Hi All! I am looking for opinions on when and why to take an oc measurement in a stock sv lens. In my experience, the standard 1/2 the 'b' is sufficient for most people, but I would like to know if there are any circumstances where it will truly help someone to have it taken. Also, can there be any drawbacks if someone has had many pairs of glasses made without one taken, then a new Optician takes one?
You are a dispensing optician in NC without the requisite knowledge of when and how to take an "oc measurement"? Seems more likely you are a consumer...
On the "off chance" that Kwill212 isn't right...
The OC is AS IMPORTANT as the PD for all the same reasons, but in the vertical sense (lens centering).
The ONLY drawback of someone who has had many pairs made without it is: The lenses will be thicker (myopes only) at the bottom than they are used to.
Hyperopes won't notice.
Good luck.
We always take an OC so that way if they decide to switch to a digital SV or a progressive lens we don't need to retake their measurements. On reading and computer glasses I almost always use half the B, they aren't looking out of them at the normal OC location and it keeps the thickness of the lenses down.Originally Posted by MEB
If the OC is yoked at half the B, the customer will probably not have an issue with it as long as their power is under 2 diopters. From the guru of guru's himself:
http://216.144.236.77/cecourse.php?url=prism_thinning/
"A study was performed a few years ago exploring the effects of vertical yoked prism on wearer acceptance.* This study showed that a group of test subjects was not significantly affected by 2.0 Δ of vertical prism (and no significant postural adjustments were made). However, 4.0 Δ of vertical prism was rejected by almost all of the test subjects. Consequently, the limit of prism-thinning for most wearers will probably lie between 2 to 4 prism diopters.* Sheedy, J. and S. Parsons. "Vertical yoked prism—Patient acceptance and postural adjustment." Journal of Ophthal. Physiol. Opt. Vol. 7 (1987), pp.: 255-257."
Most modern stock lenses are aspheric at the least, better designs are atoric. The best way optically to think of these designs is that they are like a bullseye. The curvature is either increasing or decreasing (dependent on if the power is + or-) from the OC, or the center of the “bullseye “.
Think of a bullseye target with different rings out from the center. If you don’t put the bullseye at the given OC hgt, you are neglecting the patient of the benefit of aspheric/atorics…. Better balanced vision along with peripheral performance (thinner designs are just a bonus). Accurate vertical is every bit as important as binocular horizontal (PD) placement.
I agree that it is important, I take an OC on every SV Distance pair I make. But I don't know about the statement, "It is every bit as important as a binocular horizontal (PD) placement." I haven't read a study where 2 diopters of BI prism can be tolerated but there is a study like that for yoked vertical prism.
My post had nothing to do with prism, but lens design for optical performance. ( Though you don’t necessarily want to view through yoked prism either, read up on “LCA’s). From Darryl’s Ophthalmic Lens Design paper….
….Put simply, an aspheric surface is a surface that departs from being perfectly spherical. Aspheric base curves are surfaces that vary gradually in surface power from the center towards the edge, in a radial fashion (meaning the asphericity is the same in every meridian of the lens—like the spokes of a bicycle wheel).Unlike a spherical surface, which has the same curvature in any direction across the entire surface, a typical aspheric surface becomes progressively flatter (or, in some cases, steeper) away from the center of the lens—i.e., the tangential meridian of the lens. However, the aspheric surface changes very little around the circumference of the lens, which is the sagittalmeridian of the lens perpendicular to the tangential meridian.This difference in surface curvature (and power) produces surface astigmatism, which means that the surface literally produces cylinder power away from its center. Furthermore, this surface astigmatism is used to counteract and neutralize the oblique astigmatism produced by looking through the lens off-axis. Essentially, the difference in surface power on an aspheric surface cancels out the difference in off-axis focal power produced through the lens by oblique astigmatism. An aspheric surface departs more and more from a spherical surface away from its center, just as oblique astigmatism would normally increase more and more when looking away from the center….
I used the term, “bullseye target”. Darryl used “like spokes on a wheel” to describe an aspheric design, both are correct. Aspheric (and atoric) designs’ curvatures are changing from the OC out to the lens’ edge. If you don’t place these designs both horizontally and vertically in reference to where a patient’s pupil position is in a given frame you are defeating the lens’ intended design…to manage off axis oblique astigmatism.
Using Darryl’s analogy, spokes on a wheel, how well would a wheel spin around if you put it’s axle up into the spokes rather than where all the spokes intersect at the center?
Last edited by optical24/7; 03-09-2023 at 05:26 AM. Reason: Clarity
If you made a pair of SV glasses and put the OC 3mm below the intended OC but had correct PDs you would get a passable pair of glasses. I understand it is not the best optics, your point is well made. This is why I always take an OC. However, if you made a pair of glasses with the correct OC but 1.5 off in each eye in the PD are you going to get passable glasses?
I think we are discussing two different things. For the best optics both are equally important, but to have a passable pair of glasses (especially with smaller Rxs) PD > OC.
Let me pose this a little differently. If I have an Rx of -5.00 w/4D of base out prism (or in or up or down), I can take a stock lens and block it up for edging with the OC moved 8mm’s and induce the prescribed prism. Now then, if the lens is spherical, it will make no optical difference. But, if it’s Aspherical it will…Think of Darryl’s spoked wheel. The very center of the wheel, where the axle goes is the OC of a stock (non-surfaced) lens and it’s the center of the aspherical design.
If you move the OC of stock lenses, off pupil up, dn, in, out, you also move the center of the aspherical design. Defeating, even exasperating the intended benefits of the design. (Better optics on off axis viewing.)
Sure, low powers (sub +/- 1D) make little difference in OC placements off negligible amounts (1.5mm/sub hopefully ;)
but powers 1.50 or so on up and the design of aspheric’s need to be more precise to give the effect of design. We’re particular on PAL vertical placement, we should also be with aspheric SV designs..
NAIC, I know you know all this, it wasn’t directed at you, I simply wanted to clarify for the newer opticians ( possibly the OP?) why we treat aspheric stock lenses different from a standard sph lens.
Simple, front-aspheric designs are, in fact, more limiting, with little optical benefit compared to conventional CC LENSES.
Don’t bother. They are functiinally obsolete.
B
Moving an OC, without a VD, AND without the compensating knowledge of the ametropic-related position of the CoR (Center of rotation) is akin to tilting at windmills.
VD is virtually irrelevant when it comes to putting the OC where it belongs, which is over the pupil. Some will compensate for panto in SV lenses which is valid for CC lenses only.
But, in general moving an OC to be as close as possible to pupil height both in conventional AND aspheric designs will lead to a marked improvement in optical clarity, especially in higher indices and powers above 4 Diopters, and is almost compulsory for poly wearers.
As for "front side" aspheric designs (WTH??) being functionally obsolete... that statement is functionally obsolete. If for no other reason that it implies they were once functionally relevant and stopped being so?
I'll save that for someone with a high cylinder. Almost no SV stock lens on the market is a front side only aspheric. What a silly idea.
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