Originally Posted by tulku
The best way is to put a piece of Kleenex or a Hanky over the Objective . Point the Scope towards a light ( Bulb or otherwise like the sky ) and then Focus for your eye . The Black ( in most cases ) Reticle on a White background allows ez and exact Focusing . Try it , you will like it !
The idea is that the eyepiece focus purpose is to accommodate the particular human eye that's looking through the scope. Human eyes focus without conscious effort over a limited range, So the eyepiece focus is not exact or even constant for one person. The "best" setting can and will vary with time, lighting conditions, and eye fatigue. Don't be afraid to adjust the eyepiece focus anytime it doesn't seem right. Just remember that you only use the eyepiece adjustment to focus on the reticle, never on the target.
If you move your eye laterally without moving the scope or rifle you may see relative movement between the reticle and the target. Adjust the objective focus to minimize the apparent movement between the target and the reticle when you move your eye laterally on either side of the optical axis a short distance.
Assuming the eyepiece is adjusted for optimum reticle sharpness what should you do with the objective focus?
This will sound like heresy to some. There are are at least choices in how to adjust the objective lens focus (aka parallax adjustment)
A. set the scope objective for the sharpest apparent target image for each target distance.
B. set the scope objective for minimum target to reticle parallax for each target distance. To do that adjust the objective focus to minimize the apparent movement between the target and the reticle when you move your eye laterally on either side of the optical axis a short distance.
C. adjust the objective lens to focus at 2/3 of the distance of the maximum distance shot your might take.
There are good arguments for each.
For general hunting purposes I'd recommend (A) . It requires setting the scope for each shot at different distances. Forgetting when shooting at a distant target after setting the focus for a close target can result in large parallax error (misses). If it's used, parallax error will be
much smaller than 1/2 the objective diameter but likely to be larger than setting for minimum parallax. It's faster than B but slower than C.
For fixed distance target shooting or long range hunting (beyond 800? yards) I'd recommend B. Observing the parallax and adjusting it out takes a few seconds. Setting parallax is most precise, but slowest.
For hunting large game where quick shots are essential and you don't know what distance will be I'd recommend C. Parallax error at the target cannot be larger than 1/2 the diameter of the objective lens at any distance but will approach 1/2 the objective diameter at close range. You don't have to worry about focus or parallax during a hunt. It gives considerably less parallax error than shooting with a non-adjustable objective which is set at a fixed 100 to 200 yards like most non adjustable factory scopes.