Here's the compromise points one might consider:
Higher power means:
* more apparent reticule wiggle on target, harder to hold still.
* darker image.
* changes in mirage speed sometimes harder to see.
* lesser range band that'll show mirage clearly.
Lower power means:
* less apparent reticle wiggle on target, easier to hold still.
* brighter image.
* changes in mirage speed often easier to see.
* greater range band that'll show mirage clearly.
In observing the folks who win the matches and set the records, plus the matches I've won using scopes, 16X to 25X seems to be the best choice. The lower powers tend to make the mirage closer to the firing line more visible and that's where wind has the greatest effect on wind drift on the target.
I doubt any variable power scope will be as accurate as a fixed power one. The tolerances in the zoom lens' barrel/tube are such that they tend to end up in different places when the power changes. Which means they'll focus the target image at a different place on the reticule. You can see how much slop there is in a scope's zoom lens mechanics by putting a collimator in the muzzle, zeroing the scope on it, then changing power while you look through the scope. The reticule typically makes a figure 8 move about the collimator reticule. The only way I know of to eliminate this is set the scope to its highest power then leave it there.
Ever notice that benchresters typically don't use variables?
Tube diameter has nothing to do with how bright the image is. Image brightness is determined by how large the objective (front) lens is. All it does is pipe light from the target inside the scope focusing it at a fininte point. It doesn't matter how far away the tube walls are from that point; it's as bright as the front lens allows. If tube diameter did have anything to do with image brightness, then it would be part of the "relative brightness" equation used to determine exit pupil diameter.
Image quality has nothing to do with the objective lens size. Consider the 200 inch diameter telescope at Mt. Palomar, or even the 100mm (4-inch) Unertl team scopes used in rifle competition for spotting shots and reading the wind; nothing's as bright and sharp as they are.
Last edited by Bart B.; March 17, 2011 at 04:36 AM.