I prefer a normally lit room with white walls to provide contrast to the reticle and increasing magnification to max which keeps anything in the background (the wall) from being in focus.
For those who may be lost in the discussion, we’re talking about aligning the reticle (crosshairs in a basic scope) so that they are adjusted to your eye, which may, and probably will be different than other people who have occasion to look through your scope.
Many newer rifles provide a fast focus feature at the rear of the occular (closest to the eye) with a “+” and “-“ but most scopes require you to loosen a threaded lock ring and turn the entire occular until the reticle is sharp then tighten the lock ring again.
The reason for the quick glances is to keep your eye from concentrating and forcing it to focus thereby adding strain. When crosshairs are extremely out of focus you will see a second crosshair that appears as a shadow. Once aligned however the crosshairs will be sharp.
Back to the discussion at hand, Walkalong is correct that using AO or a “Side Focus” knob won’t guarantee that your target is in focus when parallax has been eliminated, but should be very nearly so if you take time to focus your reticle beforehand. Some AO rings are equipped with a small EASY TO ACCIDENTALLY STRIP friction style set screw (meaning not aligned to a recess in the inner housing) that will allow you to rotate the outter housing such that focus and yardage are coincidental but you will need to know the distance to your target before attempting this adjustment.
And, if you are lucky enough for all of those stars to align, you’ll hopefully have had the foresight to know your bullet drops and you can use your scope as a rangefinder. Even without yardage indicators, it’s easy enough to add a piece of tape with yardage marked to the knob’s body or use a Sharpie fine paint marker to do the same.
The lock ring which must be loosened on most scopes to allow the occular housing to turn and focus the crosshairs. My fat finger is pointing to the thin ring not the zoom ring which changes magnification.