There isn’t a universal diopter position for “perfect vision.” The viewfinder diopter is a personal adjustment, even for people with normal eyesight, and it’s also useful if you sometimes shoot with glasses.

The best approach is to set it by eye:

• Put the camera on a tripod or hold it steady.
• Autofocus on something bright with fine detail.
• Look through the viewfinder and turn the diopter control until the view appears sharp and comfortable.

Some cameras may have a center or default mark, but that’s usually just a reference point, not a guaranteed correct setting. What matters is where the viewfinder looks clearest to you.

If the built-in range isn’t enough, optional corrective eyepieces are available for some cameras.