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Message: <blockquote data-quote="denco@comcast.n" data-source="post: 3321752" data-attributes="member: 25300"><p>Twilight Factor is important as long as you have enough light to illuminate the FOV. The formula is helpful if you have a 4mm exit pupil or bigger. In my experience magnification makes a big difference in how much detail you can see in low light. If you have a bigger image you don't need as much light to see detail. TF is just about how much detail you can see in low light. Your right in that a bigger exit pupil will be more comfortable and brighter but with less magnification that image will be smaller. It would be interesting to see if a 10x50 or a 9x56 would show more detail in low light. I bet they would show about equal detail. If you had young eyes the 9x56 would show smaller brighter images and the 10x50 would show larger dimmer images which you could probably see as much detail in. I would bet for someone over 50 years old the 10x50 would show more detail in low light because the 5mm exit pupil is about all the light your eye could take in and the images would be bigger. Sometime compare an 8x42 to a 10x42 binocular at dusk. You will be surprised.</p></blockquote><p></p>