In general the bigger objective lens the more light you get (that's the last number on a scope). So a x40 gives you less light than a x56. Larger lenses also means more weight. I don't know if the same hold for tub size or not, actually. The lower the magnification (first number) the more light you get, and this also true for zoom lenses (first two numbers), so a 4-12 has more light at 4x than at 12x. Magnification is part a personal preference and a function of what you want to do with your gun. Magnification amplifies perceived shake, it reduces your field of view, and the eye box gets smaller so you to be more careful with how you look through the scope. I use my rifle for pesting under 30 yrd, and like @Saltlake58 I never use more than ~16x. People that hunt may prefer lighter zoom scopes for mobility and flexibility, or heavier scope to get the most light at dawn or dusk, people that shoot benchrest may prefer high magnification scopes (say, 50x or 60x).
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