OK, there are two ideas out there about variable inset.

Let's clear any confusion and set aside variable inset by distance lens power (i.e. less inset for minus lenses, more inset for plus lenses).

The one I want to talk about is variable inset based on distance monocular p.d.

For example: a patient with a 64 distance p.d. will have a near p.d. of about 61 @ 40cm. If the orbits are symmetric, then the split p.d. will be 32/32 for distance and 30.5/30.5 for near. That means that the ideal inset for the near zone would be 1.5 mm each eyeball.

But look at this freak of nature: 64 distance p.d., but the monocular p.d.s are 34/30. The near p.d. will still be 61, so the midline would still be 30.5/30.5 monocularly. But while the left eye has to make a puny mere 0.5 mm convergence (virtually no inset required) the right eye will have to make a colossal 3.5 mm convergence (more inset required).

Pair this "unequal split p.d." issue with a high power add with a naturally more narrow zone width ergo less margin for error, and you have potential for trouble. (In fact, I used to gripe about this phenomenon, but could never understand it.)

So...how many individualized PALs will take into account this factor (and add to it the other variable inset parameter of distance lens power)??

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