Retinoscopy is especially useful in prescribing corrective lenses for patients who are unable to undergo a subjective refraction that requires a judgement and response from the patient (such as mentally handicapped or non-verbal people). It is also used to evaluate accommodative ability of the eye and detect latent hyperopia.
Retinoscope works on a principle called Foucault's principle. Basically it indicates that the examiner should simulate the infinity to obtain the correct refractive power. Hence a power corresponding to the working distance is subtracted from the gross retinoscope value.
Static retinoscopy is performed when the patient has relaxed accommodative status viewing a distance target; dynamic retinoscopy is performed when the patient has active accommodation from viewing a near target.