Im being told that I should not enter an OC height on bifocals, just the seg height. What difference would it make if the glasses are bifocals, when the patient is looking through the distance portion of the lens? Wouldn't it still be prudent to get the distance focal point over the pupil, especially on higher power perscriptions just as much as always?
Many thanks!
Consumer
??Originally Posted by optical24/7
The question sounds pretty technical to me to just be a consumer.....
But, in my experience, surface labs generally don't pay much attention to customizing the location of the distance OC in a segmented bifocal no matter what you specify.
Straight Tops generally do not require an OC, unless the SH is placed in an non-standard location. Typical calculation of the OC is generally 5-6mm above the seg for Bifocals, and 2-3mm above for tris. Consider the relationship of the pupil to the lower eye lid, which averages 5-6 mm when the SH is fitted properly.
ABSOLUTELY FT require an "OC" position.just like ANY SV.
Sheesh!!!!
B
All the way with Barry..................
All the way with Barry........................ 100 %
If the seg location is fairly standard, the default OC location will be fine for 99.5% of all patients. If there is an unusual seg location (some folks like it really low), I would absolutely include an OC height. This latter occasion will be rare enough to merit a heads-up call to the lab.
I'm Andrew Hamm and I approve this message.
agreed
The default position is usually 2.5mm to 3.5mm above the top of the segment for bifocals, and 1mm above the seg for trifocals.
I don't usually specify the distance OC vertical position for segmented multifocals except as noted above, or when there are unusual tilt values (see Martin's Rule for tilt), and for anisometropes, where the distance OC is usually alligned closer to or coincident with the primary gaze, minimizing vertical prism imbalance.
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.
No, im not a consumer, im a relatively new duel licenced optician. Thats what I like about this site, I can learn the whole cross section of perspective on an issue!. Ive also learned lens placement is not all that cut and dry. Part science and part art. So maybe to rephrase the question slightly, is there any harm in taking the OC into consideration? I was a bit taken back by the 'directive' so I was concerned. (...newish you know and trying to do the right thing, which is oddly not always perfectly clear. ) Additionally I had a guy who wanted his glasses mainly for driving and the segment mostly to see the info on his dashboard, speed, radio, etc. So I was thinking with a deep frame his eyes are looking mostly out and above center. Didnt want the segment too far down to be practical but maybe an extra mm or 2 to maximize the usefulness for mostly distance since that is what he wanted and he specifically said he wanted the segment lower than his current ones. ...as they say the rest of the story. I do draw this for people so they see where the segment is going to be. Thanks for the feedback, i agree most people most of the time it will be about 4-5ish mm above the segment and the lab makes that call ; but there seemed to be some assumption that because they were bifocals that the wearer was looking more down most of the time and not out. Im not so sure that can be assumed without some conversation hense a directive is , err, hmm, a bit short sighted.
Your patient is not going to be terribly happy with those glasses unless the seg power accounts for the increased viewing distance. That's a very specific seg height requirement, as well; you may literally have to get in the car with him to determine it. I've had to do that with bikers and their motorcycles, but never a car.
I'm Andrew Hamm and I approve this message.
Sorry iok, I thought I was responding to another thread above.....OC height on FT's need to be taken into account when we're dealing with outside the *norm* frames and or Rx's. In your above scenario, it may be prudent to raise the OC to minimize yoked prism, but on the other hand, as Robert points out, if the patient is an anisometrope you'll exasperate the vertical prismatic imbalance in the seg (without a slaboff). One other thing to keep in mind is the power needs of the patient. Again with the above scenario, unless the patient is in an extremely low add power, he's not going to have clear dash/radio viewing in a bifocal if the near distance was prescribed for a 40cm work distance.
+1
If the default OC in the distance portion is 3 mm above the seg line, and the seg is fit at the typical 8mm below pupil center, you have the OC about 5 mm below the pupil center, which is pretty close to correct for 10 mm panto.
(You can stretch any number in the above statement one way or another and be pretty close to what I said.)
From the geometric center of most FT blanks to the center of the seg line the measure is (5, 5) most the time thats where the OC is placed if not specified. In pediatrics the seg often splits the pupil so the OC should be on the seg line for the PRP to coincide with the OC. Specify when you need to especially with dissimalar Rxs. Of course tilts and prism is going to move OC's anyway and the number of opticians capable of adjusting for all factors probably isnt large enough to warrant all opticians to start inputting OC heights, it would be just as likely to create issues if you dont know what youre doing when specifying OC heights, thats why youll probably hear from labs to not mess with OCs in most cases.
If the lab doesn't place the OC at the height I want on a FT bifocal or trifocal, the job goes back and won't be accepted until it is ground properly. I see you are an optometrist in NY. I was on the NYS liscensing board for 20 years and co-authored the dispensing practical. That question of the optical center was part of the exam. Try to get a -5.00 myope to accept an optical center that is 5 mm off!
I hope the message above is the final nail for this coffin. .........................
I hope the message above is the final nail for this coffin.
Even if they were properly trained, there would still be strong pushback from the muckety mucks, in the name of throughput and profits. Oh well.
Right, but see the above.
Best regards,
Robert Martellaro
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.
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