The science of optics is basically a sort of balancing act. Think of it getting something good on once side and loosing something on the other.
But first lets talk about power or magnification.
The Mark 1 Mod 0 Eyeball (Your eye for those in Rio Linda) when young will resolve approximately 1" at 100 yards. That is if you take a bottle cap that contrast with the back ground an place it out at 100 yards and your eyes are in good shape or corrected to 20/20 you should be able to see it. (Children will find this excercise easier than adults.)
1" or about 1 minute of Angle (MOA) is a measure of this. If the rifle will shoot 1 moa then you should be able to hit that bottle cap at 100 yards with open sights, or a 2" jar lid at 200 yards and a coffee can lid at oh say 450 to 500 yards with open sights.
That means that a trained infantryman should be able to take a "tactical target" out at 600-800 yards most times if he exercises care.
Now magnification enhances this equation by making the "target" appear larger.
At 4x magnification, the bottle cap will appear the same size at 400 as it does at 100 with the eye. 8x and it will appear at 800 as at 100 yards. This assumes such things as perfect optics, no distortion by the atmosphere(mirage or haze), conditions that hardly ever exist.
Oh, so just add more power to the scope and compensate for these problems. Well, sort of. However often adding more power or magnification to the scope begins to cause resolution to decrease, and often aggravate problems such as mirage and distortion. The aforementioned balancing act. So with a big variable, you can just reduce the power. Whoa! Not so fast. There are other considerations like weight and cost. Therefore, addition of too much power is not always the answer.
So when picking a power consider the target. For a hunting scope in years past 4x was considered adequate. Varmints usually require 12 to 15x, with 12x considered plenty for normal varmint shooting to say 600 yards.
If we consider a 1" kill zone on a prairie dog at 700 yards, 14x will .5 inches resolution at that range, if I didn't make a mistake in my math.
This should be plenty to take out PDs for most hunting of varmints. Lower powers on variables such as the typical 4-14x allow a 4x value that is very useful for area spotting and this can be turned up to zoom in on an identified target.
Functionally, people today tend to to use too much power on their variables and sacrifice resolution and optical clarity when faced with mirage effects in warm weather. Ok, so why would they make 22x variables and 40x fixed target scopes.
Well, we are Americans right? Big is good, bigger is better, biggest is... well you get the idea.
But people like John Unertl, Weaver, and others knew the true skinny 50 years ago. The USMC could pick any scope it wanted and often settles for 10x or less for precision shooting. Get the idea?
