Not sure if your pulling my leg or not. But I only can test in spectrums barely visable to dogs. All my equipment is set up for 800nm or above. I am not sure of how valid my tests are. I just play around with my hunting gear after work. I do have a Back reflection test set at 1200nm that is pretty cool. I get like -20db back reflection on a pine ridge 2X I got and get like -45dB on a ziess conquest. I am not sure what spectrums apply in twilight. These guys who work in this field have the equipement to change coatings nad test at the exact lamda at twilight and morning. I just play. I am not even remotely in thier applications. Mine is all freespace data transmission and such. I just take an interest in this stuff.

wow, some great info here. Thanks everyone. I had pretty much decided on a 40mm, but wanted to see the argument for the two, and the info here settled mythoughts and debatesI have had with our hunting buddy.I have 50mm on my .260 and my Encore 50cal of all things...simply b/c one was free and the other was a gift from my father, who was went with the 50mm due to a couple people giving the "it gathers more light" argument. I have been eye-balling the Leupold VX-III in 3.5-10 X 40, and I think that is what im going with....and going to see if the guy will cut me a deal on 2 leupolds so I can replace the 3-9X50 Tasco that sits atop my .260.

Checking the leupold website the VXIII 3.5x10x40 is listed as having exactly the same field of view as the VXIII 3.5x10x50.In fact this seems to be the case with all of the leupold variables that I bothered to check.

That's correct, a larger objective lens has nothing to do with the field of view.

Field of view is determined by howeach piece of glassis ground and contoured which effects how the light is bent when it enters the scope.

I ain't gunna try to get technical so just go with 40mm. Otherwise,you have to go with high rings and you lose natural fit so it ain't worth it.

Wow!!! the physics here I can admit are far beyond me. My 2 cents would be buy what feels most comfortable to you. See if you can shoulderyou gun orthe same gun at a gun dealer with both a 50 and a 40mm scope. I think it's most important what is right for you.It alsodepends on what you are scoping for. If it is for deer within 300 yards I don't think it will make much difference ifthe scopes are the same model from the same manufacturer. Over the years I've seen some guys buy the biggest and the best butwith out the time in the bush their harvests have been no better than the guy who spent 100's if not 1000'sless. I prefer the Nikon scopes but I'm no expert, just another hunter that has become comfortable with the equipment that I use.