This is one of those questions that came up in a recent class. Some people never looked at this setting. Some just assumed to set it at zero. Some said they always wondered, but were afraid to ask. Some thought that it was too obvious to ask. Just about everyone was wrong.
The diopter control helps you focus on the viewfinder in the camera. If you wear eyeglasses you are probably familiar with the term diopter. Your eyeglass prescription may be described in diopters. If you are nearsighted your eyeglasses will have a negative diopter adjustment (i.e., -1.5). If you wear reading glasses they may be marked with a positive number (i.e. +2.25).
Camera viewfinders can be adjusted through a range of diopters, but the range may be different between models and brands of camera. -2 to +1 is a typical range.
So..., just how do you adjust this dial?
The first temptation is to look through the viewfinder and try to adjust it so that the image in the finder looks its best. Nope.
What you want to do is adjust it so that the items in the viewfinder (the etchings on the focusing screen, for instance) look clearest. Pay no attention to the image you are pointing the camera at when adjusting. Put the lens in manual focus mode, point the camera at a plain white wall, and throw the image completely out of focus. Now you can concentrate on the focusing screen. Does it have a circle etched in it? Or small boxes to show the autofocus points?
Put your diopter control all the way at one end of its range. Look at those etchings on the screen and slowly turn the dial until the screen is sharply focused. You might find two or three click settings that all look pretty good. It is like sitting in the chair at the eye doctors where they examiner is saying #1 or #2... #2 or #3...? Which looks better? Sometimes there is a big difference. Sometimes it is very subtle. Pick the one that looks the best.
Setting the diopter will not affect autofocus. But it will affect manual focusing. If set wrong, an in focus image will look out of focus in the viewfinder. You cannot make an out of focus image look in focus by adjusting the diopter.