How much actual difference in brightness is there in low light situations between scopes with 40mm objective lenses versus the same model of scope with 50mm?

Not enough to justify having to mount the scope higher which makes it very difficult to get the proper cheek position on the stock.If you use quality optics you don't need a 50mm lens.

Check out www.opticstalk.com and those guys have more info than you can possibly digest. What it comes down to is that the human eye can only use so much light and scopes have a definite exit pupil size, there's formulas to figure it out. In essence though, the 50mm is usually overkill and the cons don't outweigh the pros. As stubblejumper mentioned, they are heavy and you do need to mount them up higher. Also, in the field it may be much more difficult to keep the 50mm clean as opposed to the 40mm, there's just that much more surface area, it will collect dust, etc.

What do you consider "quality optics" in a 3-9x40 in the medium price range? Leupold? Nikon? I have a 2 1/2-8 VariX lll, and a 3-9 VariX ll, and they both seem to run out of light before the end of shooting time--especially on cloudy days.

Most people don't know this, but scopes are individuals as guns are. It depends on the yeild. As in any manufacturing situation. Some lens exceed the Quality spec, some on the edge. You could pick up a 40mm that is brighter than a 50mm due to the index matching they coat these with. Or pick up a 50mm that knocks the socks off another 50mm. Fact is the 50mm does have the edge. And if you are a person that looks thru scopes or microscopes all day, you would be able to see the difference. If you a typical hunter, or even above average typcial hunter, you can't.

The one thing the 50mm give you is a much larger Field of View. But the bottom line, if you need that much a field of view, you need practice throwing a gun up picking out a target. People that count on see thrus I think are in that catagory. Good practice is taking your unloaded gun out and throwing up on birds flying at various distances.

But stubble is right on with his observation about mounting too high. And they do make a nice compact gun look a tad dorky. I could see thier use in tactical situations of observations. Or some jump hunting like they do in Europe at night.

My personal favorite in the medium price range is the ziess conquest 3x9x40.I find it brighter than the 3.5x10x40 leupoldvxiii yet it costs about the same.

Actually, I have been leaning toward the Zeiss Conquest. Is there a noticeable difference in brightness in low light situations between the 3-9x40 and the 3.5-10x44?

I don't know a lot about scopes................but was doing a comparison the other night of my Zeiss Diavari VM/V 3X9X42MM , 1" tube .........against a Kahles 3x12x56 mm , 30mm tube that one of my buddies was thinking about buying ! At pretty much dark is was hard for us to tell a difference in brightness between the two and in both you could easily see the crosshairs ! They're both expensive scopes but I really can't imagine asking for more than my Zeiss can do...........and I have a compact rifle and love the fact I get such great performance out of a scope that fits it nicely.................my buddies Kahles would have looked like a telescope on top of my gun !