Scope Parallax Adjustment - An Alternative Approach


FCC Amateur Extra N7DEY
Scope manufacturers' instructions are woefully inadequate. WRT parallax error, they generally advise adjusting objective focus until the target image is sharp. However that requires that the ocular (eyepiece) is already focused on the reticle plane. And their procedure for focusing the ocular is itself dicey: "Point the scope at the blank sky, glance quickly at the reticle (don't stare at it!)", adjust focus, repeat, etc. You are required to fight you eye's innate ability to bring the reticle into focus over a very wide range of adjustment.

There's a more precise, comprehensive approach, based on first principles: The only condition under which parallax error will be zero and target image sharp is when both objective and ocular have been focused on the reticle plane.

The easiest way to adjust for both zero parallax error and sharp target/reticle focus (making no assumptions about the scope's adjustments whatsoever) is as follows:

1. Adjust objective (side focus or AO) for zero parallax error, i.e. no apparent reticle movement on target while moving your eye around behind the eyepiece. (Disregard target image sharpness or focus.) Now the objective is focused on the reticle.

2. Adjust ocular (eyepiece) for sharpest target image focus (and, coincidentally, reticle focus - since they are now in the same plane). This is much easier than focusing on the reticle alone with a bright blank background, but you should still glance at the image for short periods, and trust your eye's first impression, which will also avoid eye fatigue.

Usually, a second iteration of the above two steps pays dividends in fine adjustment, since detecting parallax error (reticle movement on target) is easier once the image is more sharply focused.

Once you get the eyepiece focused on the reticle plane, and locked, thereafter focusing the target image for sharpness should also minimize parallax error, but I never trust that on the bench when accuracy testing - I always move my eye around behind the ocular to make darned sure there is no parallax error.