Maybe I'm overthinking this one, but here goes!

So I was having a pleasant catch-up with a gentleman who works for Hoya, and we ended up discussing that golden oldie, the Hoya ID. Specifically, how the add is achieved by plus-cyl at right angles to each other between both surfaces (in effect, a customized cross cylinder), so the front near portion is convex in one meridian, and rbe back is convex along the perpendicular meridian.

So far so good, that part wasn't a problem for us.

Then we started wondering... for full back surface PALs, the front base curve must be sufficiently convex so the back remains concave. Every case where a flatter blank has been used on such designs (leading to a biconvex near portion), has led to complaints and an eventual rejection of the lens in favour of a move convex front with the concave back. This is logical due to ocular rotation etc, of course.

Is it as simple as combined plus cylinders between both surfaces behaving very differently than a lens that is biconvex in all meridians?

Granted, we both have dispensed many pairs of ID, the patients generally love them as much as we don't let our fitting slip up... so it's more an academic curiosity if nothing else.

So I was having a pleasant catch-up with a gentleman who works for Hoya, and we ended up discussing that golden oldie, the Hoya ID. Specifically, how the add is achieved by plus-cyl at right angles to each other between both surfaces (in effect, a customized cross cylinder), so the front near portion is convex in one meridian, and rbe back is convex along the perpendicular meridian.

So far so good, that part wasn't a problem for us.

Then we started wondering... for full back surface PALs, the front base curve must be sufficiently convex so the back remains concave. Every case where a flatter blank has been used on such designs (leading to a biconvex near portion), has led to complaints and an eventual rejection of the lens in favour of a move convex front with the concave back. This is logical due to ocular rotation etc, of course.

Is it as simple as combined plus cylinders between both surfaces behaving very differently than a lens that is biconvex in all meridians?

Granted, we both have dispensed many pairs of ID, the patients generally love them as much as we don't let our fitting slip up... so it's more an academic curiosity if nothing else.

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