50 MM objective -vs- 40MM just means you are going to have a wider field of view.
ENTIRELY untrue, though the scope companies would have you believe this. Don't believe me? Here are the specs to a Leupold VX2 3-9x40 and 3-9x50. You'll find they EXACTLY the same field of view.
Leupold VX2 3-9x40 specs
Leupold VX2 3-9x50 specs
The 50mm lens well gather more light that a 40mm lens.
If glass and internals are the same, 50mm always beats 40mm for brightness.
These statements are true only to a certain extent. First and foremost, the largest diameter beam or shaft of light the human eye will accept is about 8mm. So, one can roughly calculate how much "brightness" the human eye will realize when using a scope. This is done by dividing the diameter of the objective lens by the power of the scope. Thus, a 3-9x40mm set at 3X, will permit a 13.33mm shaft of light, but we only realize, at most, 8mm of that amount. Set the scope at 9X and we're talking a 4.4mm shaft of light or less than the human eye can detect/use. That is why we can look through a scope, turn the power ring from 9 down to 3 and see a much brighter "picture"...dial it back up and the picture becomes somewhat dimmer.
So, using the 8mm shaft of light as our reference, this means the 3-9x scope with the 40mm objective is at its best up to about 5x where it still permits the 8mm shaft of light while the 3-9x50mm will give the same relative brightness at 6.25X.
Bottom line is if the optical quality of the scopes are precisely, exactly the same, the scope with the 50mm objective is brighter, but only at
slightly higher magnification than the 40mm. IOW, the 3-9x40mm has the same brightness at 5X as the 3-9x50mm does at 6.25X.
Personally, if I can see it/do it at 6.25X, then I can likewise do it at 5X. I've never liked scope with huge objectives on a nice, trim hunting rifle. To me, it's like putting custom mag wheels and big tires on a Rolls Royce. But, to each his (or her) own!
35W