I still remember when I first heard of the diopter adjustment on my DSLR.
I’m one of those people who never read the instruction manuals… so I had completely missed the point of this tiny dial near the eyepiece of my Canon EOS 1D Mark II. Sometimes I wear glasses when I take pictures and sometimes I have my contact lenses on – I have noticed a minor difference in my vision with contacts so I always try to remember the diopter adjustment on all my cameras before I start shooting. Even if you don’t wear glasses you should check if the dial is adjusted correctly (believe me you will notice the difference if it wasn’t!)
It works just like the device an eye doctor puts in front of you when checking your vision. That device has lenses of various concave and convex curves to adjust for nearsightedness and farsightedness. They adjust for an eye that can’t focus at a reasonable distance, such as for close objects like reading this text. That little dial on a camera typically has an adjustment range of -3 to +3, with 0 being nominal 20/20 vision, more or less.
To set the diopter correctly, you will need to set your camera on something solid, like a tripod, and point it at something flat with enough texture to be easily seen. Press your shutter release halfway down to force your camera with auto-focus.
This test relies on the idea that your camera focuses properly, obviously. Without moving the camera, turn the diopter control until (with or without glasses) the image is in focus to your eye. That is all there is to it!
Read the full article over at Digital Photography School.
Source: Digital Photography School
I have been using mine for years.
I’ve used it on any camera I owned that had it and on others like my Xpro i have a screw on diopter for my prescription .