C53436: Progressive power lenses - part 2

The iso-astigmatism and iso-mean power plots of typical progressive power lenses represent the effects of Minkwitz astigmatism and the method chosen by the manufacturer to blend the distance, intermediate and near zones together. They can be looked upon as ‘fingerprints’ which are helpful in explaining the main design characteristics of a progressive lens. In particular, their positions immediately inform upon whether the design is hard or soft as depicted in figure 1.

Figure 1: Minkwitz astigmatism for modern progressive designs

These plots are frequently given to identify the characteristics of the various designs. However, it should be borne in mind that they often represent the characteristics of a single surface, normally chosen for the prescription, plano, add +2.00 D, in which case the DP curve is in the region of 5.50 to 6.50 D (figure 2) and the back surface, or prescription surface, is assumed to be spherical.