There are a couple issues here.
1. Resolution
Your basic Mark1 Eyeball when it is young and fresh will resolve about 1 inch at 100 yards. So if you wish to maintain this and the quality of the optics is perfect, (basically not the case.) then you would need 1 power for each 100 yards. So 5x at 500 yards, 7 at 700, etc. until you got to 10x at 1000 yards. If the target is large like an Elk, this relationship would work fine.
If the target is small, like a varmint or coyote, then more power is desirable. If you have a 15 power scope at 700 yards you should be able to resolve .5 moa or 3.5 inches, or at 300, say 1.5 inches.
To test the resolution of your eye sight put a bottle cap on a post and walk away from it and see how far away you can see it. Kids generally have no problem seeing it at 100 yards. Adults tend to have less ability.
The ability to see colors is important as well. If the bottle cap is blue and the post is brown, you will not see the bottle cap as well as if it had been white against this dark background. Color discrimination is very important in hunting where game animals and varmints tend to wear great camouflage suits.
In my opinion, this issue of resolution is the difference between inexpensive optics and their more expensive brothers.
2. Mirage
Heat waves rise. These rising heat wave may result in bending of light and distortion of the target. The effect becomes more pronounced as the power is increased. For this reason on days when this mirage is present, you may be limited to 10x to 12x to minimize apparent blur of the target.
More expensive optics can with their higher resolution minimize or reduce this effect. Recently, for these reasons I have tended to buy better optics.
You can test these features best by putting two scopes on the target and see which scope appears clearer. What you are really measuring is resolution.
You will note as is my normal practice, I have not mentioned any brands. Rather, I suggest you go to the store and compare several different scopes by looking at a distant object such as a street sign. A good scope should easily read lettering on street name sign at 300 yards.
D.
[ 12-24-2003: Message edited by: DMCI ]