So what does a 50mm lense do that a 40mm lense wont. If you had a choice which would you get.

Let's in more light, better for lower light shooting situations. I have a 50mm on my ar, cause i shoot a lot at night hunting coyotes and pigs.

If you read the independent studies and comparisons they will tell you that the perceptible difference in light transmission between a 40mm and 50mm is almost imperceptible to the human eye.

With that said, save the $100 or so bucks that you'd pay for the 50mm over the 40mm and buy a better quality scope with better glass. The better the glass the more light will be transmitted through it.

50mm lense and 30mm tupe will have more light going thru, better than 40mm with one inch tupe size..my 2cen

http://www.leupold.com/leupoldtv/videos-by-topic/product-showcase-videos/

Scroll to the video titled: "Boone & Crockett: Large Objective Lenses". It's a short minute and a half long and gives a good explanation.

50mm objective = more light transmission

30mm tube = stronger tube (cause it's bigger than 1") and more veritcal travel for the elevation turret

LandPirate is right. A 1" tube scope will only allow a certain amount of light to the eye and a 42mm objective is the optimum size. To check this out take 2 exact brand scopes one with a 50 and one with a 40. Slide a card from the bottom up until there is no more light . The card will be covering about 2/3 of the 50mm lens and all of the 40mm.
Cool test. A true 30mm tube will let in more light with a 50mm objective but that is a different animal.

the 30mm tube is more important than the 50mm objective. Of course you could always go with a 34mm tube and 56mm objective!!!

If you look through a very high quality scope, maybe a nothing that would make any difference but just a higher mounted scope!

You will not tell the difference between light gathering capabilities bwtween a 40 or 50 mm objective.

50 looks cooler. Light transmission is in the tube.

30mm tube doesn't have anything to do with improving low light performance, more internal adjustment and it's stronger (never seen a 1" fail personally)

assuming the same glass and at the higher magnification levels a 50 IS brighter than a 40

i.e., a 3-9x40 vs 3-9x50
at 6x, probably no percievable difference, but at the top end, yes

You can't test during the day, take scopes out in the field and watch deer at dusk into darkness

Larger tube providers room for larger adjustment range. Assuming the tubing isn't made too thin to compensate for weight, larger tubes are also generally stronger.

Larger lenses can gether more light, but that doesn't mean that they do. Lense quality and coating both play a major role. Cheapos are about 90% efficient. Quality stuff is more like 95% and the spendy stuff is more like 98%.

However, exit pupil is also key to light and often overlooked. You want an exit pupil diameter that's at least as big as your eye will dilate under the circumstances. To calculate exit pupil size, divide the objective lense size by your chosen magnification level. You want a result that is no less than 4mm. The younger you are, the more your eyes can dilate, up to around 7mm will work for the kids.

Examples:
40mm scope at 3x, 6x, 9x gives exit pupil sizes of 13.33mm, 6.67mm, 4.44mm.
50mm scope at 6x, 9x, 11.26x gives exit pupil sizes of 8.33mm, 5.56mm, 4.44mm.

You get to run 11x instead of 9x for the same exit pupil size on a 50mm versus a 40mm.

What magnification do most use when hunting at 100 yards. Ive got a cheap bushnell 4-12x40 and the last 3 deer ive shot (105, 120 yard shots) i had it magnified at 6x. I want to get a nikon and im considering either the 4-12x40 prostaff or the 3-9x40 monarch. This is on a 270.

Check here these guys have a lot of tools for learning about scopes. Also use their search engine for best product and price.
http://www.opticsplanet.net/how-to-choose-riflescope.html

Also check here after you found what you want and the price you want and see if these guys will beat or match the price. I believe they are a local company.
http://swfa.com/default.aspx

I have been using a straight 4x for about 25 years for general hunting under 200 yards.

I have been using a straight 4x for about 25 years for general hunting under 200 yards. [/quote]

I know a few old timers that have hunted Texas and Colorado for more than 40 years that swear by fixed 4x or 6x scopes. It's hard to argue with them. 6x is more than enough power to shoot 200 yards