As I understand it, stopping down the binocular objective means using the "best" part of the objective lens (lowest aberrations in the center). So I guess that it will not be equal to using a smaller objective bin?
With a stopped-down bin set-up I think it might be difficult to show that the cause of potential increased perceived sharpness is the smaller exit pupil rather than using the central part of the objective.
Dawes limit (R = 4.56/D) would also suggest that the max. theoretical resolution will decrease stopped down (smaller D), not necessarily a good thing?
A slight brightness loss also can make things look more "contrasty" if the current light is good.
So I just wonder what the explanatory model would be for a smaller exit pupil bin as a 10x32 would look sharper than a 10x42? Angle of incidence on the cornea? Circular area of the cornea used? wouldn't it be identical due to different usage distances? And wouldn't it be more of a question of the eye relief?
Current eye pupil size will also have an effect on the max sharpness the eye could reach I guess. With a smaller pupil the eye will be sharper. A 42mm bin will gather more photons, so total amount of photons will be larger with a larger bin, photon density might be not. A 32mm bin with less glass in the light path could have better transmission on the other hand. Any effect on the pupil size there? Definitely in low light conditions, though the exit pupil effect on sharpness would be most relevant in daylight.
Just wondering..
:smoke:
Lets keep it metric and use millimeters so the Dawes limit is 116/D. A 10x42 would therefore have a theoretical resolution limit of 116/42= 2.76 arcseconds. For the last ten years to qualify as a high quality binocular it only had to be better than 5.8". I was told that Zeiss, for example, usually used 5.2" as a cut off, but I've measured quite expensive models atm5.6" and relatively inexpensive ones at 3.5". This year ISO 14133-2 was changed and a 10x42 only needs be better than 6.4" now. The same standard means that an x32 limit is 7.5" and an x25, 11". In my view, a rediculously lax standard, particularly for smaller objectives, as I'll try to explain.
Optimum visual acuity occurs in fairly bright conditions when the pupil of the eye is around 2.5mm in diameter. This is when we make the highest demand on a binocular's optical resolution. When using a 10x42 with a 4.2mm pupil in bright conditions the light from the perimeter of the objective is blocked by the iris of the eye from reaching the objective. Only the light from the centre 2.5x10= 25mm reaches the retina and it's only the resolution of that 25mm proportion of the objective that is relevant. The Dawes limit for 25mm is 4.64" or about 2.4x better than the ISO standard for a x25 binocular.
Fortunately the effective resolution of most binoculars improve as it's effectively stopped down and may even approch the diffraction limit like Henry's Zeiss or the Kite Bonelli 2.0 I reviewed. However very few binoculars are that good.
Someone with a fairly typical 20/15 or 90" acuity potentially could see down to 9" with 10x magnification and 20/10 down to 6". It means that someone with average eyesight will comfortably exceed the the ISO standard of 11" for a 10x25 binocular. It explains why so many compact binoculars are rubbish, but there are exceptions of course. From my own stopped down measurements most x42 binoculars would exceed the acuity of users with 20/15 acuity but it's much more of a lottery for those with 20/10 vision.
So after that lengthy preamble, some reasons why restricting the objective diameter in bright conditions can be beneficial. Firstly, much of the stray light in the optical path originates at the periphery of the objective and stopping down will often block it and improve contrast. How much will depend on the design but I've seen a useful difference with a couple of Swaros.
The second reason follows on from that Watson paper on the target angle of view. As I mentioned before, even in fairly bright viewing conditions, the actual luminance in shaded areas can be low causing your pupil to dilate and your acuity to decrease. That paper illustrates that even a brightly lit target in front of that dark background will make little difference to your pupil diameter. You will still have reduced acuity and now the target itself will be over exposed or glarey. In that situation stopping down the objective can improve the level of detail you see several fold in particular situations but more often the benefit is more modest, but never the less, worthwhile. It's something I first noticed when positioning test charts at different locations round the garden and realised it had a practical benefit in normal use.
Hope that makes sense.
David
