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Old August 16, 2006, 10:25 AM   #1
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What the numbers on your scope mean.

I don't remember where I copied this. But it's a pretty good breakdown of riflescope nomenclature.

Objective lens: front end of scope, i.e., what you "point" at the target.
Objective diameter: the size in mm of the front glass.

Exit pupil: circular beam of light that comes out of the eyepiece of the optics. If you hold your optic at arms length and look at the eyepiece, you will see a bright circle of light on the eyepiece. The diameter of that circle of light is the exit pupil.

The human pupil can dilate to about 7mm -- so any exit pupil larger than 7mm is wasted. An exit pupil smaller than 7mm, on the other hand, will not be good for dim light conditions.

To calculate exit pupil, divide the diameter of the objective lens (front glass) by the magnification. For instance, a 10 X 50-mm scope setting has a 5 mm exit pupil.
(50 mm ? 10 = 5mm).

If you had a 20 power scope, and a 40mm objective lens, the exit pupil would be 2mm in diameter. If the objective lens were 80mm, the exit pupil would be 4mm. And so on.

A small exit pupil means the scope (or other optical instrument) will be less than effective in dim light -- when the pupils of your eyes are dilated.

Other factors -- such as glass quality, coatings, and so on -- can affect light transmission. But all other things being equal, an exit pupil of 7mm gives all you the light you can handle. If that's what you want. But most people don't do any target shooting at twilight.

For most purposes, a smaller magnification is the solution to the problem. For example, if you have a 3X9X32 scope, you can use the 9X setting during the day, and turn it down to 4X or so for twilight shooting.

Optics and brightness are secondary in importance in a scope. Normally, you use binoculars or a spotting scope to FIND your target, . In those instruments, clarity, brightness, and so on are paramount. You use them sometimes for hours at a time -- and tiny flaws can create headaches.

The riflescope comes into play only when you find game, so ultra high quality optics aren't that critical.

What IS critical in a riflescope is reliability. It has to stand up to rough handling and not lose its zero.

Most target shooting is done during the brighter portions of the day and the scopes ability to gather light isn't as critical. Under hunting conditions, at first or last light, is when this becomes critical (or at night if you are a tactical type of shooter). Most hunters prefer a variable and this is one of the good reasons for it, the increased field of view another.

If you have a 4-16X56 scope you can use the 16X during the bright day and back it down to 8 during twilight and still maintain a 8mm exit pupil, ensuring that your eye has as much of the light that comes out of the scope that it can use. Then you can back it down to 4X to increase the field of view when you are moving and more likely to get a quick shot and have to find your target very quickly.

Note that a larger exit pupil will allow coarser eye to optics alignment. This is not of much importance when plinking or target shooting, but if you are trying to acquire a sight picture in a hurry on that once-in-a-lifetime critter under less than optimum conditions, having a larger exit pupil can more than make up for the added expense and weight of a larger objective.

For those who would argue the parallax issue, if the critter is so close that you are taking a snap shot, parallax will be negligible. In fact, for large critters parallax is negligible at all ethical hunting ranges. See
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Old August 16, 2006, 03:38 PM   #2
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Thanks for the excellent info.

The human pupil can dilate to about 7mm -- so any exit pupil larger than 7mm is wasted.
Well, yes and no. It's wasted if your eye is perfectly centered. But the larger the exit pupil the further you can go with your eye left, right, up, and down before blackout, so if shooting the scope a little off kilter, so to speak, it's not wasted. When you do this of course, there is parallax error, but in close in targets, the P. error doesn't matter a great deal, so having "too much" exit pupil can be particularly useful in snap-shot type rifles.
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Old August 25, 2006, 04:49 PM   #3
Czar 6
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Is there any analogy between the exit pupil size and the light transmission percentage of the scope? IOW, if the scope is not transmitting 100% of the available light through the scope-which I don't think is possible-does having a larger exit pupil diameter have any effect on the perceived brightness of the scope?
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Old August 30, 2006, 04:49 PM   #4
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Yes. Let's assume that the lenses in a scope will transmit all light they receive.

The 'maximum usable' exit pupil is 7mm. But the exit pupil is calculated by finding the ratio of the scope's fron lens size to the magnification of the scope.

So a 1X scope would really only need a 7mm lens in front. That isn't particularly practical, but you get the point.

If you wanted a 40X scope with all the light you could use, you would need a lens of 280mm. It sure would look funny

Next time you are at a gun store, ask to see a 6-24 or an 8-32 variable scope. Turning the dial, and looking at a dim corner, you should be able to see the scope picture getting dimmer as you increase power.
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