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 or 20 centimetres. What is long-sightedness? In the case of long-sightedness, [...] technical reasons, it cannot be used during the 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