Check out this
Shooting Times Article on "exit pupil" to understand what power magification does to light transmission and how it is perceived by the human eye...
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A significant characteristic of an optical viewing instrument is the diameter of the exit pupil. The exit pupil is the circular patch of image-forming light the instrument presents to your eye. If you point a riflescope toward a brightly lighted wall or a patch of clear sky (but not at the sun!) and position your eye about 10 inches from the eyepiece, along the optical axis, you will see a bright disc of light in the center of the field. That disc is the exit pupil. The larger it is, the brighter the viewing will seem, because more of your eye will be bathed in light.
All other conditions remaining constant, changing the magnification to 6X reduces the exit pupil to 3.3mm.
You can calculate the size of a scope's exit pupil by dividing the effective objective diameter in millimeters by the magnification. For a 4X 32mm hunting scope, divide the 32mm objective size by 4 and you find that the exit pupil is a generous 8mm in diameter. With a 6.5-20X 50mm target/varmint variable scope, the exit pupil ranges from a large 7.7mm at 6.5X to a smallish 2.5mm at 20X. In a low-light situation, all other factors being equal, a lower magnification setting will provide seemingly brighter viewing than a higher one.
It is tempting to conclude that the largest obtainable exit pupil is the most desirable. But that's not always the case. The catch is that the pupil of a normal human eye opens to a maximum diameter of 5mm to 7mm, depending on the individual, even in extremely dark surroundings. Exit-pupil diameters that exceed about 7mm deliver more light than your eye can accept.
On the other hand, large exit pupils offer advantages beyond simply flooding your eye with light. With riflescopes, one of the blessings of a large exit pupil is greater freedom to position the eye with respect to the optical axis. When you must mount the rifle quickly for a now-or-never shot, you don't have the luxury of placing your eye behind the ocular with the exquisite precision a target shooter can lavish on finding the sweet spot of a 2mm or smaller exit pupil. You'll be truly grateful for the chance a large exit pupil gives you to see what you need to see, even if your eye is less than ideally located.
Looking through a 1.5-6X 20mm variable-power riflescope set to 1.5X reveals a sizeable 13.3mm exit pupil (crosshair is too out of focus to show in photo).
Another benefit of a suitably large exit pupil is the ability to see the scene clearly, without eyestrain. If you compare two scopes of equal optical quality, the one with the larger exit pupil will probably strike you as more preferable. Both will give you the same visual information, but one will make your eye work less.
Having praised generous exit pupils, I must confess that the riflescopes I use most often are fixed-power target models with exit pupils from about 1.1mm to 1.6mm. In daylight ranging from heavy overcast to bright sun they do what I need done.
One cautionary note regarding the exit pupil. There is no correlation between the size of a scope's exit pupil and overall optical quality. Some superb scopes have small exit pupils and some real dogs have very large ones. Exit-pupil calculation is a useful tool in selecting scopes, but it isn't the whole toolbox. You should consider everything and then ultimately believe your own eyes.
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Exit Pupil on a typical 4.5-14x40mm Scope
@ 8X = 5.0mm
@ 10X = 4.0mm
@ 14X = 2.9mm
Now look at the exit pupil on the 50mm Scope of the same power
@ 8X = 6.3mm
@ 10X = 5.0mm
@ 14X = 3.6mm (notice all are 25% larger and that the 50mm objective is 25% larger than a 40mm objective.)
If you had a 56mm objective and cranked it up to 12X (typical highest power on one), then you'd have a 4.7mm exit pupil. In that case, you could crank it up to 12X and have the same "brightness" as a comparable scope with 11X on 50mm, or 8.5X on 40mm.
See the advantage of large objectives from this? Range at dusk - that is all they're good for...
It is believed that the maximum usable exit pupil for the "average" human eye is about 7mm. I have checked, and mine appears to be around 5mm for perceivable difference.
On a 40mm scope, once I get above 8X magnification, the image in the scope begins to get slightly darker with increasing power. For the same brand 50mm scope, I can go up to around 10X before seeing the same difference. This is what makes objective diameter important. You can go to a higher magnification with the same usable light if you have a larger diameter scope.
This is UNIVERSAL for ALL OPTICS. PERIOD. If you are comparing two scopes of the same brand with identical glass but one has a larger objective lens, then that scope will appear to be "brighter" at high-power settings during low-light conditions.
Secondly, light transmission through the lenses is key in determining how "bright" a scope is. You cannot compare VX Leupolds with Zeiss Conquest on a level playing field in this respect. Take a Zeiss and Leupold of the same objective diameter and same tube diameter outside at dusk (Mark's will let you do this, BTW...). Put the scopes on the same power setting. Look through each and see which one seems brighter. Don't worry, it will be obvious.
It's good that you personally don't need the extra magnification for your situation and your hunting style. Just because it's a good fit for you doesn't mean that it's a good fit for everyone, though.
The KEY point to take away from this is that you must KNOW how you hunt and what it would take for you to do it effectively with confidence. If you only need 7X or 9X, then by all means go with a 3-9X scope. I personally have the opportunity for 250+ yard shots, and I enjoy a precise bullet placement, so I opted to get a scope with large objective, high power range, and excellent glass so that I could crank the power up as high as I might need during legal hunting hours should the opportunity, or need occur.
Scope Clarity:
One - focus. Every person's eyes are different. Magnification, well, magnifies the differences. You must focus a scope to your eyes. On some scopes, that means that you have to adjust the eyepiece (screw in or out) to get it focused. On Zeiss, you have a diopter adjustment on the eyepiece to make it quicker. Also, if you have adjustable objective or parallax adjustment, this can slightly affect the focus of what you see through the scope. If the target is 75 yards and not 100, then don't have the objective ring set for 100....
Two - glass quality. This is the money-shot, so to speak. You usually get what you pay for, and this is what makes scopes cost what they do. The impurities in the glass and the errors in the grinding/polishing process (and to a smaller degree, the coatings on each lens) is what determines how "clear" a scope is when it is properly focused. To get ultra-pure glass, you have to pay for it. To get precision ground and accurately polished lenses also requires money. In other words, you have to spend the money to have the clarity to see detail under magnification at long distances.
With that said, you may not need a high power or large objective, but just better glass and get it properly focused. However, the only thing that it's gonna help you on is being able to count points at 100+ yards, and you can do that better with binoculars (if they are the same magnification as a comparable scope and good quality glass) since humans are built for, and the brain processes binocular vision better than just looking through one eye.
Best bet would be to go look at some used scopes and even new ones at some dealers.
What Power:
Do you feel comfortable making a 250-yard shot with 9X magnification?
Would you rather have 12X or 14X?
I prefer at least 12X for that far. 14X makes it a decently easy shot with a good rest. 9X? It's a maybe for me, but I'm picky on my shots.
Worth noting is that if you take nearly identical (same manufacturer/make) scopes, but one is a 3-9x40 and the other is a 4.5-14x40 and put both on 9x, then they will look identical and perform identically as long as the lens count is the same internally. With that said, the only difference in them at that power is the price - since the 14X scope will cost more, but give you the flexibility to use the higher-power magnification should you choose to do so.
Remember that with the higher-powers, unless you go to a 50mm lens, then you'll either have to crank down on the power or you'll have to stop hunting a little earlier. Basically as it gets darker, your effective useful range will go down from 250, to 200, to 150, etc a little earlier than with a 50mm objective.
Find out what the max power is that you want and get the best scope you can that has that power on it in a 40mm objective and you should be OK.
To determine which one is best - well, you have to look through a bunch and think hard about that checkbook. It's all about compromise, really. $400 is your budget, so get the one that is best for YOU at or below that price.
If I were you, I'd worry about that during the off-season summer months. Then, I would start looking at magnifications and scope brands, etc. Find what you want and make a choice. Start shopping around May and watch Local Outdoor Shops, Pawn Shops, eBay, and Gunbroker. You'll find that prices can drop quite a bid during the summer, and that will also give you time to get it mounted, zeroed, and get used to the new glass before hunting season.
By the way, I've owned VariX-II's, VariX-III's, Bushnells, Bausch & Lomb, Burris Fullfield, Swarovski 30mm and 1", Kahles, Zeiss 30mm and 1" Conquests, etc, etc, etc.... Everything from 3-9X40 to 3-12X56 to 6-18X40 to 4.5-14x50. It took me a while to realize that I shouldn't buy the most expensive or the highest-power or biggest lenses, etc, and hope that it works. I changed scopes like some people change shoes. I eventually got smart and did some research and settled on the Zeiss that I have now. Got it ~7/8 years ago when that model first came out and it was half the price they are now. I don't have any intentions of changing a thing, either. I found what makes ME happy, and that's all that counts on a deer rifle.
