Okay, so I wanna see how many people follow the "for every 2 degrees of tilt, lower the OC 1mm" rule. And my question is, why haven't I heard about this until studying for the Advanced NOCE. It, of course, makes some sense; but I've never seen anyone raise a pt's chin so that the frame front is parallel to the floor nor have I seen anyone compensate for tilt. Is this just that the it's an ancillary task that people don't pay attention to, or do people just not know about it?
I always considered it part of the fit. When patients try on frames you can see where the GC would fall in relation to the pateints pupil. If the frame fits well that means that I will be able to place the OC within a position that I can adjust the frame with panto when dispensing to provide optimal vision.
Probably a bit of both, however with the advent of freeform lens designs and digital processing, the need for more refined measurements has become more critical.
Remember, "Opticians don't need no real education, just do it like we always done it." OR as I overheard from one ECP to another..."My optician gets all the education he needs when he takes Vision Monday into the bathroom in the morning." OR "Having a license is just about the money."
HAHA, true, true... I'm just upset because I hadn't even heard about this until recently. I've been doing this for over 5 years and worked under countless opticians. Why is it no one has either known this or imparted this knowledge? Even when I was studying to be Basic ABO certified, it didn't show up in any of the books I read.Originally Posted by CuriousCat
Do you have the Optical Formulas Tutorial? It's in there.
Another little known optical pearl is that you need to adjust a PD a quarter mm per diopter of prism away from the base. (Harry, is it .28?) i.e. PD 31/31, 4 D.O. OU, adjust to a PD of 30/30. (Prism displaces the pupil.)
Applies more so to PALs, simply put the idea is that you want to match up the corridor with the patients gaze. Of course that's when blocking the lens.
Aspherics too (SV), right?
Aspherics, absolutely. System for ophthalmic dispensing lists it as .33 mm per diopter displacement towards the apex. Of course they could be rounding, but if it really were .28, why not round to a quarter? Someone have the math on this?
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
Light is deviated 1cm at a distance of one meter. The distance from the center of rotation of the eye to the cornea (about 15mm) plus the vertex distance (about 13mm) averages about 28mm (the stop distance), hence a deviation of .28mm per diopter of prism. I round to a quarter, slightly undercompensating in most cases.
Last edited by Robert Martellaro; 08-13-2010 at 07:33 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.
Thank you, Robert.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
Odd, it duplicated my post. Better delete it before someone says, "You can say that again, Wes!"
Last edited by Wes; 08-13-2010 at 11:24 AM.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
Yeah, that's one of the books that I read this in. In fact, it's been in just about every book I've read since studying for this exam. I guess the materials that I studied when I took the Basic didn't bother mentioning it.
Right.
That's the math your looking for.
I think I posted something about that more than a year or so ago, George has the mind of a steel trap. Robert's figures are accurate as far as the average vertex and average center of rotation. Doesn't need to be too accurate, since there is no way to accurately measure teh center of roation on the spectacle end.
It never hurts to be more accurate in every facet of the job you do and it does show your patients will not always be able to express why the glasses you make are somehow more accurate but they are and they'll keep coming back.
A lot of great posts in this thread.
I'll add one comment regarding the fitting relationship between pantoscopic tilt and the optical center position, which is done is to ensure that the optical axis of the lens passes through the center of rotation of the eye. Although it will probably not impact vision significantly in weaker prescriptions, unless there is a gross departure from the recommended relationship, this relationship becomes especially critical in higher prescription powers, particularly when an aspheric design is used.
Best regards,
Darryl
Darryl J. Meister, ABOM
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