required for reading travel timetables. Other minimum values include: 0.4 for watching television, 0.5 for reading the newspaper and 0.7 for reading a telephone directory. Is the ability of the eye to read [...] 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 or 20 centimetres. What is long-sightedness? In the case of long-sightedness [...] emitted from a light source oscillate in waves which are scattered in all directions. A polarisation filter in a spectacle lens filters out all oscillating light waves which are not vertical, so that only vertically