Diopter Adjustment
A viewfinder correction that matches the eyepiece to the photographer's eyesight.
Measured in
diopters
In depth
The diopter changes only what the eye sees, never what the sensor records — but an incorrectly set diopter makes manual focusing impossible and makes autofocus appear to be missing when it is not. It is set by looking at the viewfinder's own overlay data, not the scene, and adjusting until the numbers are crisp.
Examples in practice
A photographer who has just started wearing glasses re-sets the diopter and discovers their lens was never back-focusing at all.