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Message: <blockquote data-quote="typo" data-source="post: 3394712" data-attributes="member: 83808">I think this is referring to a very common misunderstanding about how binoculars work.Leaving aside minor factors like variation in coating and glass absorption, in normal bright conditions the apparent brightness of a 8x42 and a 7x50 will be exactly the same as an 8x20 a 10x25 or even a 20x50.How much light enters the eye is regulated by pupil diameter. In very bright conditions the iris will constrict to produce a pupil of arounf 1.5mm with the iris blocking the residual light. As light levels reduce the pupil dilates and optimum visual occuity normally occurs at a pupil diameter around 2.5mm. Young children might have a maximum dilation of around 9mm in very low light, but this maximum decreases with age. There is quite a bit of individul variation but the average for a 60 year old is reported to be around 5mm. For normal daylight viewing maximum brightness will be delivered by all binoculars with at least a 3mm exit pupil. 4.2mm will normally get you quite far into the evening. If you are young enough to have a 7mm pupil then you should do well by starlight.If the cararacts have a more peripheral component, which is fairly common, I belive it is possible that larger exit pupils than required for the light levels might actually increase glare and reduce contrast, but  you would need an opthalmologist to examine individual cases.David</blockquote>