A. Excessive cupping of the optic nerve is a possible sign of optic nerve atrophy. Glaucoma (with or without high eye pressure) is one possible cause of atrophy. There is a certain amount of normal variation in the size of the optic nerve cup from person to person and eye to eye. The AF standards are cup size of no more than 0.4 and a difference between the two eyes of no more than 0.2. It sounds like you are right on the border with your measurements.
The measurement can be done in ways to minimize the subjectivity of the result. What I can’t give you a good answer for is if your measurement exceeds 0.4 but all other findings such as field of vision, eye pressure, etc, are normal, what your chances of getting a waiver on the cup size are.
Q2. I am currently a cadet in ROTC trying to get a pilot slot on the next board. Only one problem: My vision is HORRIBLE! 20/200 in both eyes. I meet all of the pre-op requirements for PRK, but how difficult is it to get a waiver for this? No one at my Detachment can give me any info. Is there somthing I can do to start the process now? A website I can visit?
A. The AF website on this subject has been down for a couple of months. I can see by your e mail you’re at LSU. I think your medical support is through Keesler. I recommend having your Det call the Keesler Flight Surgeon’s Office to get a hard copy of the waiver guidance. (It was a USAF/SG policy letter with attachment). If you meet the preop criteria, it takes a year minimum from procedure to start of UPT.
A. I’m Active Duty and just had PRK at WHMC in San Antonio. I’m 8 weeks post op and doing well. The active duty waiver is a laborious process. Check out the PRK website under special flying programs on the afpc web site. This has specific info on the post op requirements for waiver approval. Also there is a limit to the number of applicants that can be accepted who have had PRK. This number is limited to 10% across the board! To me it’s worth the risk, but carefully consider the benefits vs. the side effects.
Check out the Official USAF PRK Website (link updated 5Aug07).
Q3. I’m Active Duty and had Air Force approved PRK at WHMC. I’m pursuing pilot training and need to complete my Flying Class I physical. The physical is complete with the exception of the updated vision portion. I read the PRK policy letter thoroughly for FCI waivers and am a little confused by all the different time restrictions mentioned. The way I interpret the policy is that three months post PRK I can apply for the waiver assuming I have no other complications pertaining to the surgery. I would appreciate your interpretation or the AETC/SG interpretation of this policy.
A. Here’s my understanding of the rules for those who have had PRK and are applying for pilot training: You may begin the waiver process no earlier than three months after having PRK done (this includes a full examination including your “new” visual acuity, with and without corrective lenses). You’ll need a second visual examination at least three months after the first post PRK evaluation (i.e., at or after six months post PRK). You’ll need a full ACS PRK evaluation in any case thereafter. You will be able to begin UPT no sooner than twelve months post PRK.
Q4. A few years ago I took an Army flight physical; failing the eye refraction test. Shortly after I went to a civilian doc with my physical paperwork and was told that the Army doctor had made a mistake in his calculations. During this period my window to branch aviation had passed. Now I have the ability to branch transfer to aviation. I was wondering what refraction is? Could a mistake be made in diagnosing it? How is it tested for? Does refraction change under certain conditions ie dehydration, fatigue, time of day, age? Can it be fixed? Can a waiver be obtained for it?
I am planning on scheduling an appiontment with another Army doctor. I would just like a little education on the topic. Thanks for the help.
A. Refractive error is expressed in diopters, sphere (correction for far or nearsightedness) and cylinder (correction for astigmatism).
Q: I was wondering what refraction is?
A: As used here, it’s the refractive error, if any. Put another way, how many diopters of correction do you need to meet standards?
Q: Could a mistake be made in diagnosing it?
A: Yes. In addition to clerical/math errors, there is some subjectivity in the test, but mostly on your part. (Which looks clearer, A or B?)
Q: How is it tested for?
A: There are several ways. One is for the optometrist/ophthalmologist to focus on your retina and see what lens setting is needed to make it clear. Another is to put lenses in front of your eye and have you see which makes your eye focus better. For flying class I, this is done with the lens unable to move (so it can’t play). This is done by putting in eyedrops that temporarily stop the muscle that changes the lens shape from working.
Q: Does refraction change under certain conditions ie dehydration, fatigue, time of day, age?
A: It should not change except slowly with age. If done with the lens able to change shape, fatigue of the muscle that changes lens shape can be a factor. (If you have out of control diabetes, your vision can change with blood sugar level.) As you age, the lens becomes less elastic and less able to change for near versus far vision. This is why people start needing reading glasses in their 40s.
Q: Can it be fixed?
A: Yes, that’s what RK, PRK, LASIK, LASEK, etc do. Remember, only PRK is approved (under very limited conditions) for flyers!!!
Q: Can a waiver be obtained for it?
A: No, but you can apply for an exception to policy (not commonly granted).
Extract (with translation to nonmedical terms) from AFI 48-123 Table A7.1. VISION & REFRACTIVE ERROR STANDARDS.
Sphere: +2.00/-1.50 (Far/nearsightedness)
Cylinder: 1.50 (Astigmatism)
Anisometropia: 2.00 (Refraction difference between eyes)
Q5. I was curious about refractive errors. I know that you need to have 20/20 uncorrected reading vision so I was wondering where the +2.00 and -1.5 come from. If you have a plus correction I thought that meant that you couldn’t read 20/20 and minus was for distant vision but I seem to be confused by these numbers. Thanks for your help.
A. The question had to do with the plus/minus numbers in the refractive error standards. Short answer, in the case of the standards for FCI, the allowed correction (plus or minus) applies to distant vision because you can meet standards while requiring correction for distant, but not near, vision. For other classes, it is the maximum you need, whether to correct near or distant visual acuity.
The numbers are the strength of the lenses used to correct your vision in diopters. The plus or minus does not have to do with near or distant vision. The plus indicates a convex lens equivalent and the minus a concave lens. The easy way to think of the sign of the number of the lens strength is whether it is for correcting “nearsightedness” or “farsightedness.” This is not to be confused with near visual acuity (tested at 16″) and distant visual acuity (tested at 20 feet).
The numbers referred to as sphere are lenses that correct equally in every visual plane (up/down, right/left, diagonally). Cylindrical lenses correct along a given axis and are used for astigmatism. This is when the person’s lens has different strengths in different visual planes. Without correction, an “X” or ~ez_ldquo+ez_rdquo~ would have one limb blurred and one in focus.
In a simple prescription, you have one (spherical) lens strength per eye. Add astigmatism and you get a second lens strength (cylindrical) as well as the axis along which it is oriented. Add bifocals, you get yet another spherical lens strength for the reading portion.
In the case of FCI standards, the correction is only applicable for distant vision (near vision must be 20/20, uncorrected).
Q6. During my FC1 my vision was 20/50 but my refractive error was -1.75. I was wondering first if this will DQ me and second if waivers are ever granted for refractive error if you meet acuity standards but not refractive standards and if they are how hard is it to get an ETP.
A. Refractive error is out of standards, but only by 0.25 diopters. This is DQ, however. It would require an exception to policy and not a waiver. It is really hard to predict ETP likelihood, but with it so close and the acuity within standards, you might get lucky.
Q7. I took a Flight Class I Physical a month ago and I was just informed that my “accommodation” was out of standards and I would need to go back and have the test done again. Can you tell me what this means and what test was used to determine this?
A. The accommodation test measures how close to your eye you can focus. As you age, the lens becomes less flexible and the minimum distance you must have something from your eye to see it in focus increases. (This is why kids can hold an object really close to their eyes to get a magnified view and if an adult tries this, it only looks blurry).
The test involves a card that is moved slowly away from your eye along a ruler. Each eye is tested separately. The result is determined by the distance at which you can read type that is 1mm high. The passing score is a function of age (as you get older, the minimum distance at which you can focus tends to increase). The results are expressed in diopters, but the easiest way to look at it is the minimum distance at which your lens can bring an object into focus.
Q8. On the Accommodation standards for Flying Class I, is being less than or more than the diopters for your age disqualifying? Example: The diopters for a 25 year old person is 6.9. Do you have to have less or more than that.
A. You want to have that much or more accommodative power. Having less than required for age is disqualifying. In this measurement, bigger is better. It represents the eye being able to focus even at very short distances. Or another way of looking at it is you are closer to needing reading glasses than you should be at your age if your accommodation power is less than standard. The way to think of accommodation is you can get a “bigger” view of an object by holding it closer to your eyes. This works until you get it so close that you can no longer get it in focus. When you are young, your lens is very pliable and can change shape to be a fairly strong magnifying lens. Kids can get a magnified view of an object by putting it a few inches in front of their eye. They can still focus on it and it does not look blurry. It would look big but blurred to an adult. As you get into your 40s, you will probably need glasses (or longer arms) to read. And note how low the accommodative power standard is at 45.
Q9. I am a Junior in ROTC. I go for my AF Flying Physical on the 24th and i have a question regarding my eyes. I have deep Optic Cups, so I have been asked many times if there is a history of glaucoma in my family (NO), and i had a visual periphery check and pressure test and all that, which all came back normal. So what can I expect the Flight Surgeon to say about this?
A. There are standards for the width of the optic cup (an indentation in the optic nerve head as seen looking in the eye). There are no standards for the depth. As long as the width is within standards and there are no other abnormalities, I think you should be OK.
Q10. In the AFI148-123 14Nov2000 it states that for UPT flying class I one must have 20/70 Uncorrected distance vision and no more than -1.5 refraction. As I understand it one must meet Class I standards to get into UPT. Currently my right eye needs -2.75 and left -2.5 correction. As far as the 20/70 line I can get about 50% of the letters right. On the civilian side I have about 800hrs flight time along with my single/multi engine commercial instrument ratings along with CFI, CFII, and MEI, which means I can teach others to fly. Looking at class II requirements the max refraction is -4.0 and 20/400. I definitely meet class II requirements. My question is it possible/likely that I might get some sort of waver to the class I requirements to gain eligibility to UPT? I should also mention that I have 5 years in the Army reserves at E-5 and am currently 24 years old.
A. I hate to have to be the bearer of bad news, but this is outside the medical waiver process. You do not meet standards for FCI and need to apply for an exception to policy, not a medical waiver. The approval authority for exception to policy for vision and refraction standards for UFT is the Chief of Staff of the Air Force. (AFI 48-123 A7.7, Note 10).
Q11. I recently took my Flying Physical for UPT and I can’t pass the depth perception test (even with correction to 20/20). I have been told that I don’t have “perfect” 20/20 vision, it is a weak one and just barely get it. I have read the AFI 48-123 on stereopsis and some of it is greek to me? Can I still go to pilot training? The waiver process sounds really intimidating. Do you know of anyone that has received this waiver before? Or anybody in Microtropia Study/ Management Group? Thank you for your time…greatly appreciated.
A. The bottom line (FCI, FCIA, FCII): You need to either pass the depth perception test or get a waiver for substandard depth perception if you don’t. A brief simplified explanation of the depth perception test:
Depth perception screening tests the ability to tell which object is closer/further by the angle between your eyes. For this to work, both eyes must simultaneously focus on the object (visual fusion). In some people, one eye is sufficiently “dominant” that the information from the other eye cannot be used simultaneously. You can see OK with both eyes (tested individually), but when using both eyes together the “weak” eye gets semi-ignored so you don’t get the angular information processed by the brain to tell depth. In other people, the brain switches rapidly from one eye to the other and never actually sees with both at precisely the same time. In either case, depth perception may be substandard.
If you fail the screening depth perception tests (including testing with corrective lenses), you need an extensive evaluation of your depth perception using a battery of tests. (See A7.11.2 below).
The tests can show
Normal depth perception
Substandard depth perception (of various degrees)
Lack of depth perception
If you have normal depth perception by the more extensive testing, you’re pretty much good to go (with extra paperwork, of course). If you have substandard depth perception of a mild degree, you are a reasonable waiver candidate. If you have absent depth perception or substandard depth perception worse than mild, you are not likely to get a waiver. I do know people who failed the screening depth perception test and went on to be approved for FCI. Here’s the waiver guide info:
I. Overview. Microtropia and monofixation syndrome (MFS) represent defective forms of binocular vision in which there is preservation of peripheral extramacular fusion but the absence of central macular fusion and fine stereopsis. This results from subtle misalignment of the eyes (microstrabismus), but can also occur in some individuals whose eyes are straight. Patients with this syndrome have the inability to use both foveas simultaneously (bifixation) and must resort to fixating with one eye at a time (monofixation). Failure to have simultaneous bifoveal fusion always results in degraded development of normal stereopsis.
Diagnosis is based on the presence of a facultative macular scotoma, a stereopsis deficit (though it may be mild), and a tropia of less than or equal to 8 prism diopters of deviation. Such tropias can be intermittent and some may degrade under the rigors of the flight environment and fatigue. It may be present, either with good visual acuity in the deviated eye, or amblyopia. Near stereopsis tests should never be used alone to qualify any aircrew, since many microstrabismics may have defective distance stereopsis but normal near stereopsis and vice versa. However, distance stereopsis is the main aeromedical concern.
There is usually no indicated treatment for this diagnosis.
II. Aeromedical Concerns. The functional aeromedical impact of this condition concerns the associated constant or intermittent disruption of stereopsis. A thorough evaluation must be performed due to the prevalence of associated defective stereopsis, anisometropia, macular scotoma, and amblyopia as well as to establish the etiology and rule out correctable causes such as uncorrected refractive errors. By definition, the degree of microtropia is equal to or less than 8 prism diopters of tropia. Larger deviations are called small angle strabismus and usually have more significant performance decrements.
III. Information Required For Waiver Submission. During initial and annual flying physicals, stereopsis testing on the VTA-DP or its newer replacement, the Optec 2300 (OVT-DP) is required. A near stereopsis test is never a substitute for assessment at distance. A local ophthalmological evaluation can help delineate the specific diagnosis. Experiences at AETC and the ACS have shown that the majority of these cases are not adequately worked up in the field to address the aeromedical concerns. Already trained aircrew subsequently identified to have decreased stereopsis, microtropia, or MFS will need an ACS evaluation. The ACS has considerable expertise in evaluating and diagnosing this condition as well as managing and occupationally assessing a given aircrew member with respect to performance in the military aviation environment. An active USAF/SG Study Group of trained aircrew with these conditions is maintained by the ACS, and a new Defective Stereopsis Study Group for mild defective stereopsis identified in UPT applicants has also been established in order to prospectively evaluate aircrew performance-based outcomes associated with mild stereopsis deficits.
IV. Waiver Considerations. An average of 15 aviators each year are seen at the ACS with a diagnosis of microtropia, over half of which are being evaluated primarily for another diagnosis. Another 8-10 per year are being identified at Enhanced Flight Screening-Medical (EFS-M) medical screening. Over 90% of all evaluees with microtropia have been given waivers based on retrospective analysis. No prospective studies have been done to date to evaluate the impact of mildly defective stereopsis as a cause of UPT attrition or overall airmanship. Aircrew with these problems typically present by failing the depth perception test and usually have a history of this on careful review of the record. Consequently, a new mildly defective stereopsis study group for UPT students has been established to determine if fine stereopsis requirements can be safely modified in future standards.
Duane, TD, Jaegar, EA. Clinical Ophthalmology. Philadelphia: Harper & Row, 1993;3:14.1-14.12.
Clarke, WN, Noel, LP. Stereoacuity testing in the monofixation syndrome. Journal of Pediatric Ophthalmology and Strabismus. May-Jun 1990;27(3):161-3.
Hahn, E, Cadera, W, Orton, RB. Factors associated with binocular single vision in microtropia/monofixation syndrome. Canadian Journal of Ophthalmology. Feb 1991;26(1):12-7.
A7.11. Depth Perception/Stereopsis.
A7.11.1. Flying Class III (other than Inflight Refuelers and individuals required to perform scanner duties). No standard.
A7.11.2. Flying Class I, IA, II-Flight Surgeon Applicant and III-Inflight Refueler Applicants and individuals required to perform scanner duties. Failure of the Vision Test Apparatus (VTA-DP) or its newer replacement, the Optec Vision Tester (OVT), screening depth perception test with uncorrected refractive errors should be retested with refraction correction in place, regardless of level of unaided visual acuity. Failure even with correction is disqualifying, but may be considered for waiver consid-eration by higher waiver authorities, only after completion of a full evaluation by an ophthalmologist or optometrist, to include all of the following: ductions, versions, cover test and alternate cover test in primary and 6 cardinal positions of gaze, AO Vectograph Stereopsis Test at 6 meters (4 line version), AO Suppression Test at 6 meters, Randot or Titmus Stereopsis Test, Red Lens Test, and 4 Diopter Base out Prism Test at 6 meters. These tests are designed to identify and characterize motility/alignment disorders, especially microtropias and monofixation syndrome. The results of these tests done locally are considered to be preliminary, but will be used by waiver authorities to determine whether a candidate should be permanently disqualified without any waiver consideration, to identify if there are potentially correctable causes, and to determine whether further evaluation is required. NOTE: A prospective Undergraduate Flying Training (UFT) Microtropia Study/Management Group is established at the ACS with minimally defective stereopsis secondary to monofixation syndrome or microstrabismus that are considered appropriate for waiver consideration. Potential Study Group members must meet the criteria established by the ACS to be eligible for this Study/Management Group. All potential candidates must be evaluated at the ACS Ophthalmology Branch if recommended and approved by HQ AETC/SGPS. AETC/SGPS is the waiver authority.
A7.11.3. Flying Class II and III-Inflight Refuelers and individuals required to perform scanner duties. A new failure of the VTA-DP or OVT requires evaluation by an ophthalmologist or optometrist to determine the cause of the failure and to rule out correctable causes, i.e., refractive error and ani- ometropia. If any new failure still is unable to pass the VTA or OVT with proper optical correction, then all of the motility tests listed above under Flying Class I in A7.11. must be accomplished as a pre-requisite for any further waiver consideration.
A7.11.4. If the aviator has previously failed the VTA or OVT, and has previously been evaluated, and has either, normal motility or a stable previously known waivered motility disorder, and can pass another stereopsis test, such as the Verhoeff, Titmus, Randot, or Howard Dolman, no further work-up or waiver is required. However, such cases should already have been granted an initial waiver for this consideration. If not, a waiver is required. NOTE: If the local flight surgeon feels that the degree of depth perception may not be compatible with the present aircraft or duties of assignment, further work-up and waiver will be required. Consultation at the ACS is indicated for any rated aircrew member with defective, questionable or change in stereopsis or depth perception or a significant change in the level of stereopsis performance.
Q12. I failed the OVT depth perception test, however just recently took an alternative test, the Verhoeff test, at a local Navy Base for my UPT physical. Is this good enough? Or do I need to still be evaluated? I hope I’m in the clear on this issue and can get my physical off to the National Guard Bureau asap. Thank you once again for your time and your interpretation of the Regs.
A. The bad news: You still need a bunch of eye tests.
The good news: If you pass, you should be OK.
The quote from the reg (note: you need ALL the tests mentioned performed, except they may do either the Randot or Titmus stereopsis test). Failure of the Vision Test Apparatus (VTA-DP) or its newer replacement, the Optec Vision Tester (OVT), screening depth perception test with uncorrected refractive errors should be retested with refraction correction in place, regardless of level of unaided visual acuity. Failure even with correction is disqualifying, but may be considered for waiver consideration by higher waiver authorities, only after completion of a full evaluation by an ophthalmologist or optometrist, to include all of the following: ductions, versions, cover test and alternate cover test in primary and 6 cardinal positions of gaze, AO Vectograph Stereopsis Test at 6 meters (4 line version), AO Suppression Test at 6 meters, Randot or Titmus Stereopsis Test, Red Lens Test, and 4 Diopter Base out Prism Test at 6 meters. These tests are designed to identify and characterize motility/alignment disorders, especially microtropias and monofixation syndrome. The results of these tests done locally are considered to be preliminary, but will be used by waiver authorities to determine whether a candidate should be permanently disqualified without any waiver consideration, to identify if there are potentially correctable causes, and to determine whether further evaluation is required. Good luck.
Q13. I am sweating the color vision thing again and had a couple more questions:
- Any chance of getting a waiver from the Air Force, or an exemption from the requirement to pass the color vision (plates) exam (i.e. already commissioned, status as a naval aviator, can pass the FALANT, etc.?)
A: The AF has become very reluctant to waive substandard color vision. The AFI specifies that the AF no longer accepts the FALANT. The fact that you’re commissioned won’t help, but the fact that you demonstrated success as a naval aviator may help. What will happen in your case – only time will tell.
- I assume the reserves fall under the same standards as the rest of the USAF. Do you think I have any chance of heading to UPT if I can’t pass anything other than the FALANT?
A: FCI/FCIA/FCII are the same standards AD, ANG, and AFRC. You’ll need a waiver for UPT if you can’t pass the pseudoisochromatic plates.
- Any suggestion of who I should contact if I get medically disqualified?
A: Request a waiver. This involves extensive color vision testing and submission of the results with an aeromedical summary.
- Are Navy flight surgeons able to give the Flying Class I/IA exams…. or do I really have to have another long form exam done (latest longform/ekg/eye exam was 2 February 2002).
A: Yes, USN flight surgeons can do FCIs that the USAF will accept. If you had your “long” flying exam in your USN capacity, this is probably the FCII (remain qualified as pilot/nav) exam, not the FCI (become a pilot). The FCII leaves a few tests out, so you may have to add to the exam already done or do the whole thing over.
Q14. I am in the USAF, flying for many years with glasses. Recently diagnosed with Keratoconus. I can still see 20/20 with glasses, but the next physical could be a problem. I have researched this problem, and I think I now know more than my eye doc on base about it. I looked into it on the web, and the Brooks AFB site says there are 85 of us (AF pilots) out there that wear RGP’s to correct their vision. I have a feeling my doc has no idea about this, or how to treat it, and I have just gone with the flow, as he scratches his head in confusion over my eyes. Any advice? Ever hear of anyone out there with my prob? I still see very well with my glasses and am very confident when flying, but I have tried on RGPs, and my vision is far superior.
A. First, the (probably) good news. In many people with keratoconus, the disease progresses slowly and you can probably continue to fly (with a waiver) as long as you can be corrected to standard. Yes, I’ve seen people with this condition. The fact that you can be corrected with just glasses is a helpful sign, but there are other considerations.
Now the bad news: The diagnosis alone (even if mild) is disqualifying and you will need a waiver. RGP contact lenses are not authorized in the “usual” contact lens program. They may be authorized for a person with keratoconus, if that’s what it takes.
If you have been no-kidding diagnosed with keratoconus, you should have been grounded and plugged into the waiver process already from how I see it. If you have a “topical pattern suggestive of keratoconus” as opposed to actual keratoconus, this makes a difference. (It may or may not ever progress to actual keratoconus but still needs to be followed).
Keratoconus is a disease of the cornea that results in it weakening and changing from a shape that is like a tangential cut from a sphere (to a first approximation) to a more protuberant cone shape (hence the name). In severe cases it can be so pronounced as to be visible when looking at the cornea from the side. It is thought to have genetic predisposition, but may affect the one eye and not the other or to different degrees. The amount of progression is variable. It might not progress at all (reasonable waiver candidate). In slowly progressive cases, you may just need a new prescription every couple of years (also reasonable waiver candidate). In more quickly progressive cases, you may need a new prescription every few months (this is a circumstance in which you would almost certainly not get a waiver approved). In severe cases, you’re looking at a cornea transplant (which would not be waived).
Q15. On the FCI physical, how does your normal eyeglass prescription (sphere/cylinder) differ from what goes on SF 88 under cycloplegic refraction. I notice it reads BY___ S.___ Cx___. What exactly are these values? Also, I had heard that the opthalmologist is supposed to record the prescripiton in positive cylinder. Could you explain this?
- Q: On the IFC1 physical, how does your normal eyeglass prescription (sphere/cylinder) differ from what goes on SF 88 under cycloplegic refraction.
A: First of all, on any given day, if you have your eyes checked, you might get slightly different results. Secondly, when your lens can’t play (the muscle that changes its shape is temporarily not active due to the eyedrops they put in for a FCI or FCIA), you might get a different result. Thirdly, there are two ways of expressing the correction for astigmatism and your glasses prescription uses one convention and the exam uses the other.
Here’s what “BY___ S.___ Cx___.” means:
The S is sphere (near or farsightedness) correction, the Cx is cylinder (astigmatism) correction with the axis of the cylinder in degrees.
By S. Cx
+0.25 +0.25 090
In the example, the individual had the best vision in this eye with a + 0.25 sphere correction and a +0.25 cylinder along an axis of 90 degrees (which could also be expressed as +0.50 -0.25 0 degrees).
- Q: Also, I had heard that the ophthalmologist is supposed to record the prescription in positive cylinder. Could you explain this.
A: If you have astigmatism, there will be two correction numbers for lens strength and an axis for the second number.
– Sphere (S), for a spherical lens equivalent, which is equal correction in all axes.
– Cylinder (Cx), for a cylindrical lens equivalent, which corrects along one axis and has no strength at 90 degrees to that axis.
The Cx correction can be expressed as added to or subtracted from the S correction. These are two different ways of expressing the same information. Traditionally glasses are ordered in minus sphere and the exams are reported in in plus sphere. Here’s an explanation:
6.1. Transposition and Ordering Spectacles:
6.1.1. Transposition: Transposition is the process of changing a spectacle prescription from minus to
plus cylinder, or the reverse.
6.1.2. The rules of transposing are:
22.214.171.124. Add the sphere “power” to the cylinder power:
126.96.36.199.1. If the signs are the same, add the two powers.
188.8.131.52.2. If the signs are different, subtract the smaller number from the larger and use the
sign of the larger of the two numbers.
184.108.40.206. Change the sign of the cylinder (plus to minus / minus to plus).
220.127.116.11. Change the axis by 90 degrees (do not use degrees greater than 180 or less than 0).
6.1.4. Ordering Spectacles: All lenses, either for single vision or multifocal lenses, are ordered in
terms of “minus” cylinder.
Sphere | cylinder | axis
Original: +2.75 s. -1.75 cx 179
Transposed: +1.00 s +1.75 cx 089.
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