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Message: <blockquote data-quote="typo" data-source="post: 3322202" data-attributes="member: 83808">Lets be clear about twilight factor. It works providing the exit pupil or the pupil of the eye is not limiting. Holger explores it nicely (even if you can't follow the maths) in his paper which can be downloaded from this page. <a href="http://www.holgermerlitz.de/bino_performance/bino_performance.html" target="_blank">http://www.holgermerlitz.de/bino_performance/bino_performance.html</a>A 5mm exit pupil is probably adequate for most twilight birding (or hunting) needs. At this light level our eyesight would be typically 5 to 10 times worse than in bright conditions. You will need 5 or 10 times the magnification to see the same detail you would unaided in daylight. The 25% extra magnification of a 10x50 compared to an 8x40 entirely translates directly as an observable benefit in detail. With such poor eyesight the detail loss due to shake is effectively zero.  The difference in transmission between a top of the range binocular and relatively cheap one is generally less than 10% these days which is not nearly so useful as increase in magnification. A cheap 10x50 (TF 22.4) will beat an expensive 8x40 (TF 17.9) when light or age dictates a 5mm pupil size. The best I can get to these days is about a 5.5mm pupil. By moonlight my 10x56 (TF 23.7) allows me to see detail about 4 fold smaller than my best 8x42 (TF 18.3). My 12x45  has a TF of 23.2 but is easily beaten by the 10x56 in moonlight because the 3.75mm EP is limiting, however it still can compete with the 8x42 on detail due to the extra magnification. A young person who can manage a 7mm pupil will see more detail with an 8x56 (TF 21.1) in the lowest light than I will with a 10x56 because my pupil and the binocular exit pupil is limiting.Hope that makes some sense. David</blockquote>

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