40mm lens vs. 50mm lens
Thanks for posting. This is a question we get a lot and one that takes some explaining.
The short answer:
In practical terms, the 50mm scope will be brighter than a 40mm scope only under a combination of the highest magnifications and the very darkest conditions. Otherwise, a 50mm scope is delivering more light than your eye can use. In addition, a 50mm scope will pull your head up off the stock and also be heavier as well as more expensive than a similar model in 40mm.
The big objective is a hard myth to bust, because we have become so conditioned to bigger is better. On a riflescope, though, a bigger objective does not always guarantee a brighter image. Here's why.
The long answer:
The exit pupil on a riflescope is the actual width of the beam of light that leaves the eyepiece. If the beam of light that leaves the eyepiece is larger than the opening in your eye, the riflescope is delivering more light than your eye can use. You can calculate the width of this beam of light (exit pupil) in millimeters on your riflescope by dividing objective size by the magnification setting. A 3-9x50 set at 5x produces a 10mm exit pupil (50 divided by 5). A 3-9x50 set at 8x produces a 6.25mm exit pupil (50 divided by eight) . The higher the magnification setting, the smaller the exit pupil (beam of light) your scope produces.
In order for your eye to open to its maximum, you must be exposed to conditions of total darkenss for at least thirty minutes. Most people do not hunt under these situations, but for the sake of argument, let's say that they do. Under these conditions, the average widest a young persons eye can open is about 7mm (some individuals 8mm), but most folks who hit their forties will find their eyes can open no more than 5 or 6mm. By age 50, you're doing good at 5mm. It's part of the aging process.
According to the math, if your eyes can, and are, open to 7mm, a 3-9x50 delivers more light than your eye can use at magnifications below 7x. For eyes that are open to 6mm, magnifications below 8x waste light. For eyes that can only open to 5mm, magnications below 10x waste light - in other words, all magnifications on a 3-9x50 when your eyes are only open to 5mm, deliver more light than your eye can use.
Since very few people hunt under total darkness, they will not be shooting with eyes that have opened to 7mm. Under typical low light situations, your eyes will be open to 4 - 6mm at best.
Once again, do the math. For eyes that are open to 5mm - typical for low light adapted eyes - a 40mm riflescope is still delivering more light than your eye can use at magnifications below 8x. Since most deer are shot at much lower amgnifications than 8x, a 3-9x40 will do anything you need to do in terms of low light shooting. If you want to spend more money, you are dollars and performance ahead to invest in a better quality 3-9x40, rather than jump to a 3-9x50. Quality will have a greater impact on performance.
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And the other part of all this is optical coatings. A scope made by a low budget company telling you that their objective lens is fully multicoated while most of the internal lens are not (but they don't actually explain all of that part) which has less of that light transmitted to your eye is going to be darker and if you are looking at something with a bright spot in it have a lot of flare or glare. A scope made from a good company that costs more money and has all lens that interface to air with a good coating or multilayer coatings is going to lose less light and transmit more of it to your eye. So a 50mm objective lens on a cheap scope with poor AR coatings may still get less light to your eye then a 40mm scope with the best AR coatings on all of the lenses. Some brands tell you up front that their lenses transmit 90%, 91%, 92%, 94%, 95% and so on. Its not just the loss of brightness its the loss of contrast as well.
This is an absolute fact. With an uncoated glass surface, even with the best optical quality glass, 4% of the light is reflected back from each surface. you don't add up the lost percentage you multiply each interface times 0.96. So to go through one lens with no coatings on either side your are down to 0.9216 of the original light. On most zoom lense rifle scopes you have between 12 to 16 lens air interfaces in multiple lense groups and the reticle.
So if you are at 10X and under, buy the best quality optics, good lenses with good AR coatings and not the biggest objective lens you can find. If you are going above 15X then you really need to think about both but still quality of lens and coatings outweights size. If you are above 20X I believe you must have the best quality lenses with fully multicoated optics and then the 50mm lens to be worth it. Here is where you can easily seen the difference between the serious rifle scopes and the pretenders, you are not going to see high contrast and good brightness at 15 to 20X (or more) out of a low budget scope, no matter how big the lens is. I have looked through many, they just don't have what it takes. This is where the Leupold VXIII, Nikon Monarch UCC's, Bushnell 4200's, Zeiss, and other top line scopes shine. When you put them next to a cheap scope and compare them at distance, it is literally like night and day.