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deerndingo

Hi All,
I had a question about how to determine the better size for low light and twilight shooting.

Here's an extract from http://www.lynxoptics.com.au/FAQ/question_7.html that I bumped into:

The two popular reference systems assisting buyers to select scopes and binoculars for use in low light and at night are Relative Brightness system and Twilight Factor system (Dämmerungsfaktor). Both systems are useful and both have anomalies to be aware of. To use the information you need to know that an instrument's exit pupil is the bright round disc of light seen in its eyepiece when it is held at arm's length - and that the diameter of the exit pupil in mm is obtained by dividing the instrument's magnification into its front lens diameter i.e. an 8x56 scope or binocular has an exit pupil 56/8 = 7mm diameter.

Relative Brightness (RB) assigns numerical ratings to binoculars and scopes e.g. 25RB for a 7x35, 50RB for a 7x50 - telling the user that a 7x50 is twice as effective as a 7x35 at night when his eye pupil is 7mm in diameter. The RB numerical ratings are the squares of the exit pupils of the two instruments concerned, i.e. 5x5 for the 7x35 and 7.1x7.1 for the 7x50. The weakness of the RB system is that it does not take into account the instrument's magnification - and magnification has a big influence on target visibility at night. For example a 6x42 and an 8x56 riflescope both have exit pupils of 7mm (RB rating 49) but the 8x56 is almost 50% more effective at night when the user's eye pupil is 7mm.

The Twilight Factor is the square root of the product of magnification and front lens diameter, i.e. for an 8x56 scope the Twilight Factor is (square root 8x56)=21.17. For a 6x42 it is 15.87. The "weakness" of the system, if indeed it can be called a weakness, is that it gives misleading results for high power, small diameter instruments - a 20 power scope with a 20mm front lens diameter has a Twilight Factor of 20 but in practice the user would see almost nothing through it at night. A user of this system must stay aware of the major importance of 5mm, 6mm and 7mm exit pupils for conditions ranging from dawn/dusk to starry darkness. May 1998.
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