How do dioptre adjustment, focus and parallax differ?
When looking through a riflescope, you see two things at once: the reticle and the target image. They may not appear equally sharp. The dioptre adjustment at the rear of the scope adapts the appearance of the reticle to your eyesight. This is not the same as focusing the target image.
When looking through a riflescope, you see two things at once: the reticle and the target image. They may not appear equally sharp. The dioptre adjustment at the rear of the scope adapts the appearance of the reticle to your eyesight. This is not the same as focusing the target image.
Parallax means that the reticle appears to move relative to the target when your eye position changes. Here is a simple example: hold a finger in front of a more distant object. If you move your head sideways, the finger seems to shift against that object. A similar effect can occur in a scope when the reticle and target image are not optically in the same plane.
A sharp image and an image with no noticeable parallax are therefore not exactly the same thing. Depending on the design, a side knob may affect both. Its distance scale is only a guide, not a built-in rangefinder. The scope’s manual explains what each control actually does.