In the first part of this article we discussed the main requirements for providing clear vision at different distances (distance, intermediate and near) and pointed out that not all lenses are suitable for all possible tasks. For example, a pair of bifocals will suitably correct distance vision and normal reading and writing at a desk, with the near segment fitted around or just below the centre line. A pair of 'standard' progressives fitted in the normal way (fitting cross at pupil centre) will also provide a good visual correction for the same general task. However, if the near task is not situated at the 'normal reading position' (line of vision about 30 deg to 50 deg below the horizontal) the near portion of the bifocal/progressive will not be in the right place. Tasks that cause these problems include working underneath a car and painting a landscape canvas - both require clear vision for close work above the horizontal line of vision. Normal bifocals/progressives will not be suitable. The same problem arises if the task being undertaken is not within normal working distances. Traditionally these were accepted to be: Near vision (reading/writing etc) - up to about 25 or 33cm; intermediate vision - from the near zone to about arms length or slightly beyond; and distance vision - from the intermediate zone to infinity. Taking these distances as the starting point, it was possible to provide suitable corrections for each of the three zones.
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