One additional tip on making viewfinder diopter adjustments quicker and easier with optical viewfinders in SLR cameras...
The only purpose of viewfinder diopter adjustments is to bring the camera body's focusing screen into focus for your individual vision. The lens and any image from it seen in the viewfinder is irrelevant. I've found it helpful to remove the lens when making adjustments as it eliminates any background image distractions and facilitates in focusing your eye's attention on the focusing screen itself. Getting the focusing screen sharply focused for our eye is the only thing we're looking to accomplish with this adjustment.
Remove the lens; point the body's lens mount opening toward a light source to give a bright contrasty image; then adjust the diopter focus control to bring any of the black frame or focus indicator lines, circles, brackets, boxes, etc., contained in the screen, into sharpest focus; and you're done.