<https://www.birdforum.net/threads/daft-question-about-low-light-performance.458307/>
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"I apologize in advance for asking a question that I feel I should know the answer to, but I'm confused about the interplay of objective diameter and exit pupil in determining the low-light performance of a pair of binoculars.  For the sake of the argument, let's assume that the viewer has pupils that dilate to 5mm and a bunch of binoculars with identical optical characteristics. I feel relatively confident that a 10x42 will have poorer low-light performance than a 12x50, since they have similar exit pupil diameters so the objective diameter determines performance.  Am I ok so far?  But then it gets hazy for me...what about a 10x50 vs the 12x50?  Is the determinant of low-light performance simply the objective diameter, or does the larger pupil (matching the maximum size of the viewer) of the 10x50 give better performance into dusk?  Then how about the more extreme examples -- 8x50 or 7x50?  Would these give the viewer a better view into the gloom than the 10x or 12x, even though the exit pupil diameter exceeds the viewer's pupil size?  Help!!  And thank you very much!\n\nMilton"
<https://www.birdforum.net/threads/daft-question-about-low-light-performance.458307/#post-4656102>