By the Power of Greyskull...I Have the Effective Power!
Sorry - He-Man reference. Optics can be interesting. I just had a patient come in with a pair of glasses (Rx written by a different doctor) and their complaint was: "THIS IS SO WEIRD! I see blurry through them but if I hold them out away from my face about 3 inches...then I see PERFECTLY!! How is this possible?" Ah. We already know what's wrong: she's over-minused. WE EVEN KNOW ALMOST EXACTLY HOW OVER-MINUSED SHE IS! How do we know? She told us! Step right in and I'll explain. The "effective power" of a minus (nearsighted) lens INCREASES the CLOSER it gets to the eye. Conversely, it DECREASES the FARTHER away from the eye you take it. Incidentally this is why some people's contact lens Rx numbers are different from their glasses numbers. Patients tell me that their contact lens Rx is "weaker"...no, it's not. It's "effectively" the same Rx as your glasses...we just accounted for the lack of a "vertex distance" (distance between your eye and the lens). I always joke that patients and even other medical care professionals think refraction and optics is easy baby stuff and we're just constantly Rx'ing "stronger" glasses...but it's really not all that simple. ANYHOO...the aforementioned patient says that at 3in farther out, her glasses are perfect. When she pulls the "minus" glasses away from her face, she is DECREASING the effective power of the glasses. In English that means she's wearing "too much" and she needs "less". How much less? 3 inches is approx 76mm or 0.076 meters. Currently her glasses sit approx 14mm from her eye (vertex distance). So she's telling us that with a vertex distance of 90mm (76mm + 14mm, or 0.9m) she sees great. A diopter is a reciprocal meter, so 1/.9m is 1.1 diopter. SO! She's about a diopter over-minused. Sure enough when we refracted her we came out with 0.75 diopters less minus than she was Rx'd (why not 1.1 diopters less? Probably b/c vertexing matters MORE when an Rx is STRONGER and less when it is weaker, plus she said "about" 3inches, not "exactly" 3in). So we're Rxing her "less"...pretty much just like she told us to at the very beginning!