The next set of attributes have to do with optical characteristics of the scope. The clarity or
resolution of the scope is determined by the quality of lenses and their coatings. Image brightness
and low-light performance are determined by lenses, coatings, and the objective lens diameter versus
the magnification. The exit pupil is the size of the image coming out of the ocular lens. (Ocular
means it's the lens closest to your eye.) If you look at the ocular lens of a scope from a foot or
more away, you will see a small area of transmitted light surrounded by black. This is the exit
pupil. A larger exit pupil can provide a brighter image to the eye. The maximum size of the human
eye's pupil is about 7 mm, so a scope exit pupil larger than that size has no benefit in low light.
One advantage of a larger exit pupil not often mentioned is that a larger exit pupil gives more
flexibility in eye position to see a full size picture. This is helpful because small head
movements do not create the black fringe in the scope view. The other benefit is that it is easier
to retain the sight picture through recoil so the shooter can spot his own bullet impacts.
Long-range optics have to provide a mechanism to precisely specify elevation hold-over to compensate
for bullet drop past the rifle's primary zero. This can be done by using demarcations in the
reticle, by dialing an elevation knob, or by using a combination of those two methods. Reticle
design to specify elevation can be as simple as a mil-dot, or as complicated as the Horus H-25
reticle, which has small hash marks every fifth of a mil. Elevation knobs have several properties
we care about: click size, total travel, and if it has a zero-stop. For practical long-range
shooting, a click size of 0.25 - 0.50 MOA, or 0.1 - 0.2 mil is appropriate. Some target scopes have
eighth MOA clicks which are too fine because you'd need 250 clicks to get a 308 to 1000 yards! To
emphasize the importance of this point again- the reticle and the knob clicks should be in the same
units system: MOA and MOA, or mils and mils!
This S&B 3-12x50mm PMII provides 120 0.1-mil clicks per turn of its two-turn elevation knob; its windage
knob also has 0.1-mil clicks.
Next, it is important the elevation knob be able to provide enough travel to get the bullet to the
maximum distance the rifle can shoot. Thirteen mils or 45 MOA is enough to take nearly any 308 load
to 1000 yards. At Denver altitude, .338 Lapua Magnum takes just over 13 mils to get to 1500 yards.
This is where scope tube diameter comes into play; a larger diameter tube allows more erector
movement, which in turn allows more elevation.