 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Medical Forum / General / Vision / December 2006Astigmatism + Myopia vs Astigmatism alone
Is it true that when calculating the error of an eye, it is ok to say that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to an astigmatism of -1.00? If not, which option (-075 + -0.25 or -1.00)would be considered as "better vision" of the two? Dear Pershing, An astigmatic value can be converted into a spherical-equivalent. What is done is to take 1/2 the astigmatic number and add it to the spherical value. Is it true that when calculating the error of an eye, it is ok to say that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to an astigmatism of -1.00? Otis> No. That would be like stating that -1.00 diopter spherical is the same as -1.00 astigmatism. Otis> It would be more correct to say that the spherical equivalent would be -0.75 - .25/2 or -0.87 spherical equivalent. If not, which option (-075 + -0.25 or -1.00)would be considered as "better vision" of the two? Otis> The spherical part is determined first, for instance if you read the Snellen at 20/70. A Stronger minus lens would be used to clear the "best" line possible. Once that is done, a cylender lens is used to determine if your vision can be made sharper, say to 20/15, say a -0.5 diopter lens at 80 degrees. That is how your refractive STATE is determined with a phoropter. Best, Otis > Is it true that when calculating the error of an eye, it is ok to say > that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to > an astigmatism of -1.00? If not, which option (-075 + -0.25 or > -1.00)would be considered as "better vision" of the two? CORRECTION: Is it true that when calculating the error of an eye, it is ok to say that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to an astigmatism of -1.00? A prescription is stated spherical, followed by astigmatism and angle. Thus -0.25 diopters, -0.75 at 80 degrees (a typical prescription) Spherical equivalent is -0.25 - 0.75 / 2 = - 0.62 spherical equivalent. Hope this clarifies how you convert spherical/astigmatic into spherical equivalent. Best, Otis > Dear Pershing, > [quoted text clipped - 34 lines] > > an astigmatism of -1.00? If not, which option (-075 + -0.25 or > > -1.00)would be considered as "better vision" of the two? > CORRECTION: > [quoted text clipped - 18 lines] > > Otis If someone has +0.25 myopia and -1.00 astigmatism its spherical equivalent is -0.25 right? Thanlk you for your help. That would be correct. Perhaps Mike could check your calculation. Otis > > CORRECTION: > > [quoted text clipped - 23 lines] > > Thanlk you for your help. > If someone has +0.25 myopia and -1.00 astigmatism its spherical > equivalent is -0.25 right? Yup. +0.25 - (1.00/2) = -0.25. -MT > > If someone has +0.25 myopia and -1.00 astigmatism its spherical > > equivalent is -0.25 right? [quoted text clipped - 4 lines] > > -MT Good thanks, In that case I have some more small questions. Given http://www.toronto.drdc-rddc.gc.ca/medical/visreq_e.html In section 8 there is a standard for Pilot applicants in regard to Myopia, Hyperopia and Astigmatism. I would like to make sure I am interpreting the table correctly. 1. It says that for Astigmatism "Cylinder power not more than 0.75D in either eye". If there is no sign in the number, are we talking about max being + 0.75 and - 0.75 or there is only a limit on the positive side (in which case -1.00 would be acceptable)? 2. For Myopia "Not more than - 0.25 spherical equivalent in either eye". This is a reference to the actual spherical equivalent calculation of (Cyl/2) + Sphere as described in above post or is it the spherical equivalence of (-0.25/2) + Sphere? 3. Is the scale allowing for values between 0 and 0.25? I am trying to see if (-0.75/2) + 0.175 = -0.25 spherical equivalence is a valid combination. Thanks again. well your posts do not clarify what your cycloplegic refraction is. you said "+.25 myopia" which doesn't make sense. The plus sign means hyperopia, not myopia, so the question is self-contradictory. the rule in the link you give is pretty plain: no more than .75 astigmatism (+ or -), and no spherical equivalent of myopia greater than -.25. If your astigmatism is 1.00, you're out. If it's -.75 then the sphere power must be +.12 or greater + (0 or any minus would disqualify you). If the astigmatism is +.75, then the sphere could be as much as -.62, which is the same thing, just written in different form. Those silly standards are set in stone? Too bad if you're just over. You might try again, as there is some variation from measurement to measurement. If you try again, make sure they fully cycloplege you (ask for 2 drops per eye, 5 min apart), and don't do any reading the day of the exam. As an old ex USAF optometrist, I'd far prefer a pilot who's -.50 (fails, even if 20/20) to one who's +2.50 and 10^ esophoric with even 2^ vertical deviation (passes). Unfortunately, we had to go by the book. That's one reason I didn't stay in... w.stacy, o.d. > > [quoted text clipped - 35 lines] > > > well your posts do not clarify what your cycloplegic refraction is. you > said "+.25 myopia" which doesn't make sense. The plus sign means [quoted text clipped - 19 lines] > > w.stacy, o.d. My current readings from yesterday are: 1. Right eye: +0.25 (Sphere), -0.25 (Cyl) * 150 , uncorrected vision 6/6 and no other problems 2. Left eye: 0 (Sphere), -1.00 (Cyl) * 180 , uncorrected vision 6/7.5 and no other problems I was tested on another occasion about 6 months ago and my reading was 0 (Sphere), -0.75 (Cyl) in the left eye. Both times the people who performed the tests were unaware of the standards I posted, they simply made tests. You say that a +0.12 (Sphere), -0.75(Cyl) would qualify me. Is there a lot of difference between 0 and +0.12? Could this difference be associated to measuring instruments? I mean if I try it another time could the reading be that different? My eyes have been getting some major pounding for the last 3 months (Comp Sc student here) so I little bit of brake before the exam may have some influence too. The results were not yet sent for official review so I don't know how strict they are. I don't know if for example my good right eye could compensate for the left eye being slightly worst. I prefer to not take any chances and I will try to present my Opt with the standard and see if something can be done. Thanks for your help >My current readings from yesterday are: > [quoted text clipped - 8 lines] >made tests. > Ok then the R eye is obviously fine. The left is the problem. Yesterday's numbers would disqualify on both counts (over .75 cyl and sph equiv =-.50), but the other occasion you would pass on the cyl and barely fail on the sph. equiv. >You say that a +0.12 (Sphere), -0.75(Cyl) would qualify me. Is there a >lot of difference between 0 and +0.12? Could this difference be >associated to measuring instruments? Absolutely. .12 is well within the accuracy of the instrumentation, the examiner, and your responses. >I mean if I try it another time >could the reading be that different? My eyes have been getting some >major pounding for the last 3 months (Comp Sc student here) so I little >bit of brake before the exam may have some influence too. > > Could help. >The results were not yet sent for official review so I don't know how >strict they are. I don't know if for example my good right eye could >compensate for the left eye being slightly worst. I prefer to not take >any chances and I will try to present my Opt with the standard and see >if something can be done. > The military doc may well have a bias, and I would tend to think they are a bit on the lenient side. Your doc on the other hand if anything is biased the other way, esp. if he/she also fits glasses. Sounds to me like you'll make it. Avoid as much computer work and heavy reading as you can until the official exam. w.stacy, o.d. > The military doc may well have a bias, and I would tend to think they > are a bit on the lenient side. Your doc on the other hand if anything [quoted text clipped - 3 lines] > > w.stacy, o.d. Other than taking time of books/computer, are there any "eye exercises" I can perform to optimize my chances? A quick google query returns some results such as this http://www.eyesercise.com/ ... but when it looks too good to be true it usually is. Thank you again Dear Pershing, As suggested by some eye-docs, you could consider using a simple reading glass for all close work for several weeks prior to an examination. See: www.chinamyopia.org and www.myopiafree.com Best, Otis > > The military doc may well have a bias, and I would tend to think they > > are a bit on the lenient side. Your doc on the other hand if anything [quoted text clipped - 10 lines] > > Thank you again > Dear Pershing, [snip] Otis is our resident 'Net Loon. Please ignore him and you'll be just fine. > <>. but when it looks > too good to be true it usually is. right on. the best thing you could do right now is get as much physical activity as you can. Aerobics are definitely in order, and make sure your nutrition is good. There may be no discernable effect, but as Groucho says, it couldn't hoit. > > <>. but when it looks > > too good to be true it usually is.right on. the best thing you could do right now is get as much physical > activity as you can. Aerobics are definitely in order, and make sure > your nutrition is good. There may be no discernable effect, but as > Groucho says, it couldn't hoit. Roger that. Could passing the exam early in the morning, mid day or later at night have some positive/adverse influence? Obviously no reading in all cases would apply. Also when you talk about nutrition, do you have anything specific in mind? There is the suppl/Vitamin A myth (or reality?) going around. Is there any type of food one should avoid? As usual, thanks again for your input. Pershing wrote: > Is it true that when calculating the error of an eye, it is ok to say > that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to > an astigmatism of -1.00? > Otis> It would be more correct to say that the spherical > equivalent would be -0.75 - .25/2 or -0.87 spherical equivalent. Wrong again, Otis. Thank you for playing. The correct answer is -0.62. > That is how your refractive STATE is determined with > a phoropter. Based on your extensive experience with phoroptors. -MT |
Reply to this Message |
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.