If you do not have 20/20 vision, you do not have to worry about wearing your glasses or contacts while wearing your camera. If it's a DSLR or a mirrorless camera, you can better match the performance of the viewfinder to your eyes.
Next to the viewfinder is a small dial, the diopter dial. This adjusts the performance of the viewfinder and allows you to optimize things so that you will look good, whether you are short-sighted or long-sighted.
By default, the diopter adjustment dial is set for 20/20 Vision. You'll know that it's not set properly when looking through the viewfinder, the information displayed is blurry or when your camera tells you that the focus is good if the image still looks blurry – at least for your eyes. If you focus manually, you will find that your images are not properly focused during the later review.
Setting the diopter is an experimental process. There is usually a slightly larger mark to indicate the center of the control wheel, so you can easily restore it to normal.
If you adjust the diopter correctly, you will know best when the information displayed in the viewfinder is clear. If it's easy to read, the viewfinder should show the scene in front of you. Turn the Diopter dial back and forth until you find the setting that best suits you. It may be easier to remove the viewfinder cup so you can turn the dial.
The diopter adjustment only affects the viewfinder – whether optically or electronically; You still need to wear your glasses to focus with the Live View screen.
Finally, the diopter adjustment wheel does not engage. When things get blurry through the camera again, you run the risk of accidentally hitting the dial. Adjust it again.