According to FAA standards, there seems to be no diopters limitation for a first class medical. I'm Japanese. JP CAC limits it. UK CAA also does. I would appreciate it if you could tell me whether my understanding is mistaken or not.
The FAA bases their eyesight limitations on total acuity and not on diopter correction strength. Although many people equate these as being the same thing, they are not.
Diopter correction is based on focal length. How well an image is focused onto the retina of your eye. An image not being focused on the retina is usually due to a distortion of the cornea. Either it is too curved or not curved enough. The image focal point can be on, prior to (cornea too curved), or after (cornea too flat) the retina.
The cornea might also not be completely spherical. More like a rugby ball and less like a soccer ball. This is called astigmatism.
Sometimes the issue could be with the lens. The stiffening (reduction of flexibility) of the lens due to aging is usually the cause of the need for bifocals and reading glasses. The lens can no longer adjust its own focal length as well as it used to. Making it harder to focus up close.
Misalignment of the focal point with the fovea due to a tilting or shifted cornea, lens, or retina is a prismatic issue. The inability to physically align the eye (lazy eye or amblyopia) can also cause this.
Acuity is based on how well the image can be distinguished clearly regardless of focal length. An image can be focused on your retina (0 diopter correction), and still not be clear or distinguishable. Eye diseases like cataracts (fogging or frosting of the lens), glaucoma (increased internal pressure of the eye pressing on the retina, causing damage), macular degeneration (damage to the fovea of the retina), etc. can degrade your acuity without affecting your focal length. No glasses of any diopter strength will improve acuity in these cases. Even eye injuries like a corneal ulcer or a scratched cornea can negatively affect the acuity of a person with normally perfect vision.
Although all of these issues affect acuity, all of them together add up to total acuity. Using only one measure can not accurately determine a pilot’s ability to see correctly. Diopter correction alone can not tell the entire story.