Thread: Light gathering and twight factor

Years ago an old friend who was the test pilot who sorted the P51 Mustang and who later had a great interest in hunting gave me a simple formula for assessing optimum x power for optics and scopes in particular. Apparently the human eye in perfect condition can discern light to a factor of 7. He told me that the formula to use was to divide the diameter of the objective lens of a scope by 7 to determine the optimum magnification power for best light gathering. For example with a scope like say a 3-12*42. Divide 42 by 7 = 7. Thus in dull light the greatest light definition for a human eye would be to wind the scope back to 7 power. As we age and visual acuity diminishes, we require more light entering our eye in order to see so an even lower magnification would be appropriate. In other words a fixed 4 * 32 scope will allow optimum light for a good eye. With a 3-12*56 scope the maximum light available would be 56/7 = 8 power.
With a 2-5 * 20 scope the optimum light sensitive magnification would be only about 3 power. A scope on 16 power would require a 112 mil objective lens to gain maximum usable light; so having your scope on high magnification in dull light may handicap you.
All that being said, I do not understand the european term "twilight factor" Can anybody clearly explain how this rating applies to scopes?