When you’re setting up your new scope to your rifle, you want to be able to shoulder that rifle, open your eyes and see a nice clear and full picture provided by the rifle scope.
All too often when you look through the rifle scope there’s a large blackened circle encompassing the picture, or rather than a nice round full picture, you see one that is partially eclipsed or darkened. Then you find your self shifting your head forward or backward or changing your cheek weld to the butt stock in order to get that full, clear bright image you want.
So why do you mount some scopes and easily see that image though the rifle scope while other scope and rifle setups take longer to get just right? It all comes down to something called the exit pupil. The rifle scope is projecting and image toward your eye in a cone shape. That cone shape becomes increasing larger as it gets farther from the rifle scope until right at the exact distance, a perfect image is formed. The distance is referred to as eye relief, the size of the image is called exit pupil. A larger exit pupil is more forgiving to the placement of your head and location of your eye than a small exit pupil as well as provides more light… but more light only in dimmer conditions as we’ll explain.
It sure would be easy if all rifle scopes just came with a large exit pupil, but do to some laws of of physics and some limitation to the human eye, it’s just not possible and sometimes not beneficial either. First off, the human eye has a pupil that ranges in diameter from about 2mm to 7mm. Your pupil gets larger in dimmer light to allow more light in and smaller in bright light. On the other hand, the exit pupil given by the rifle scope is a calculation of taking the objective lens diameter and dividing it by the magnification. Ideally, the exit pupil should match your eye pupil size, but with changing light conditions and magnification levels, it’s just not possible.
As an example we can look at two rifle scopes both set at 12 times magnification. Rifle scope A has a 50mm objective lens and rifle scope B has a 40mm objective lens. The exit pupil on rifle scope A is 4.17mm (50 divided by 12) while the exit pupil on rifle scope B is 3.33mm (40 divided by 12). If outside conditions are bright and your eye pupil has become smaller, lets say 2.5mm, both rifle scopes will provide a bright clear picture since both exit pupils are larger than your eye pupil. In dimmer conditions, when your eye pupil has increased to be larger than the exit pupil, scope A with the larger exit pupil should provide a brighter picture.
If the exit pupil becomes exceedingly larger than the eye pupil, then some light is actually lost and may actually provide less picture. So in dimmer conditions or large magnifications it may be worth the added expense of looking at a larger objective lens, but beware, a quality scope with fully-multi coated lenses and a higher percentage of light transmission will out perform a lesser quality scope with a larger objective as well as reduce weight and maintain the ability to keep the scope mounted closer to the rifle.