If you’ve got perfect eyesight you may not need to make any correction, but most of us haven’t so this dial is here to make adjustments for short or long sight. To use it, look through the viewfinder with your eye relaxed – don’t strain to force the display into sharp focus. With your eye still relaxed, turn the diopter adjustment dial to bring the display into sharp focus. This is something you’ll only need to do once unless your eyesight changes or someone else uses the camera.