Objective size and magnification go hand in hand. If you divide the objective size in millimeters by the magnification, the number you get is he diameter of the light beam coming out the back of the scope. Known as the "Exit Pupil". You want a minimum of 5 for low light. But for most people anything much over 5 is wasted. Certainly, no more than 6 or 7.
A scope with a 50mm objective set on 10X, a 40mm scope on 8X and a 20mm scope set on 4X all allow the same amount of light through them.
