In a classic eye test you read letters or numbers (optotypes) of different sizes on a chart. This gives the visual acuity – a value of 1.0 (100 %) counts as normal. It is complemented by objective measurement with an autorefractor and subjective fine-tuning, in which you compare different lenses ('better like this – or this?'). This produces the values sphere, cylinder and axis (for astigmatism) plus the near addition.
An eye test at the optician is used to determine values for glasses and contact lenses and takes about 20–30 minutes. However, it does not replace an eye doctor's examination: conditions such as glaucoma, cataracts, retinal diseases or diabetic eye damage are only reliably detected by an ophthalmologist using additional equipment (eye pressure, retinal exam). With sudden loss of vision, pain, double vision or flashes of light you should see a doctor, not an optician.
An eye test makes sense if you see blurred, often get headaches or tired eyes at the screen, see worse at dusk – or simply as a check every one to two years. For a driving licence an eye test is mandatory; opticians with the relevant accreditation may carry it out.
In Switzerland the eye test for a prescription is often free at the optician if you buy glasses there; the driving-licence eye test costs roughly CHF 20–40 depending on the provider (guide values, no guarantee). A medically indicated examination by an eye doctor, by contrast, is usually billed via basic insurance (minus deductible and co-payment).