Ophthalmologists usually use a machine used to provide an objective estimate of refractive error. The patient would be asked to sit down with their chin on a "chin rest" then focus on the image inside the machine. The doctor will get an estimate of the objective refractive error.
The doctors will use objective and subjective refraction to measure the refractive errors. First, the doctors will use objective refraction. They will use a tool called a retinoscope or auto-refractor to measure your refraction. Then the doctors may use subjective refraction. A phoropter is used to measure your subjective refractive error. You will be asked to sit behind the phoropter and look at an eye chart. The doctor will change lenses until you can see the letters on the chart clearly.