Diopter

The diopter is the focus ring built into the eyepiece of a scope, and its single job is to make the reticle appear perfectly crisp to your eye. Because every shooter’s vision differs, this adjustment compensates for that personal variation so the aiming reference reads sharp rather than soft. To set it, you look at a blank, bright background and turn the ring until the reticle snaps into focus at a glance, before your eye has time to strain and compensate.

It helps to understand that the diopter focuses the reticle, not the target image. Distance focus and removing parallax are handled separately by the objective or side-focus control, and the two adjustments do different things through the same ocular lens. Set the diopter once for your eye and it generally stays put, while the parallax control gets touched at every new range.