Hi Myo,
Otis> Thanks for your kind commentary. I respect medical people in so many ways. Except for the issue of self-prevention. Some of them know that it would be wise, but realize that most of us will not "stand for" prescribing a plus for prevention. I understand that issue and respect it. But that means, if I want prevention - I will have to do all of the measurements and judgments myself. In that sense, I am indeed biased. But I am thinking of full empowerment of the person himself, who does not want to get into intricate measurements using a Jackson-Cylinder, and complicated measurement system. (That are very time-consuming, and in my judgment, not very accurate or effective.)
Myo> BTW, although I highly respect Mr. Otis Brown I personally can't agree with his (a bit biased) recommodation for ALWAYS using the SE because it only caught me on a plateau.
Otis> I mean use a SE for your measurements of refractive status. When I say this, I am thinking that the person wants to get rid of wearing any lens for distance. But that means, at self-measured 20/50 to 20/60, he will have to get his own "reduced" lens from Zennioptical (and save arguements with an OD about what you are doing).
Otis> My feeling about mild astigmatism is this. All normal eyes have a slight amount of it. It is a totally normal situation. It is true that with ONE EYE, you can probably measure -1.0 diopters of it. But that ignores the fact that we look at objects with BOTH EYES OPENS.
Otis> The brain is a sophisticated system. The two images are over-laid, and the imperfections are cancelled out. Therfore, there is no need to, "correct" for a mild amount of it - even if you measure it. My bias is to establish (from 20/50) I can read the 20/20 line THOUGH a -1.25 diopter spherical lens. I do this for several reasons.
Otis> If I can read 20/20, then my eyes have no medical problem at all. There are too many people who believe that a lens is a medical device, for a medical problem. I deny this completely. But I do recommend you personally know how to make this check.
Otis> I can do this check with two spherical minus lenses, of -1/2 and -1 diopter. (I can create -1.5 diopter by putting the two lenses together.)
Otis> Only if I could not get 20/20 though a minus lens, would I even consider checking for astigmatism. But, yes, the desire is for simplicity, and "do measurements yourself", is part of my personal bias on this subject.
Otis> I know that each of us must choose his own path to 20/40, and then to 20/20. I only express this idea as I have 20/20, and type this with no glasses on. (But I do wear them to prevent myself from going negative again.)
Questions for the forum:
1. Does it make sense that different cylinder levels are needed for reading and distance glasses? If so, what is the physics explanation for this?
Hi warnbd,
the optical physics about that difference shouldn't be hard to understand:
When reading, as a myope you easily can approach to the "circle of least confusion" distance which is equal to a full spherical equivalent (SE) prescription.
For doing the same at far you would need to be emmetropic or hyperopic in sphere to focuse at that intermediate meridian.
BTW, although I highly respect Mr. Otis Brown I personally can't agree with his (a bit biased) recommodation for ALWAYS using the SE because it only caught me on a plateau.
After searching for emmetropization and astigmatism it showed there are several papers suggesting while there always being a direct compensation for spherical errors the response on astigmatic aberrations seems to be different.
Particulary in primate studies it was found emmetropization only picking one of the 2 astigmatic principal meridians.
Because the SE works by increasingly overcorrecting half of the meridians it raises the question what if hormesis would pick the "wrong SE' overcorrected" ones.