What is the difference between a 50 mm and a 40 mm scope?

Diameter of the end lens.

To answer your question is the size of the objective (front) lens...

No guys i realize a 50 mm is bigger than a 40 mm but what does the bigger lens do? Does it just let more light in?

More light and a bigger field of view.

10mm (50mm - 40mm = 10mm)

YEP it does ...
But more importantly its the coating on the lens that increases light transmition...
a 36mm scope that has fully-multicoated lenses will let in as much light as a 50mm lens that has no lens coating at all...

Larger objectives gather more light and allow a wider field of view. I personally like larger objectives and have a 56mm on my Diavari.

I called Leupold about this a few years back....They said a 50mm allows about 3% more light than a 40mm...If the glass is the same...Now, when you go from a VX-II to a VX-III a 40mm transmitts more light (I think it was 3-4%) than a VX-II with a 50mm...The difference between a VX-II and a VX-III at that time was about 6%, when both had 40mm....The current line of VX-IIs have the same glass that the VX-IIIs had 3-4 years ago...Now...I have 2 VX-II 40mm (with the older glass)...my brother has a VX-III with a 50mm (with the older glass)...I have hunted with his gun....MY eyes can't tell the difference...probably because I'm 50...When you get past 40, your pupils don't dialate as well as they did at 20....So you may not be able to tell a difference, because your eyes are the limiting factor.

With my Leupolds I can see to safely shoot about 10-15 minutes past legal shooting time (30 minutes after sundown in NC)...depending on stand location in relationship to sun, if I'm in the woods or field and if its a sunny day or cloudy.

A friend of mine fell on his gun and damaged his scope a few years back, I took my Leupold VX-II 3x9x30mm compact off my Remington 541 and installed on his .260, he used it during the rest of the deer season...One evening we were hunting together and I heard him shoot, it was right at the cutoff of legal shooting light, the buck was about 120 yards away and he made a good hit on the deer, had no problems picking the deer up in the scope.

I prefer a 40 mm because a 50mm is mounted higher off the barrel, so I feel I get a better cheek weld with the stock...If you are varmit hunting with the moonlight, a 50mm might help, but I doubt it.....

With the higher quality scopes I feel a 40mm is all you need.

Thanks nchawkeye, that was exactly whatI was wanting to know.
You guys on here are great.