Originally Posted by NJS
In our society bigger is better... right? Well not always... The difference in the image from a 42mm scope and a 50mm scope is only barely noticeable the last couple of minutes in the early morning or late evening. If you do a lot of shooting or hunting at night or the first or last minutes of the day I'd go for a 50mm objective. If not, you are paying for capabilities you won't use, plus the negatives of higher mounting, larger & heavier unit, and additional cost. Unless you hunt at night, I'd stick to the 42mm size objective. NJS
Good response - I'd add this:
Ideally, during low light use you would want an exit pupil of about 6mm. In the 42mm objective that would be achieved with the power set on 7X. With a 50mm objective scope that would be achieved with the power set on 8.3X.
Sooo.. you could get the same amount of usable light from either scope except that the 50mm will allow you to get it at slightly higher magnification. Naturally, the comparison assumes that the two scopes are essentially the same except for the size of the objective lens.