Alan, I do know that camera lenses have higher resolution when stopped down than at max aperture - usually there is some slightly stopped down aperture at which the resolution peaks. I'm not certain why this is so. Isnt the image from most bins generally sharper and less distorted at the centre than the edge? If you are using the inner 21mm of the available 42mm lens, perhaps you would get a sharper image with greater resolution than using the entire 21mm of a 7x21. Just pondering .....
-
Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.
Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
Please read if you are about to buy high end bins (1 Viewer)
- Thread starter Blincodave
- Start date
mike60 said:
The resolution of a camera lens does not always increase as the aperture is stopped down, though that is indeed the usual case. The reason is that at wide aperture various aberrations - spherical aberration, chromatic aberration, coma and so on - destroy the image quality. Stop the aperture down, and the various aberrations tend to reduce, and hence the resolution increases. There is though a point at which the aperture is so small, that diffraction sets in and the image quality starts to degrade. Diffraction can be crudely thought of as the light rays passing close to the aperture blades bending slightly.
Camera lenses tend to have a sweet spot between F8 and F11, which is why landscape photographers sometimes use cheap lenses but stop them down to ~F11.
AlanFrench said:
It is true that binoculars do not use the full resolution of the objective. However it is quite easy to show that for example a Leica 8x32 BN has a lower resolution than a Leica 8x42 BN.
Does stopping down the aperture really have the same effect as stopping down the exit pupil? I doubt that, though maybe someone with more knowledge can correct me if I am wrong. It cerainly does not agree with the observation that an 8x20 glass has markedly lower resolution than an 8x42 glass.
AlanFrench said:
Absolutely.
AlanFrench
Well-known member
Leif said:
I have been down this road before. It was rough and stormy trip, but there is absolutely no doubt that stopping down the exit pupil is exactly the same as stopping down the aperture.
There are two ways of looking at this. First, consider what the exit pupil actually is. It is an image of the light coming through the objective, and there is an exact correspondance between the light at the aperture and the light in the exit pupil. The outer part of the exit pupil is the light from the outer part of the aperture, and the light from the inner part of the exit pupil is the light from the inner part of the aperture. You can demonstate this very nicely. Hold a finger in front of the objective and see where it is in the exit pupil. It is easy to see that by stopping down the exit pupil you are throwing away the light from the outer part of the objective, so you losing both light and aperture.
Second, consider what happens in the optics. Diffraction from the objective determines the potential resolution of the lens. A larger objective has the potential for better resolution (assuming it is high quality and diffraction limited, and used at a higher magnification that you get in birding binoculars). If your eye's pupil is larger than the exit pupil, the light enters the eye without further diffraction. If, however, the eye's pupil is smaller than the exit pupil, then additional diffraction occurs in the eye, and the potential resolution of the objective is lost.
If 8x42 binox do indeed have better resolution than 8x32 binox, you need to look at some other explanation. Personally, I have seen no real good, objective tests of binoculars. Most comparisons are quite informal and very subjective. I suspect the detail you see is largely determined by optical quality and magnifcation. Increasing either will improve resolution.
Clear skies, Alan
Last edited:
Can i suggest that one possible reason for reduced resolution on smaller bins (and i have noticed this effect myself) is that although the manufacturing production tolerances that can be held from part to part is a fixed quantity, it but becomes a bigger percentage of the smaller bin's dimensions.
AlanFrench
Well-known member
mike60 said:
Leif nicely answered the question about camera lenses.
The on-axis, center of the field view, is always sharper in any optical system. Off-axis, you always have some aberrations that do harm the image - essentially a consequence of optics that are no longer symmetrical as seen by the light rays passing through it.
Here's something to think about. A typical binocular objective has a view angle of about 6 to 8 degrees. A typical binocular eyepiece has a view angle (apparent field) of about 50 to 67 degrees. Which do you think has to "work harder" to get relatively aberration free images across the field? You can proably see why many binocular objectives have only two lens elements, while eyepieces can have quite a few more.
Clear skies, Alan
Leif said:
Alan is correct. Stopping down the exit pupil has the same effect on transmission and angular resolution as stopping down the aperture (entrance pupil), as the exit pupil is an image of the entrance pupil.
Not sure how you would be able to detect lower resolution in a 32mm lens against a 42mm lens with a magnification of 8X? Angular resolution (resolving power) of a 32mm lens is approx. 3.5 arc seconds, for a 42mm lens, approx. 2.7 arc seconds. A difference of 0.8 arc seconds is not visible at such a low power. Any visible difference in image quality between an 8X32 and an 8X42 binocular from a manufacturer like Leica is not resolution, but contrast increase due to minimising edge aberrations.
A similar point was raised in an earlier posting about resolution increasing when a lens is stopped down. In a perfect system resolution is NEVER increased by stopping a lens down, because resolution is a function of aperture. In an imperfect system (virtually everything made), stopping a lens down improves (as was said) edge and other aberrations, resulting in an improved image. But, it is important to distinguish between the effect of suppression of aberrations, and changing angular resolution in an image by increasing or decreasing the entrance pupil. The former tends to mask the latter, and the latter is very difficult to see unless the power is very high.
best regards
Chris
ukbraychris said:
I disagree. I can quite clearly see the difference in resolution between many premium ~8x32 binoculars and ~8x42 binoculars when testing against a suitable target. One reason I like the Nikon 8x32 SE so much is because in terms of resolution and brightness it matches a premium 8x42 binocular. They seem to provide as much resolution as the observer can cope with. NB: I am referring here to centre field resolution, though 'sharpness' is remarkably good over most of the field in the case of the Nikons.
If you look at the Better View Desired web site, you will see graphs plotting the distance at which the author could resolve a target for various binoculars. His target is IIRC a dollar bill. I have done similar tests for binoculars that I have owned and obtained similar results.
For example I can quite clearly see a slight softness of the image when observing with a Zeiss 8x30 BGAT (which incidentally is a rather nice binocular).
I presume the reason why resolution is nowhere near the theoretical value is due to the sum total of manufacturing imperfections i.e. from objectives, prisms and eyepieces. Binoculars are after all quie 'cheap' given the number of optical components. For example, even the most expensive roof prism binoculars show obvious chromatic aberration given the right lighting e.g. a dark bird against a bright sky.
AlanFrench said:
You are right that aberrations get worse off-axis. However, in the best lenses image quality can be remarkably even across most of the image field, even wide open. That is why some people buy Leica and Zeiss lenses. As a group they are a step up from most photographic lenses in terms of both optical quality and price.
I have seen plots of resolution in lines per mm as a function of aperture and distance off axis for typical photographic lenses, and even the centre field resolution usually increases markedly as the aperture is shut down to ~F11.
It often surprises me that binoculars are so cheap given that high quality telescope eyepieces are ~£150 and more. And a binocular has to include two eyepieces, 4 prisms and 2 objectives.
Leif said:
Yes you can see differences between binoculars from the same family, such as 32mm and 42mm, but the differences are not due to resolution. The eye is physically incapable of seperating two points of angular seperation 0.8 arc seconds unless the magnification is at least 40X per inch of aperture (or at least 1.75X per mm of aperture, whichever you prefer). These are well known physical laws in optics, regardless of what your brain is telling you.
You mention above that you can tell the difference in resolution between 32mm and 42mm binoculars on a suitable target, then in the next sentence you say you like the Nikon 8X32 SE because in terms of resolution and brightness it 'matches a premium 8X42 binocular'. One statement contradicts the other. Which is true? You can see the increased resolution of a 42mm over a 32mm, or that the resolution of a 32mm matches a 42mm?
Whether you describe resolution of a lens by angular resolution of point sources (resolving power), or by lines per mm, the 'gain' in resolution of 42mm over 32mm is not visible with the eye at a magnification of only 0.25X or 0.2X per mm of aperture.
Any differences in image quality between the binoculars you test are differences due to 1.) greater or lesser contrast, 2.) greater or lesser aberration control, 3.) astigmatism in the eye of the observer, 4.) internal reflections etc. the first three could well be due to diameter of eye pupil against exit pupil, a point we have covered before. Which ever of the above are responsible for the image differences, none have a direct association to resolution.
Alan's posting yesterday at 23.29 sets it out very well.
Leif, if you need to read up on the subject of image formation and the diffraction effects of circular apertures, can I suggest a book title? 'Amateur Astronomers Handbook' by Sidgewick. It is an ideal read on introductory optical theory for the layman. I think the third or fourth edition is currently in print, and some libraries may have a copy.
best regards
Chris
ukbraychris said:
I'm fully aware of that and I nearly made the same point myself but I thought it was a bit obvious to say to you given your background.
ukbraychris said:
Okay that's a fair point and I will clarify. Firstly when I say that one binocular has higher resolution than another, what I mean is that using it I can see more detail on a test target. (That obviously is not the same as using a camera and very fine grain film to take a picture of the image formed by a binocular, and then using a microscope to measure the lines per mm resolved. The human eye is thus part of the equation.)
Secondly I have used various 8x30 and 8x32 roof prism binoculars - Zeiss 8x30 BGAT (2), Leica 8x32 BN (1), Nikon 8x32 HG. I have also tried various 8x42 binoculars including a Nikon 8x42 Egret (cheap), Nikon 8x42 HG and Swarovski 8.5x42 EL. Many of these instruments I have owned (or still own). In careful testing using a suitable target and real world tests, I see that the 8x32 roof prism binoculars and the Nikon 8x42 Egret all have similar resolution. To my eyes they all have a very slight softness of the image. This slight softness is only apparent when for example, in a hide, and I am resting my arms on a shelf. However, it is noticeble. To my eyes the Nikon 8x42 HG and the Swarovski 8.5x42 EL have noticeably higher resolution, the latter no doubt helped by the slightly higher magnification. The difference is not that subtle.
To my eyes my Nikon 8x32 SE behaves rather like my Swarovski 8.5x42 EL in terms of brightness, resolution, edge quality, contrast and so on.
What I find interesting is the large variation in perceived brightness between instruments that does not depend directly on objective size, which is for me anyway unexpected, and clearly due to differences in optical design and quality. To my eyes the Nikon 8x32 SE is unexpectedly bright, due presumably to improved optical quality. (It's easier and hence cheaper to make a Porro prism of a given quality than a roof prism, but then you know that anyway.)
ukbraychris said:
I don't see how what I have said contradicts what you say above. Aberrations in the various components of a binocular will as you say reduce the performance. I am interested in how real instruments behave, not theoretical ones, and hence I am interested in what I see, which involves the human eye.
To state the obvious binoculars are manufactured to certain tolerances and use various coatings. The optical design, tolerances, and coatings all go together to determine how an instrument behaves. Thus the Nikon 8x42 HG provides a much brighter sharper image than the Nikon 8x42 Egret. (It also has better edge definition, more eye relief, higher contrast and so on.) I suspect that binoculars are a huge compromise as they have to be portable, affordable and yet provide useable images. Also manufacturers are not going to make the components to a higher tolerance than need be, as that would increase costs needlessly.
ukbraychris said:
Thankyou but I bought a copy 20 years ago and I also studied some optics during a physics degree. (The effect on resolution of stopping down the exit pupil is far from obvious so that I will checkup.)
I trust my eyes first and then theory. That is one thing I learnt from a Ph.D. in theoretical physics. I will continue to choose binoculars according to what I see rather than what an expert tells me I should see.
I mentioned the Better View Desired web site. Presumably you disagree with the graphs that he presents then?
AlanFrench said:
Thanks. That's interesting. Yes it is far from intuitive that stopping down the exit pupil is comparable to stopping down the objective. (Obviously the diffraction at the exit pupil is greater due to the smaller aperture, but the image is already magnified, so presumably the net effect is equivalent.)
AlanFrench said:
You say that "Most comparisons are quite informal and very subjective.". That's a good point.
I am though a wee bit confused when you mention that I need to look for some other explanation. Yes I agree that the explanation for the differences that I see must be due to differences in optical quality. I think we are talking the same language!
Last edited:
Leif said:
I think there is some confusion here over 'resolution'. What you are describing is image sharpness and contrast, not resolution. A common mistake.
The control on aberrations, collimation, flatness of field, and any astigamatism in the eye of the observer will result in the differences you see, not resolution. Resolution (whether resolving point sources or resolving linear markings) is a function of aperture. I get the feeling that the term 'resolution' is loosly banded about as a term for describing image sharpness, rather than as a term describing seperation of point sources or point source groups governed by aperture or diameter of entrance pupil.
No one is suggesting that you do not see differences in binoculars of various makes and apertures, but those differences are not differences in resolution, but in manufacturing tolerances, aberration control, contrast, and image brightness as you state. A magnification of 8X is simply too low to see resolution thresholds in a binocular or telescope. I would have thought a student of optics would have known all this already????
As Alan hinted about not being aware of any useful objective tests for binoculars, I am also of this opinion. There are too many inconsistencies in mass-produced optics to test a low power instrument on some chart at a distance, and then assume that all other binoculars of the same model will be identical, and then announce that model A is better than model B. Certainly a low power binocular or spotting scope can not be tested for resolution on a chart. If you know the aperture you know the resolution threshold. A chart may show imperfections in the design, aberrations etc for that one particular binocular, but don't confuse this with resolution differences.
A student of Theoretical Physics who places trust in subjectivity before theory?? Hmmm. Which University did you complete your post-grad?
Anyway, as amusing as this has been, I am away for the weekend. Not available till next week.
Best regards
Chris
ukbraychris said:
Thankyou for the patronising remark. There is little point in answering given your tone.
ukbraychris said:
Thankyou for that patronising response. Again there is little point in answering given your tone. s
In response to your question, I did my Ph.D. at Cambridge University, in England.
You say: "A student of Theoretical Physics who places trust in subjectivity before theory?? "
I did not say that. You are twisting my posting. In fact you repeatedly twist what I say.
I am not interested in responding to your sarcastic and arrogant remarks. Nor am I interested in an online fight.
Goodbye.
Grousemore
Senior Member
Wouldn't it be a pity if these informed discussions on optics are curtailed by personal antagonism? 'Lecturing' often goes down badly in my experience.
Hermann
Well-known member
An interesting discussion. I personally also *believe* I can see a slight, but perceptible difference in "image quality" between 8x30/8x32 binoculars and 8x40/8x42 binoculars, but only when the air is exceptionally clear and the binoculars are on a tripod. I'm not sure it's a difference in resolution, I personally believe it's more like a combination of slightly better contrast and perhaps resolution.
An even more important factor, however, is that binoculars with larger exit pupils are somewhat "easier" to use in the field even when the eye pupil is stopped down. That becomes immediately clear when you do a comparison between 8x20's, 8x30's and 8x40's and 8x50's of similar optical quality. The thing is that the eye pupil distance isn't always the same, and actually shifts somewhat when you focus your eyes - and you always do, trying to achieve the sharpest possible image. This is in my opinion the reason why so many people like binoculars with largish exit pupils such as the Zeiss 7x42BGATP.
With handheld binoculars the effect is even more pronounced because of hand and body movements. The larger exit pupil makes it far easier to keep the eye pupil centered on the exit pupil of the binoculars, even when it's stopped down to the minimum in bright sunlight.
This also explains why many people just can't seem to get along with pocket binoculars. My own limit seem to be exit pupils in the 3.5mm region, by the way. I tried quite a few pocket binoculars over the years, but none of them seems to work for me simply because the exit pupils are too small.
Another brief comment on the debate about resolution: One point that deserves mention is that the visual acuity (Is that the right word? where's my dictionary? ..
) varies a lot. There are people who can detect slight differences in resolution much more easily than others, simply because their eyes are that much better.
Hermann
who's trying to convince himself he *really*
needs another pair of binoculars
An even more important factor, however, is that binoculars with larger exit pupils are somewhat "easier" to use in the field even when the eye pupil is stopped down. That becomes immediately clear when you do a comparison between 8x20's, 8x30's and 8x40's and 8x50's of similar optical quality. The thing is that the eye pupil distance isn't always the same, and actually shifts somewhat when you focus your eyes - and you always do, trying to achieve the sharpest possible image. This is in my opinion the reason why so many people like binoculars with largish exit pupils such as the Zeiss 7x42BGATP.
With handheld binoculars the effect is even more pronounced because of hand and body movements. The larger exit pupil makes it far easier to keep the eye pupil centered on the exit pupil of the binoculars, even when it's stopped down to the minimum in bright sunlight.
This also explains why many people just can't seem to get along with pocket binoculars. My own limit seem to be exit pupils in the 3.5mm region, by the way. I tried quite a few pocket binoculars over the years, but none of them seems to work for me simply because the exit pupils are too small.
Another brief comment on the debate about resolution: One point that deserves mention is that the visual acuity (Is that the right word? where's my dictionary? ..
) varies a lot. There are people who can detect slight differences in resolution much more easily than others, simply because their eyes are that much better.Hermann
who's trying to convince himself he *really*
needs another pair of binoculars
ukbraychris said:
Chris, some of the points you raise I would agree with, but certainly not your remarks about no objective testing of binoculars. I am sure that Leif can explain for himself, but I know that you can evaluate binoculars using a focault test chart (line pairs). How? set up the focault test chart approx at 10m distance and with the binoculars on a sturdy rest. Put several of these charts together (10) and you can evaluate resolution, edge to edge clarity and alignment using these charts.
The theoretical resolution does not say anything about the quality of the optics. Resolution (theoretical) (R(t)) = Only a measure of how much detail (pattern size) that can be resolved theoretically under day light conditions.
Last edited:
AlanFrench
Well-known member
ukbraychris said:
Chris,
Books on optics generally talk about resolution and contrast, nor "sharpness." Probably the best tool for testing optical performance would be a contrast/resolution chart. This has resolution targets at various contrast levels.
Clear skies, Alan
AlanFrench
Well-known member
Hermann said:
Hermann,
Quite true. If the exit pupil is close to the size of the eye's pupil, the odds are that you are not often getting all the light from the exit pupil into your eye.
Yes, you definitely NEED at least one more pair of binoculars.
Clear skies, Alan
