There are three basic O's in the optical industry.
Ophthamologists or OMDs as Christine mentioned go through four years of medical school, plus rotations and residencies to specialize in the eye. They are considered specialists in surgical procedures and all forms of disease management. IE the health of the eye and systematic diseases that affect the eye..
Optometrists are your actual vision specialists. They go through four years of school plus clinicals at the same time to study the visual system. They are the primary care givers of the eye. They do the refractions, basic eye health checks, visual field, some treatment of diseases and occular injury. They also are experts in the contact lens and vision therapy arenas. IE there primary focus is on vision.
Opticians are the ones help you make the most out of either doctors prescription. They help you select the best frame, lens materials, lens designs, and coatings to enhance the rx. They also take all the measurements to be able to make sure the lenses are specific for you. If you have trouble with your new glasses they are the first person to see, that way they can see if it is possibly not the rx, but some other factor affecting the prescription. (bifocal placement, pd, oc..etc...) Opticians also routinely help in the contact lens dispensing and training for the ODs and OMDs. They double check the accuracy of all manaufactured eyewear to make sure they are withing federal tolerances to the dr's rx.
They also often work in the labs as well to guide them in the manufacture of the eyewear. (This is actually where I got my start in optical..I was a lab rat 🙂 )
Along with these three professionals there are also certified ophthalmic or optometric techs. These are the ones who do the majority of the pretesting for the doctors and in the case of the OMDs, may be doing the refraction. They too are trained in the contact arena for dispensing and contact classes as well. They are also sometimes called COAs or certified ophthalmic assistants...
If you have any other questions, feel free to ask 😉
Cassandra