Q: I’m in the market for a good binocular and had just about settled on a 10×40, but the salesman in the gunshop tried to talk me into an 8×40.
He claims it has a greater twilight factor which results in a brighter image. Is this true?
Francis Angwin
A: The salesman was only partly right in his given advice. When you are assessing optics, the twilight factor is important, since it is a number combining the effects of brightness and magnification.
Twilight factor is the square root of the optic’s magnification multiplied by its objective lens diameter. So if you have an 8×40 binocular, the twilight factor is the square root of 320 (8 times 40), or 17.9.
For a 10×40, the twilight factor is 20, but the exit pupil shrinks from 5mm to 4mm. The extra magnification more than offsets the drop in brightness.
I am satisfied with a 4mm exit pupil and prefer a 10×40 binocular over an 8×40, but that is only my personal preference.
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