If you don’t have 20/20 vision, you don’t need to worry about wearing your glasses or contacts when you use your camera. If it’s a DSLR or mirrorless camera, you’ll be able to adjust the power of the viewfinder to better match your eyes.
Beside the viewfinder is a little dial called the diopter-adjustment dial. This adjusts the power of the viewfinder and lets you tweak things so that whether you’re long- or short-sighted, things will look right to you.
By default, the diopter-adjustment dial is set for 20/20 vision. You’ll know it isn’t set correctly for you if, when you look through the viewfinder, the information displayed is blurry or if your camera keeps telling you focus is good when the image still looks blurry—at least to your eyes. Also, if you’re manually focusing, you’ll find that your images are just out of focus when you review them later.
Adjusting the diopter is a trial and error process. There usually is a slightly larger mark to indicate the center of the adjustment dial so you can easily reset it back to normal.
The best way to know when you have the diopter set correctly is when the information displayed in the viewfinder looks sharp. If it’s easy to read, then the viewfinder should accurately show the scene in front of you. Twist the diopter-adjustment dial back and forth until you find the setting that works for you. It may be easier to remove the viewfinder cup so you can turn the dial.
Adjusting the diopter just affects the viewfinder—whether optical or electronic; you will still need to wear your glasses to focus with the Live View screen.
Finally, the diopter-adjustment dial doesn’t lock into position. If things start looking blurry again through your camera, there’s a good chance that you accidentally hit the dial at some point. Adjust it again.
Source: How-To Geek
