adj. (in ophthalmology) describing an optical device consisting of an opaque disc punctured with a fine slit or hole (or holes), which is placed in front of the eye in the same position as glasses and enables sharper vision in cases of gross long- or short-sightedness or astigmatism. It sharpens the image formed on the retina because it confines the light reaching the eye to one or more fine beams, which pass through the centre of the lens undeviated by refractive error. The same principle is used in the pin-hole camera.
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