John Russell
Well-known member
I followed a recent dicussion on the binoculars forum on contact lenses and there were some interesting contributions from looksharp65, who is an optometrist. I should be grateful if he or anyone else could answer the following questions.
Firstly:-
Assuming the eye does not need correction for astigmatism, at what pupil size (or exit pupil diameter) does it perform at its best? I recall that Holger Merlitz once mentioned some old military research on terrestrial scopes, in which an exit pupil diameter of 2,5 mm was considered optimal and 30x magnification the ideal compromise for the majority of conditions. The latter would, of course depend on climate and terrain but would place the ideal scope for purely visual use at 30x75.
Secondly:-
At what exit pupil diameters does the depth of field of the eye obscure astigmatism of, say -0,5 diopters or -0,25 diopters? I believe some amateur astronomers use magnifications, which result in exit pupils of around 0,5 mm, which would make cylindrical correction unnecessary for most. On the other hand, one manufacturer of astronomical eyepieces offers cylindrical correction lenses for the longer focal length eyepieces. For birders the practical lower limit of exit pupil would be around 1,5 mm, beyond which things start to get rather dim and it interests me whether this could obscure some degree of astigmatism.
John
Firstly:-
Assuming the eye does not need correction for astigmatism, at what pupil size (or exit pupil diameter) does it perform at its best? I recall that Holger Merlitz once mentioned some old military research on terrestrial scopes, in which an exit pupil diameter of 2,5 mm was considered optimal and 30x magnification the ideal compromise for the majority of conditions. The latter would, of course depend on climate and terrain but would place the ideal scope for purely visual use at 30x75.
Secondly:-
At what exit pupil diameters does the depth of field of the eye obscure astigmatism of, say -0,5 diopters or -0,25 diopters? I believe some amateur astronomers use magnifications, which result in exit pupils of around 0,5 mm, which would make cylindrical correction unnecessary for most. On the other hand, one manufacturer of astronomical eyepieces offers cylindrical correction lenses for the longer focal length eyepieces. For birders the practical lower limit of exit pupil would be around 1,5 mm, beyond which things start to get rather dim and it interests me whether this could obscure some degree of astigmatism.
John