As many of you know, measuring the OC on single vision Auto II lenses is required (or at least desired) by the lab.
This has me wondering how many of you routinely measure OC heights for SV jobs?
How many of you feel it is important in some or all cases for regular SV lenses (not including the Auto II SV)? We already do mono PD's on all jobs, but do not take OC heights routinely for a run of the mill SV Rx.
Thanks!
John
99% of the time we do not measure OC HT for SV specs. Proper frame selection and adjustment take care of the need to.
Where we all should measure oc heights as we do multifocal heights, I doubt few do.
I do observe all patients as to where centers line up, and do request special palcement when needed. Most cases, actual pupil location is going to be somewhat above center, but with general paient gaze slightly downward, and correct panto adjusted, verticle movement is rarely required.
We measure OC heights on all jobs
If appropriate frame fitting is used there should be no need to measure on average Rx’s. If you fit higher powers or high fashion frames with long b’s then measure, or in cases like autograph 2 takes that measurement to calculate the true power.
I won't on a -.50 spherical, but I do with aspherics, higher powers, and when there's power disparity at 90. I measure everything if it's POW optimized.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
So why WOULDN'T you measure every time? Is the extra 10 seconds of work too much? LOL It gives that extra opportunity to maximize our accuracy. While it's effect is certainly smaller in low power Rxs, it will have some relevance to final Rx. It's funny that with all the parroting that goes on here about that horrible Abbe monster in 90% of all optical lenses these days, that so few would bother to be as accurate as possible in not bothering to take a perfectly measured OC every time as well. :hammer::p:cheers:
This is exactly how I do it too.
I find the problem with most dispensers, is that they do not measure OCht properly. They must remember to drop the OC 1mm for every 2° of pantoscopic tilt.Originally Posted by Robert Martellaro
The easiest way accomplish this without the math is to raise the patient's chin until the lens plane is perpendicular to the floor and simply spot at pupil center.
Last edited by mullo; 04-03-2009 at 03:42 PM. Reason: cause
There are a few issues involved... I do OC's for all Aspheric lenses or you simply won't maximimize the design. I also do OC's for all powers over 3D - or 2D +.
I do them however 1-3 mm below the pupil depending on the patients height, usage and RX.
You may not NEED an OC height, but it helps cut down potential problems in higher RX's the patient gets used to the lenses faster.
Sharpstick
Here is what happens if we don't get the eye's center of rotation/panto relationship right.
Martin's formula for tilt
S'= S{1+(sin a)^2/(2n)}
C'= S'(tan a)^2
S sph power
S' new power
a tilt
n index of refraction
C' induced cyl
Axis is 180 and the sign is equal to the sphere power
Rx is -10.00DS, panto 15 degrees, Trivex, 180 line bisects the pupil. I see this happen on a fairly frequent basis, usually with a narrow frame that sits too high.
Using the above formula, the power that the eye sees is -10.22 -.73 x 180.
To eliminate this error, the OC needs to be lowered 7.5mm (1mm per two degrees of tilt). The best way to do this is to pick a frame that puts the 180 line about 5mm below the pupil, and reduce the panto 5 degrees or lower the OC to 2.5mm below the 180. If a high dispersion lens is used, one might want to reduce the panto so that the OC can be closer the pupil, reducing chromatic aberration on the distance gaze.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
as stated we need to drop hts by 1mm for every 2 degree of tilt, as most people look about 5mm above datum, and most frames are angled by about 10 deg that puts the oc ht back on datum again. thats why i dont do it unless, the vertical power is quite diff r and l to prevent any vertical prism imbalance
We take OC heights for every SV job.
Agreed some single vision lens designs need the OC hieght much more than other's, but how many times would we or one of our Opticians forget to do it when it was crucial? Aspherical lenses are being used more and more today "requiring" a OC height and lets face it most of us are habitual by nature. Good practice equals taking OC heights on everybody! Bottom line is patient gets a better pair of glasses!
On the other hand, the spherical equivalent of the error in this case is approx -10.87, which is only 0.62D off from the target rx. If we assume the normal 20 foot error in refraction is 0.16D, the net error drops to 0.46D, an amount that, in the absence of a " corresponding" Ref VD, makes all this power vetting crazy if you depend on finished spherical lenses.
b
True, but many vision care plans force the practitioner to bump the volume to make up for extremely low margins. There's a stampede of clients in and out of the front door, and the first place that corners get cut is at the dispensary. There's simply no time to get 'fancy'.
As the song goes: Don't try to understand 'em, just rope an' throw an' brand 'em, soon we'll be livin' high and wide.
One can hope (or verify) that the prescriber adjusts the final Rx for the chart distance, and that short of a written doctor's vertex distance, the clients facial structure will allow a close fitting frame to be more or less equal be in vertex distance to the position of the phoropter when the client was refracted.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I measure OCs on all jobs. My co-workers do not
I've never heard that before and now, I'll never forget it! Great tip!
Hi, mullo.
When we perform this neat trick, and come up with a value, double it to find the approximate pantoscopic tilt in degrees. The formula is the stop distance (typically 27mm) times tan 1 = .5mm. In other words, one degree of ocular rotation is approximately equal to .5mm at the spectacle plane.
(The stop distance in centimeters (.27) x tan 1˚)
Last edited by Robert Martellaro; 08-16-2014 at 04:03 PM.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I'm betting that most of us would agree it's rarely a good idea to set a seg height for a multifocal or PAL differently for each eye unless the differences in ocular altitudes are extreme.
I'm curious if there's a similar concensus among you diligent OC takers regarding SV lenses? Do you split any difference and set OCs OU to the average? Do you give each eye its own monocular OC regardless? Is there a boundry of measurement differential where you cross over from the OU split to monocular values?
Is it wiser to just leave such considerations to the doc and trust they factored in any 'vertical prism' needs into the Rx itself for anything resembling a .25D or more and assume that the patient's eyes have worked it out for themselves long ago for any smaller value?
It's never a bad idea to dot the center of the pupil on any SV and use that for you're OC when blocking to avoid unwanted prism.
On that rare occasion with more than a .5mm split or if it's a high rx (+/-4) I'll do that. Especially when I use the auto 2 sv. usually I put the oc mark at the lower of the two or will split it by .5mm.
I've been told that using the mono oc is always best and I have to say I'm doing it more and more. At on place I just cut at I did that for one of the opticians and she loves it. granted she is around a -6 ou, but it blew her mind.
Well said,
For any lens as well, before you even dot a pt. you should adjust the frame for that person, nose pads, temple curve, pano or retro, because even with SV the stronger the rx the more drastic the result. Do the job and take care of every aspect. most lab LMS's with divide the OC height in half do to the B of the traced frame and place the OC there unless a specified OC height is input by the tech/monkey. On one hand though if all this is all missed and the pt. puts on the frame and even tho the OC height does not match the Pt. pupil as long as the prism is equal verticaly in both sides they will never notice. (Back on subject) As stated above a few seconds of attention to detail are worth more than a thousand Re-Do's.
Last edited by Outkast; 08-20-2014 at 09:45 PM.
Thank you for the scoop, Boldt! If I'm understanding you right, you use monocular OC values for any difference in OCs greater than 1mm between OD & OS. Anyone else use a different threshhold? Or the same one?
That's what I've started doing and so far so good. I've had one pt's say it was the best thing ever, but he was kinda on the dramatic side so I'm taking it wa grain of salt.
You gotta keep things simple and consistent!
1. Split distance pds with pupillometer for everyone
2. Dot the pupil with a marker for everyone.
This gives you the position of the eye in the lens, then you can do with it whatever you want...fit a BF. Fit a PAL. Fit SV. Whatever.
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