Author Topic: Mismatch between spherical equivalent of prescription & crossover of astigmatism  (Read 3078 times)

Offline johnlink

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For my right (astigmatic) eye, at a distance of about 7 inches (5.50 diopters) the astigmatism disappears. At the distance of 7 inches the image is quite sharp. From reading http://www.nfos.org/degree/opt11/module_09b.html I now understand the crossover distance to be the average between the focal distances (measured in diopters) along my axis and 90 degrees from my axis, i.e. the spherical equivalent. Thus we have the following equation describing my right eye:

SPH + CYL/2 = 5.50

My new prescription for my right eye is -2.50 -2.25 010. If we assume that -2.50 accurately corrects the spherical component of my eye then we can solve for the cylindrical component:

2.5 + CYL/2 = 5.5
CYL = 3*(5.5-2.5) = 6.0

Now the CYL in my prescription is -2.25, which is substantially different from -6.0 suggested by the calculation above. At my exam on May 13 the intern told me that I was under-corrected for astigmatism but I don’t think she said it was 6.0. I think she said 3-something or maybe 4-something. I will find out.

Let’s calculate the spherical equivalent of my new prescription:

-2.50 + -2.25/2 = -3.625

Now I’m puzzled. My prescription gives me 20/20 but its spherical equivalent is not close to the negative of the spherical equivalent of my eye. My eye has a crossover at about 7 inches while the spherical component of my prescription leads me to expect it to be at almost 11 inches (3.625 D). Can anyone help me make sense of this?
« Last Edit: May 24, 2013, 02:53:11 AM by johnlink »

Offline johnlink

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I am pleased the ODs at the clinic are helping you with this project (experiment).

Otis, you are much too kind. Or maybe you're being facetious. I had to be adamant (that word appears in the OD's report) with the ODs in order to get the slightly-reduced prescription that brings me to 20/20 instead of 20/15.

Offline OtisBrown

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Hi John,
I am very seldom "facetious", or for that matter sarcastic.  I take everyone as a sincere person - even if I must disagree with some of the concepts expressed by SOME majority-opinion optometrists.  (It was the second-opinion optometrists Raphaeson that truly FORCED me to "change my mind" about these tough issues.)  I am pleased the OD said ADAMANT - which is expressing extreme bias on his part.

This sort of "attitude" is not helpful.  If *I* am reading my Snellen at 20/40, and have passed the DMV test (or very close), believe me, I would not be wearing a -2 diopter contact if it gave me 20/15 vision.  In fact, for the DMV, I would get a -1.0 diopter, which would give me 20/20, but would wear it ONLY IN THE CAR.  For tennis, biking and all other activity - no minus lens.

I know others reading this - and confirming close-to 20/40, will have to make their choice about this "personal control" issue.  What truly amazes me is that guy with 20/200, who managed to get to the 20/50 to 20/70 level.  John,  I hope you reach that visual acuity level also - and I am not being facetious about that statement.
Otis



I am pleased the ODs at the clinic are helping you with this project (experiment).

Otis, you are much too kind. Or maybe you're being facetious. I had to be adamant (that word appears in the OD's report) with the ODs in order to get the slightly-reduced prescription that brings me to 20/20 instead of 20/15.

Offline johnlink

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Since a contact can not be "fixed" on your eye, but rotates around as you blink - it is (almost) impossible to prescribe a contact lens for "astigmatism".  In my personal judgment, all they can do is prescribe a "spherical equivalent", and hope you don't notice.

There are some contact lenses that are "weighted", so presumably you can keep the contact in on orientation - but these are  rare.  Do you have that type of contact.  If not, you are wearing a "spherical equivalent" - as my guess.

Your guess is incorrect. I wear a Biofinity Toric lens on my right eye. I presume it is weighted. It has a short line on it that radiates from the center and that line must be at the bottom of my eye.

https://www.1800contacts.com/lens/Biofinity-Toric?pn=biofinity%20toric