When should you see an Ophthalmologist instead of an Optometrist?

If you want to see and Ophthalmologist - you can't... you've got to see either an Optometrist or a General Practictioner (Family Doctor) who will refer you to an Ophthalmologist.

If anyone from the street can go to see an Ophthal, then they'll be inundated with 'simple' cases and force the urgent important cases to be on waiting lists.

If you are concerned about your eyes, go and see and Optometrist first.. they will be able to answer all your question and concerns.. If they can't, the can then refer you to an Ophthalmologist.

But to answer your question: Optometrist can diagnose, and treat simple diseases affecting the outer and front of the eyes via eye drops or minor surface surgeries. Ophthalmologist can diagnose and treat all diseases (more serious ones) affecting the eyes.

(Optometry's scope of practice varies from place to place; in my answer I'm assuming you're Canadian or American.)

In general, optometrists are the primary eyecare providers, or the equivalent of a family doctor for your eyes. You go there for routine checkups, as well as specific instances of blurred vision, red eyes, pain, etc. Ophthalmologists are specialists, and typically require a referral to get an appointment. An optometrist may send you to an ophthalmologist if it seems like you have glaucoma, cataracts, retinal problems, or anything else unusual. A family doctor may send you to an ophthalmologist for the same reasons. Most ophthalmologists don't do refractions (glasses/contact lens prescriptions) though.

So in general, you should see an optometrist for eyecare, unless you've been asked to see an ophthalmologist by another healthcare professional. Optometrists can handle all the basic stuff, and will refer you to a specialist if needed.