Originally Posted by
G. F. Peck
Thanks for that AWTECH,
Are you saying that regardless of whether we use a front surface design with 18mm minimum height or a back surface design with 18mm minimum height, like the one illustrated, both designs would work equally well??
In both cases the corridors would be 13mm and we would expect the reading add to come in at 18mm. For the same degree of occular rotation, with the front surface design there would be a direct line of sight through the reading add whereas with the back surface design the direct line of sight is restricted to a point 15.5mm down the progression - in other words the reading add is not reached.
Maybe I'm missing the point and perhaps my argument is too simplistic but if that's the case then why do we get problems with back surface designs that are usually cured simply by changing to a front surface PAL.
Ultimately, we are faced with a barrage of new designs, all promising to be better than the rest. In the interests of progress (and financial reward) we try these new 'wonder' lenses and if they work - fine! If they don't work, we either never use them again or we try to explain where the problems may be. That's all I'm trying to do.
Graham Peck
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