Thanks for that information. I've always wondered what the term diopter meant.

Question:

*Is the definition of diopter (inverse meter) the same for use with units other than meters?* One of my old manual focus lenses has the distance scale in feet. I'm just wondering.

Good point! The answer is no; the convention is that a **diopter** is specifically an inverse meter:

di·op·ter

/dīˈäptər/

noun

a unit of refractive *power *that is equal to the reciprocal of the focal length (in meters) of a given lens.

The ~*phi*~ that I referred to can be in whatever (inverse) linear units are being used in the analysis, though. So in the Geometrical Optics courses (Doug Sinclair, Rudolf Kingslake, Daniel Malacara, etc.) we would always just write out the lower case Greek letter phi --- which is extremely useful in all that math stuff. That way, the units become fungible.

I fondly recall one of the problems on our sweatin' bullets Optics MS ~comprehensive exam~ U of Rochester, back in the 1970s. Involved looking at a piece of graph paper through a plano-convex lens plopped flat side down on a piece of graph paper. The puzzle was quite unusual, because parametrics got coupled strangely: diameter, radius thickness and index got coupled through the "crossed chords theorem from high school algebra" --- who would have imagined Kingslake torturing us with that curve ball! And then, to express the answer in **diopters!**

PS: What I really liked about Rudolf: For the grad students, he would toss problems at us that were *not* specifically covered in his courses. That way, learning all the taught stuff was *not* sufficient to ace the course. You absolutely had to be studying beyond the assigned materials and sources, else have not a prayer of scoring an A. Maybe a B, but not an A. So, only a few students managed to earn A. That was the traditional British Tradition. He intended that high grades would mean ~going beyond~ I loved that; most students hated that.

**Edited by TOMDEY, 29 May 2019 - 09:52 PM.**