Re: max effective range for leupold 12X fixed scope?
Nominally, the unaided healthy eye can resolve objects down to about 1MOA in angular size. At 100yd, that's technically, precisely 1.047" and at 1000yd, that's a bit more than 10". So the 24" object should be resolvable with the naked eye out to a distance of at least 2000yd.
But taking a look at an F Class target at 1000yd, I'd say we really would need to use a more reasonable LR resolution of about 4MOA of angular distance for the naked eye, and start my magnification minimum value at around 4X.
At the other end, I find that 16-18X is about the reasonable maximum when mirage is running strong, and have been able, on the relatively rare occasions of unusually clear conditions, been able to put 24X to full use, where low magnification image acquisition is more important.
When shooting beyond 300yd, wind reading ability becomes as crucial as visual acuity, and beyond 600yd, it becomes the dominant factor. Your optics will serve you best if they can provide clear images of the wind's effects on distant terrain features, like leaves and grass, so clarity is at least as important as magnification. That's what you pay the bigger bucks for.
I like a 6-24X42 for stationary applications, and could make good use of a 4-16X42 for more portable ones.
Greg