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Unlike the old Twilight Zone, the Twilight Factor is how scopemeisters state the light-gathering ability of a scope.Take out your calculators and follow the formula to get to the Twilight factor:
First you multiply the size of the objective lens by the magnification power, then find the square root of the result.(See, I told you you'd need a calculator.) For example, the twilight factor on a variable power scope set on 10X with a 40mm objective lens is 20.
Why? Because 10 x 40 is 400 and the square root of 400 is (anybody?) 20.
The twilight factor is a purely mathematical equation that doesn't take into consideration the quality of the lens nor the coatings. So, you can leave your calculator at home when trying to decide on a new scope.