Your eyes are different from my eyes. In fact, my left eye sees differently than my right eye. This is typical in most human beings. So when you look through a microscope, your right eye is seeing your specimen with a different degree of sharpness than your left eye. By allowing the microscope user to adjust the eyepiece through the use of a diopter, the user can sharpen the focus of the specimen to a finer degree customized to their own eyes.
Adjustable eyepieces are essential to an image’s sharpness. When purchasing a microscope, you should insist upon a microscope where both eyepieces are adjustable.
The good news is that most of the high quality microscope manufacturers, like Nikon and Olympus, provide the option for adjustable eyepieces. In fact, Nikon’s current clinical line of microscopes only come with adjustable eyepieces. When buying microscopes for your lab, make sure that both eyepieces are adjustable.