I need a new scope for my rifle, and plan on  3x9 magnification at the most.



At this range of magnification, is there any great difference between the light gathering capabilities of the 40mm vs. the 50mm objective lens? From my limited research it seems like the >40mm objectives really come into their own only at the higher magnifications.



This will be for a hunting purpose rifle.

I run decent quality 50's [Vari-X III and VX-III] on both of my primary hunting rifles, with the older having been in service for over 2 decades.  With all other factors being equal in terms of glass quality, nothing beats a larger objective for reduced light performance aside from an I^2 tube.    The only real downside is that it requires the optical centerline to be slightly higher, but that is moot for the AR platform, as the optimum height is arguably around 1.4" above the bore.  Even on a bolt gun, a 50 is well manageable from a mounting perspective.  Notice that many European optics have very large objectives; guess what, they do alot of night hunting over there.

As an experiment several years ago, I tried a 2.5-8x36 with otherwise identical specs to the 3.5-10x50 which was temporarily removed; the results were dismal, and the experiment was quickly aborted.  It really sucked to be able to effectively observe the animals through the relatively mediocre 8x42 binocs in the closing minutes of legal shooting hours, but not through the riflescope

It is generally recognized that the human eye can make advantage of an EP as large as 7mm.  To achieve this with a 40mm objective requires the magnification be reduced to 5.7x, whereas with a 50, one can theoretically stay up to 7.1x.  Many hunters reduce their mag in the pre-dawn or as the sun is setting; I sure know that I do.

Both of these numbers fall within your anticipated mag range.  Is an additional 1.4x beneficial depends on where you hunt, and how important it is to ID targets, such as when rough scoring whitetails.  

I think that some folks get overly hung up on the concept that hyper high mag will make them shoot better, maybe it helps bench shooters, but in a hunting environment where weapon support is often a bit more ad hoc, 10x is the most that I have found to be effective.

Hope this helps in your selection.  

murderman hit it on the head when he brought up Exit Pupil. If all things are held the same except for objective diameter, then at higher magnifications you'll have a larger exit pupil. [exit pupil = obj. diam. / "magnification" number on the power selector ring]



As for glass having an ability to gather light, that's marketing/sales talk. Glass does not attract light energy through any magnetic qualities, electrical properties or even gravitational pull that some may allege that Light has. Light energy has to physically strike the glass for quantum mechanics of light retransmission to occur.



While it's true that a curved surface with a diameter of 50 mm is larger than a curved surface of only a diameter of 40 mm, in modern design, the amount and type of light is limited to reduce aberration. All things considered, you'll get pretty much the same light retransmission values through a 50 mm obj system as with a 40 mm system (all things held the same except for obj diam.) But with a 50 mm obj you will get a larger exit pupil for the same "magnification" value on the power selector ring. That means a larger aperture for you to look through; which you'll perceive as a brighter image. The only downside to a large exit pupil is that it's easier for the shooter to have his eye outside the center-axis of the lens system which induces more parallax error. But, if you have a good and well practice cheek weld, that won't be a big deal. Good luck with your purchase.

Most excellent point made regarding the off-axis / parallax consideration of a larger EP!