If I'm considering a 8x42 binocular and an 8x50, what differences would I be seeing between the two sizes in terms of optical quality?
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Imagine you have two binoculars with the exact same glass quality and optical coatings, but one is an 8x42 and the other is an 8x50. Let’s say your pupillary opening just happens to be 3.5 mm in the particular lighting situation you’re in. As such, you will not be utilizing the full exit pupil of either binocular (5.25 mm & 6.25 mm respectively).
At first it might seem that there would be no difference in image quality between these two binoculars, but that would be an error. Not only does aperture increase brightness (when your pupillary opening can receive it), but it also improves resolution regardless of pupillary diameter. Thus, the 8x50 in this example will render slightly more detail because the 3.5mm exit pupil you are taking into your eye is comprised of more photons of light: resolution is a function of aperture size.
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