Do these people need in fact the same glasses?!
Question: Here is the question: Patients with (i'll give you values that are the same on both eyes) sph-0.25 cyl-3.00, sph-0 cyl-3.50 and shp-1.75 cyl-0. Do all these people need in fact the same glasses as the equivalent sphere is the same to all of them? If so, what is the difference on the different values? Regards, thanks.
Answers: Here is the question: Patients with (i'll give you values that are the same on both eyes) sph-0.25 cyl-3.00, sph-0 cyl-3.50 and shp-1.75 cyl-0. Do all these people need in fact the same glasses as the equivalent sphere is the same to all of them? If so, what is the difference on the different values? Regards, thanks.
Hey patso :-) In your last question I alluded to the equivalent sphere but didn't explain it more because it can be a bit confusing depending on a person's background of knowledge. You seem to have a reasonable grasp on it though, so here is a bit more:
With the prescriptions you mentioned above, you are right that -1.75 sph is the eq sph for all of them. All of these people would see better with those glasses than without. but the last person would see much better than the other two, because -1.75 is their true prescription. The other two still have the astigmatism, which is at a high level and is interfering with optimal focus. Equivalent sphere is not the way glasses are prescribed in general, but as I said before, I think prescribing is as much an art as a science. Eq sph is something that can be considered though, but it is not used to the extremes that you offer.... Here's what I mean:
In the first and second cases, the true prescription may be difficult for a person to start out with if they have never had glasses before because the cyl is so high. Walls will look tilted, and they may get dizzy. I personally would lower the cyl for this person on their first pair of glasses, to maybe a -1.75 or -2.00, but then I should compensate on the sph to keep the eq sph the same. So I might give sph-0.75 cyl-2.00 to the first person. The vision will be sharper than if I had given them a -1.75 sph (simply the eq sph) because I am correcting some of the astigmatism, but still probably not quite as sharp as if I were to give them the true prescription. (But then, what is the point of crystal clear vision if wearing them makes you nauseous?) I would make a note in my chart to increase the cyl again on their next visit once they have become more accustomed to wearing it. This next increase should give them the clearest vision and it will be comfortable for them to wear.
That's a bit more than you asked here, but I hope you can apply it to the other question I talked to you in as well. :-)
only an Opthalmologist can answer your question..wait till some one responds...otherwise fix up an appointment with the Eye Doctor and get it resolved.
No, because the astigmatic component can be put into the glasses.
You are right about the "equivalent sphere" being the same in each case, -1.75DS, but the phrase does not imply that a -1.75 DS lens would deliver the same vision in each case.
It means that each of those patients would see roughly as well, or badly, on a distance letter chart. "The equivalent sphere" has a few other guideline uses, but it does not mean that the astigmatic component can be established, halved and added to the sphere, and then ignored.
In the last case, with a -1.75DS lens, the error of refraction in every meridian would be zero. In the second case, in one meridian there would be an error of correction of +1.75D, and of -1.75 in the meridian at 90 degrees to that. Terrible!
If, for some strange reason, the only lenses available were spherical lenses, then yes they would all get the same glasses. But persons one and two would not see very well with the glasses because their astigmatism remains uncorrected.