You could ask about the area you would be working in (depending how large the opticians is).
If somewhere like Specsavers then you could be on the shop floor helping customers, dealing with fittings, assisting with people needing glasses or contact lenses or even up in the place that deals with the making of the new lenses and fitting them in, say like Vision Express where they do them in an hour.
Some staff seem to do some of the initial tests before you see the optician themselves.
Some opticians also deal with eye problems, we have a Red Eye service up here where certain opticians will see people who have minor eye problems, like if someone comes in with a sore red irritated eye, and they can prescribe certain drops and such.