Measured Powers

[RGC_man]

Sorry if this has been done before, but even Essilor are doing it on certain varifocals now. What is the justification for supplying a slightly (or not so slightly) different sphere, cyl, and axis to the one you ordered? Zeiss once told me it is because they calculate the power the optician really wanted in the eye test, to which I replied how do they know the optician hasn't already done that calculation? I can't help thinking someone takes the finished lens, takes a measurement, then calls it the measured power. Please enlighten me.

[Darryl Meister]

Zeiss once told me it is because they calculate the power the optician really wanted in the eye test, to which I replied how do they know the optician hasn't already done that calculation?

You might need to provide some context for this question. If you are referring to an "ophthalmic" optician who has done an ocular refraction with a trial frame in the "position of wear," you could probably either note that the lenses don't require compensation or specify zero tilt and vertex change.

However, keep in mind that you might be better off not specifying the prescription in its as-worn position, since good software can account for additional factors -- including the variation in power due to lens thickness and the oblique performance of the near zone.

If you are referring to a "dispensing" optician, very, very few of them perform these compensations. They would generally have to do this compensation mathematically, since performing an ocular refraction would not be an option, and the calculations are quite involved for anything but the simplest prescriptions.