Posted on 08/16/2006 8:15:50 AM PDT by LouAvul
I don't remember where I got this, but it's a pretty decent explanation of scope 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 http://www.usoptics.com/sub_pages/parallax.php
Bump for later read.
You will need a good scope to spot any of the new planets. Ceres, maybe, is possible for dedicated amateurs.
Quality of the glass,coatings used on the lenses and how well the interior on the scope has been blackened has much more bearing on light transmission than exit pupil. It's easy to get wrapped up in optical calculations but it all boils down to buy quality optics and spend time at the range.
Is there any way to determine eye relief from the info commonly found in rifle scope specs?
I wonder, a) what power the scope is set, b) distance to target, and c) approximate size (height v length) of the ram?
If they say what the eyepiece type is it might have some use. Rifle scopes should have a very wide exit pupil so they can be used in active conditions. Most rifle scopes are designed this way. Telescopes can get by with a narrow exit pupil since the observer expects to settle into a particular observing position for an extended time.
Thanks. Stupidly, I mail-orderd a scope with a rather short barrel for my Winchester 54. After some trial and error I ended up with a mount extender that alleviated the problems greatly. Still, when I bring the gun up, I have to move my head some to get that nice full bright circle. I'm just trying to avoid this in the future if I decide to re-scope.
BTW, you're right. The 'eye relief' term I used was from my limited knowledge of astronomical telescopes.
Parallax is not a problem if you buy a good scope - Nikon, Burris, Leupold, or better. Eye relief can be adjusted on most scopes, even the cheap ones. The eyepiece adjusts. It is attached by screw threads to the body of the scope, and you can change the focus by screwing it in or out. There is a lock ring so that you can secure it when you have it set properly.
The proper way to mount a scope is with the shooter present. The eyepiece adjustment controls the focus on the reticle (crosshair.) The shooter should be able to mount the rifle and immediately see a sharply focused reticle. If the reticle is fuzzy, the eyepiece needs to be adjusted. Or the guy needs glasses (this has happened to me.)
Is there a specific rifle and scope combination you are considering?
That picture is a fake. Where is the bolt handle?
It's an M14. He's probably an SDM (squad designated marksman).
I've got a Winchester 54 .30-06 and the scope I put on it is a Bushnell Trophy 3-9x40. Not exactly in the class you were talking about. Although I do need glasses for close work, I'm fine with the reticle and naked eye. It's that bright big circle that I can't get immediately.
I meant the first picture, but now that I look at it again, I can just make out a little bit of bolt handle at the very bottom of the frame.
The problem you are having is because the scope is not at the right distance. Loosen the rings and move it a little forward, take a look, and if you have to, try moving it a little back. After you think you've got it just right, hold it down and quickly shoulder it as though you saw a 12 pointer running. When the scope looks right every time you do that, tighten the rings.
Sometimes it takes a little fiddling with it to get it right.
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