Instrument myopia--microscopy.

Looking through optical instruments stimulates the eye to accommodate more than would be necessary for naked eye viewing; this increased near power is called instrument myopia. The amount (up to -5 diopters) varies with the observer and the conditions of viewing. Using microscopes produces -1.5 to -2 diopters of instrument myopia. More instrument myopia is found with low-power magnification, inexperienced users, and when focusing form minus blur to sharp image. Physiologically, the instrument myopia appears to be associated with an accommodative equilibrium position and, psychologically, with a sensing of proximity.