It explores the terminology used when fitting progressive lenses in relation to minimum fitting height and recommended minimum fitting height.
In the early days the corridor length, and subsequently the minimum fitting height of progressive lenses, were fixed by manufacturers and only offered in one length.
If a pair of progressive lenses are measured and fitted at the manufacturer's minimum fitting height (see Figure 1), the patient could be deprived of up to 50% of the actual available reading area.
Figure 2 demonstrates the principle of the recommended minimum fitting height in relation to the manufacturer's quoted minimum fitting height.
By matching the optics of the progressive lens design to the wearer's frame size, optical practitioners can take full advantage of the available lens area, down to a minimum fitting height of 13 or 14mm.
It is important to emphasise that the minimum fitting height is the measurement taken from the progressive lens fitting cross to the centre of the reading area of the lens.
In 2010, Carl Zeiss Vision produced a useful chart which looked at the numerical relationship between minimum fitting height, the corridor length to 85% of the reading power, the intermediate height, the intermediate width and minimum near width (see Table 1).
The top image shows a lens with a measured fitting height of 18mm and a corridor length equivalent to a minimum fitting height of 18mm, demonstrating that the majority of the reading zone would be removed during glazing.
Reproduced with permission from Carl Zeiss Vision Minimum fitting height 14 15 16 17 85% corridor length 10 11 12 13 Intermediate height 5 5.5 6 6.5 Intermediate width 6 6.5 7 7.5 Minimum near width 11 11.25 11.5 11.75 Minimum fitting height 18 19 20 85% corridor length 14 15 16 Intermediate height 7 7.5 8 Intermediate width 8 8.5 9 Minimum near width 12 12.25 12.5
With fixed minimum fitting heights and corridor lengths the practitioner had to decide whether to use a short or a long corridor lens.
In addition to the established Anateo design, a new short-corridor version, Anateo Mini, will be introduced, offering a minimum fitting height