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Twilight Factor Equation
You can determine the numeric value of a scope's twilight factor by multiplying the scope's magnification by the diameter of the scope's objective lens, then finding the square root of that answer. For example, if the scope is set at a magnification of five and has an objective lens that is 40mm in diameter, your equation would read: 5 x 40 = 200, and the square root of 200 is 14. Therefore, for this particular example, the twilight factor is 14.
Objective Lens
The objective lens is located at the far end of the scope and is the entry point for all the light channeled into the scope. Generally speaking, the larger the objective lens, the more light is brought into the scope. Scopes with the highest twilight factor are those with the largest objective lens diameter; for example, the 50mm wide lens would produce a higher number then a 32mm objective lens.
Magnification Power
The first variable in determining the numeric value of the twilight factor is the magnification setting. Therefore, the higher your scope's magnification ability the greater the possible twilight factor. Variable power scopes will provide you with a range of magnification settings from which you can choose the best-suited for your particular situation.
The Best Twilight Factor Scopes
Using the variables of the twilight factor equation as your guide, the optimum scopes would have the capability of a high magnification setting and a large objective lens diameter. For example, a 6 -- 24 x 50mm scope, set on its highest setting of 24 power, has a twilight factor of 35, whereas a 3 -- 9 x 40mm scope on its highest setting can only produce the twilight factor of 18.