An optician is to an ophthalmologist and optometrist as a pharmacist is to a physician. The optician dispenses or fills the lens prescription ordered by the eye doctor. In the U.S. many eye doctors do their own dispensing, and it is frequent for eye clinics to have an optician on their premises; or, conversely, for large optical chains to have optometrists in offices on their premises.
In the UK, an ophthalmic optician is licensed to both determine a person's prescription and fulfil it. A dispensing optician may only fulfil a prescription prepared by another ophthalmic optician (=optometrist), or ophthalmologist / ophthalmic medical practitioner.
See also: Eyeglass prescription