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The reason is low light yes.
But I'm not a professional in optics..., so I'm grateful for every tips.
Maybe I can use 50mm too....
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It depends on your age. A youngster can use a 7mm exit pupil but by the time we hit 40 that's usually down to 4mm. Divide the objective diameter by the magnification and you get the exit pupil. The other thing to keep in mind is the quality of the coatings. Bare lenses reflect back a percentage of the light that strikes them reducing the amount of light passing through the scope by 15-20% or more (multiple lenses reflecting light, which also causes cloudiness). Coatings reduce the amount of light reflected off the lens which increases the amount of light transmitted through the scope. Premium scopes like Nikon's Monarch, Bushnell's Eite 4200 and 6500, Burris' Signature Select, and Weaver's Grand Slam will pass 95% of what hits the lens to your eye. Popularly priced scopes like Nikons Buckmaster, Bushnell's Legend, Burris' Fullfield II pass roughly 90-92%. As you get further down the food chain the quality of the coatings is reduced to cut costs. Still lower on the food chain they reduce the number of the lens in the scope that receive coatings. Look for a premium product from a premium brand name that uses
fully multi-coated lenses (fully = all lenses coated). You'll also notice that premium scopes produce a sharper image than cheapie.
Weaver claims that their RV9 3-9x32 rimfire scope will pass 95% of the light that strikes their fully multi-coated lenses. 32mm divided by 9x means an exit pupil of 3.8mm or about all a 40y/o can utilize. Dialing it back to 5x and you have an exit pupil of 6.4mm or just about all that a youngster can use. Just food for thought.