaps into precise focus and becomes really crazy sharp. But the image is then vignetting.

I projected the exit pupil on black cardboard, and surprise: It is 10mm in front of the eyecup in its longest position. The image is definitely vignetting in that position. On the other hand, the manufacturer´s specification for eye relief fits with my observations. That is, the eyecup is simply 10mm too short. Plus, too strong vignetting for my taste, maybe from tight baffling?

Any ideas or similar observations?

Hi Tobias:

I understand what you mean by "collimation" is perfect. However, the words "collimation" and "perfect" do not belong in the same sentence—it's a matter of acceptability. It’s semantics, I know, but it can be IMPORTANT semantics.

By "diopter" I assume you mean focus. This is puzzling because you are in control of that.

Eyestrain is normally caused by a lack of “accommodation”—usually spatial (collimation issue). Any aberration that might cause discomfort would need to be so severe as to be noted, immediately.

With your image getting crisper as you pull back—thus shrinking your field—it sounds as if you are experiencing problems with severe astigmatism—yours, not the binos. :cat:

Just a thought; have a great day.

Bill

 

[Hermann]

With your image getting crisper as you pull back—thus shrinking your field—it sounds as if you are experiencing problems with severe astigmatism—yours, not the binos. :cat:

But that doesn't make any sense. The problem occurs with ONE of six different alpha binoculars. That MUST be a problem of that particular binocular.

Hermann

 

[Tobias Mennle]

Thanks to all for their input. I checked collimation with the mirror test and it was as good as I can see with that test.

I managed to further unscrew the eyecups and secure them with tape, so they are now about 6mm longer and I´m reasonably fine for the moment because the exit pupil now falls almost into the right place and sharpness is fantastic. Will ask the manufacturer about this thing next week. The fact remains that when I´m adjusting for the exit pupil the FOV definitely shrinks below the specs (again I compared to other models with same or similar spec).

 

[OPTIC_NUT]

At this point, given your fix for the problem, my best guess would be that they
are the fussy/hi-performance type of EPs. I have a few pairs I have tamed
with a -5D plano-concave lens tacked on over the EP. The nice thing
is, I can place the curve facing front or back to nudge the EP out of
the zone where the aberration starts. I've mounted them it either way.
Makes the view much more 'comfortable'. A lens just a bit wider than the EP
face makes it so you don't lose field of view at all.

There was just one case where I found they put the field lens in backwards.
That was easy to fix, but hard to discover.

 

[ceasar]

Tobias,

Did you try using it with glasses on? Sun glasses if nothing else.

I bought an inexpensive 8x28 binocular by mail order which gave me blackouts in normal use with the eye cups extended unless I held it further away from my eyes.

My son wears glasses. He tried using it with the eye cups extended and it worked for him. Then I tried it with sun glasses on and it worked for me. I gave it to my son.

Bob

 

[WJC]

But that doesn't make any sense. The problem occurs with ONE of six different alpha binoculars. That MUST be a problem of that particular binocular.

Hermann

Faults in a binocular MAY be lessened through our attributes in the matter.

When needing to read a menu or the tiny print on the back of a DVD (without my glasses), I may put the tips of the thumb and index finger of both hands together--forming a tiny box--looking through such a tiny opening, which removes outer offending rays, often highlights what I am trying to read. It's an old trick you may try yourself. By using it, you change the perceived properties of the bino.

Does the bino become better? No. Can severe vignetting help resolution? Perform the test and decide for yourself.

Bill

 

[OPTIC_NUT]

I think it's actually a bit of a mystery to all of us.

One thing can can cause to above-mentioned defocusing, effectively:
a focuser gone loose. Just a possibility.

 

[Tobias Mennle]

Tobias,

Did you try using it with glasses on? Sun glasses if nothing else.

I bought an inexpensive 8x28 binocular by mail order which gave me blackouts in normal use with the eye cups extended unless I held it further away from my eyes.

My son wears glasses. He tried using it with the eye cups extended and it worked for him. Then I tried it with sun glasses on and it worked for me. I gave it to my son.

Bob

That´s why I checked where the exit pupil is in relation to the eyecup rim. I made the eyecups longer.

 

[Tobias Mennle]

At this point, given your fix for the problem, my best guess would be that they
are the fussy/hi-performance type of EPs. I have a few pairs I have tamed
with a -5D plano-concave lens tacked on over the EP. The nice thing
is, I can place the curve facing front or back to nudge the EP out of
the zone where the aberration starts. I've mounted them it either way.
Makes the view much more 'comfortable'. A lens just a bit wider than the EP
face makes it so you don't lose field of view at all.

There was just one case where I found they put the field lens in backwards.
That was easy to fix, but hard to discover.

Sounds interesting. Fussy EPs, that´s good. The focusing is definitely good in this sample, I had another one with an asynchronous focusing which drove me nuts. Hope I´ll get to know more the next days.

 

[kabsetz]

Tobias,

In the binoculars I have tested, the exit pupil typically falls beyond the level of the eyecup rim at its longest extension, but not by 10 mm. In a Zeiss 8x42 SF, longest extension of the eyecup from fully retracted is 12 mm, and exit pupil distance measured from the plane of the fully retracted eyecup is 17.5-18 mm, so here is almost six mm difference between them. In a Swaro 8.5 x 42 SV, the difference was 4 mm.

Kimmo

 

[Tobias Mennle]

Tobias,

In the binoculars I have tested, the exit pupil typically falls beyond the level of the eyecup rim at its longest extension, but not by 10 mm. In a Zeiss 8x42 SF, longest extension of the eyecup from fully retracted is 12 mm, and exit pupil distance measured from the plane of the fully retracted eyecup is 17.5-18 mm, so here is almost six mm difference between them. In a Swaro 8.5 x 42 SV, the difference was 4 mm.

Kimmo

Kimmo, thanks, that´s great information and again, you´re right , 10mm is not an accurate enough measurement although I did use that distance for really close observations. I did a more precise measurement indoors with the focus at infinity and really holding the bino perpendicular with the second eyecup as a marker, and it is a good 6mm difference between eyecup rim and EP, fitting well with your Zeiss SF data. In fact, my eyecup elongation was 6mm for simple practical reasons and the bino was then really easy to use. But with very strong vignetting. I still guess the baffling is too tight and the EP design is a bit "fussy", as Optic Nut remarked. Strangely, no blackouts whatsoever.

Of course the manufacturers may interpret the average facial shape of the users in a different way, and the punishment for giving them long enough eyepieces would be possible vignetting for some inexperienced users... Another variable is hard vs. rubber buffered eyecups, the latter making the eyepiece almost 2mm shorter when pressed against the face.

I have already suspected the SF eyecups to be a bit on the short side because I have a slight tendency for black outs with the SF which I don´t have with any of the others (and never had with the SE even). Your data explains why.

 

[OPTIC_NUT]

Tobias,

In the binoculars I have tested, the exit pupil typically falls beyond the level of the eyecup rim at its longest extension, but not by 10 mm. In a Zeiss 8x42 SF, longest extension of the eyecup from fully retracted is 12 mm, and exit pupil distance measured from the plane of the fully retracted eyecup is 17.5-18 mm, so here is almost six mm difference between them. In a Swaro 8.5 x 42 SV, the difference was 4 mm.

Kimmo

I called that 'forced eye relief', not where you can go but where you must go.
I have seen posts where people solve that by adding a section of bicycle
innertube. My Meopta Meopros have that issue with the EP, but it isn't a
practical problem at all because their eyecup extends very far out.
Makes for a very dark background...nice.

 

[kabsetz]

People's facial contours vary a lot. How deep are your eyes set, how aquiline is your nose and how wide, how thick are your brows. Then there is the eyecup diameter and eyecup rim contour, which are partly design choices and partly determined by the diameter of the ocular eye lens. In modern wide-angle high-eyepoint designs, the lens diameter is very large which necessitates large diameter eyecups. All of this presents quite an optimization problem for the designers, and with any given design there are bound to be individuals for whom it is unacceptable.

Having said that, there have been a few alpha binoculars with which I, with my facial features, have felt that simply having the eyecup extend further out would help, and that it should have been possible to design for that. An additional and related complaint I often have is that there should be more click-stops for the eyecup travel for a more precise adjustment, or better still, a system where you can pull out/push in the eyecup to any intermediate position, and then twist it to lock it in place there. There are monopods and tripods with leg adjustments that work this way, and it should be pretty trivial to design eyecups to do the same. Add to this an extension scale printed on the eyecup barrels, and you'd have a pretty nifty design that would be an improvement on the ergonomics of all current designs.

Kimmo

 

[Ignatius]

Sorry to revive this old thread, but I am not sure my input would merit a new one. In an article from 2019 which I just found on a phtographic site I frequent, it is stated that a physicist has finally managed to solve the problem of spherical aberration - the aberration which according to Vukobratovich in his 1989 paper on Binocular Performance And Design is responsible for kidney beaning.

 

[Tringa45]

Sorry to revive this old thread, but I am not sure my input would merit a new one. In an article from 2019 which I just found on a phtographic site I frequent, it is stated that a physicist has finally managed to solve the problem of spherical aberration - the aberration which according to Vukobratovich in his 1989 paper on Binocular Performance And Design is responsible for kidney beaning.

The cause of kidney beaning is usually spherical aberration of the exit pupil, a characteristic of some wide angle and long focal length eyepieces.
The author of the linked article was rather confused in attributing poor edge sharpness in a camera (or telescope) objective to spherical aberration.
Of course, an objective with spherical aberration will show poor edge sharpness but SA , along with CA, is the major cause of poor central sharpness.
It occurs when the parallel rays near the periphery of an objective are brought to focus closer to the objective than those rays nearer the optical axis.
It could be reduced by stopping down the objective, which is often done on a camera lens but is not a sensible option on a binocular or telescope.
SA can be controlled by the addition and spacing of other lens elements and the use of aspherical (parabolic) lens surfaces.
Field curvature, astigmatism or possibly coma would be the major causes of poor edge sharpness in either the objective or eyepiece.

PS:- A pity that Kimmo's nine year-old suggestion of infinitely adjustable and lockable eyecup adjustment has never been realized.

PPS:- Like your avatar, Ignatius, and the self-deprecating humour (had to look up "Nudelaug").

John (Kanalratte turned Piefke)

 

[Ignatius]

@ Tringa45: The author of the article I linked to explains to his photography-fiend audience the work done by 3 young scientists in Mexico, and published here and here, pertaining to the solution of the Wassermann-Wolf and Levi-Civita problems respectively. They worked out formulae to deal with spherical aberration. Which is, at least according to Vukobratovich (1989), the aberration that causes kidney beaning in binoculars.
So yes, I am a naturwissenschaftliches Nudelaug, and I cannot follow the mathematical formulae written out in papers of that nature, but I am nevertheless a social scientist (political science, modern English & American lit. and philosophy), so I can at least follow text and logical reasoning.
So I am still convinced that even though the writer in the online photography megazine may have got some things wrong, those three Mexican scientists probably haven't.
Yup, Nudelaug is a local term my Viennese wife loves using for me in such cases, especially since she is a bio-chemist with an MSc in psychology and a PhD in sociology. I always lose

Ignaz (Piefke turned Kanalratte, turned Texan, turning Ötzi)