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Message: <blockquote data-quote="Alexis Powell" data-source="post: 1173123" data-attributes="member: 5327">Assuming your pupil has dilated enough to make use of it, brightness is determined by exit pupil as well as how well the binocular transmits light (a property affected by glass type, prism coatings, lens coatings, prism type).  I'm no expert on the specifics of "twilight factor", but basically it takes two factors into consideration: brightness (as presumed from exit pupil--there is no consideration of transmission differences) and magnification.  It is supposed to be a better index of what a binocular allows you to see/discern in low light levels.  The fact is, what you can see is not governed by brightness alone, magnification is also very important because it spreads the light over a larger area of your retina, thus allowing for the detection of finer details.  Consequently, it is possible (within limits) to see details better in a dimmer (due to smaller exit pupil) higher maginification binocular than a brighter lower magnification binocular.  The twilight factor calculation is supposed to approximate the trade-offs between the two factors.--AP</blockquote>