Hence the classic Kahles 8 x 56. Huh? I hear you say.

Exit pupil can be calculated by dividing the nominal diameter of the objective lense by the nominal magnification. Thus, 56 divided by 8 equals seven. Simple. a BIGGER exit pupil is pointless because light / image transmitted outside the nominal 7mm will not enter the eye and is “wasted”. Hence, building scopes with optimum light transmission, especially in low light, as widely used in stand hunting in gloomy European forests.

Putting a supplementary aperture on the back of a scope will help the shooter stay “optically centered”; a useful thing in a world of variable optics quality and imperfectly-fitted stocks.

Caveat is that the higher the magnification of the scope, the greater chance of parallax errors, especially in “variable” optics. “Budget” low power scopes can also suffer from this, simply because corners were cut to deliver the goods at a a low price. I have seen “Mil-Spec 1.5x optics with bad parallax errors.

Theoretically, a correctly assembled, fixed power scope, built from correctly ground elements, should show very little parallax.

How do you detect parallax?

Fis the optis or the complete rifle in a scure rest and set the reticule on a target at a moderate distance, say 100 yards.

Look away from the scope, then look back and slowly move your head in a circle (NOT “Exorcist” style) but so that you are looking “off-axis” through the scope. A “good scope will still show the reticule centered on the target.

With a dubious scope, the reticule will appear to “float” around the original aiming point on the target.