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Given equal quality of glass and same magnification the 50mm allows a larger circle of light transmission to the pupil of the eye. Known as "exit pupil".

Exit pupil in mm = diameter of the objective lens divided by the magnification
With a 9x50 scope the exit pupil is 50/9 = 5.6 mm (more light),
compared to a 9x40, 40/9 = 4.4 mm (less light).

There is a practical and functional limit to this advantage. When the exit pupil size exceeds the actual diameter of the pupil the eye cannot use the additional area. So somewhere around an exit pupil of 7 you will achieve the maximum amount of light possible.
 

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Larger bell scopes are more prone to damage and getting knocked off sight..More expensive and only a little better in low light conditions..
 

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Big Ken said:
50 mm bell is bulky and awkward...Small return for the investment.
Heavier too, so if your going to carry it all day, 40mm is better.
I've got 50's and even a 56mm, but they are not on my deer rifle.
 

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A 50 usually requires higher mounts also. I prefer to use lower mounts and have the scope closer to the barrel than higher. Keeps things a little more compact
 

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Like most fishing lures..... made to catch customers not fish. 50mm imo causes more problems(a few stated above) than worth.
 

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buy a quality scope with a 40mm objective, and it will be better then a cheap scope with a 50mm obj.
 

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burk313 said:
Larger bell scopes are more prone to damage and getting knocked off sight..More expensive and only a little better in low light conditions..
I disagree with most of you on the 50mm. I actually like the 50's better.

As far as the comments on the large objective scopes being more subject to damage.....I guess the nightforce scopes probably won't hold up as a majority of those have large bells. Don't buy a POS scope and you won't have to worry about either size.

And my 50mm scopes are mounted on low rings....so it's mounted at the same height as the 40's.

I guess I'm wrong.......agree to disagre.
 

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I like the bigger objectives too. You won't miss them until it's last light and you have a monster out in front of you. As far as the 40 mm being more durable I have to wonder what some of you do to your rifles out in the field?? Each one has its merits, so personal preference is what wins. Good luck with your VX-R, they are great scopes.

Mac
 

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I don't care for anything bigger than 40mm. But, if I ever build a moose rifle it will have a 50mm on it. It seems to never be light enough when I go to shoot at a moose.
 
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