Your eyes pupil opens up in low light to about 7 mm. The simple rule of thumb then is to have binos that give the largest light gathering ability. That is why folks who need that ability/ police, military, etc. favor 7 x 50's. Divide 50 by 7 and you get a bit over 7mm. So your eye is getting maximum light. Buying a 8 x 32 only gives you a 4mm wide light output. Quality of lenses also come into play, but if you want low light binos, or a mono, you have to follow that formula, that is 7 x 50, 5 x 35, etc.