4-hour charging period. What is a dioptre? A dioptre (dpt) is the unit of measurement of the optical power of a lens. In the case of spectacle lenses, negative dioptre values correct short sightedness, [...] positive values correct long-sightedness. The dioptre is equal to the reciprocal value of the focal length of a lens measured in metres: a power of +5 dpt therefore has a reciprocal length of 1/5 metres