adapts the viewfinder to your vision so you see the subject in sharp focus

A device generally located either on the right eyepiece or as separate function of the central-focusing system of binoculars, used to correct for visual differences between the viewer's eyes.

When you look through a microscope with two eyepiece lenses, you must be able to change the focus on one eyepiece to compensate for the difference in vision between your two eyes. The diopter adjustment does this. The way to correctly adjust the diopter on a lens is to first close the eye over the eyepiece with the diopter adjustment and normally focus the microscope so that the open eye sees the image in focus. Next, switch eyes (close the open eye, open the closed eye) and without changing the main focus knobs, focus on the image by turning the diopter adjustment only. Now with both eyes open, the image should be clear with both eyes. (This technique is used with binoculars too.)

A balancing focus to facilitate any difference between your left and right eyes.