When you look through a microscope with two eyepiece lenses, you must be able to change the focus on one eyepiece to compensate for the difference in vision between your two eyes.   The diopter adjustment does this.  The way to correctly adjust this is to first close the eye over the eyepiece with the diopter adjustment and normally focus the micro [..]

Adjusts the optical viewfinder's magnification factor to suit the eyesight of the user. Look for a knob or dial next to or beneath the viewfinder's eyepiece. Not all cameras have this feature. Dithering