It's not to do with measuring. It's to do with glazing. When blocking the lenses, the prism causes the perceived placement of the uncut lens to shift, and so you must compensate for that.Originally Posted by drk
It's not to do with measuring. It's to do with glazing. When blocking the lenses, the prism causes the perceived placement of the uncut lens to shift, and so you must compensate for that.
Hmm. So you're saying that you're trying to (forgive my ignorance!) block up an uncut? And you are using that lighted grid thingy?
Scroll down to "Puzzle" and the differences between virtual and real images.
https://www.eophtha.com/posts/fun-wi...-ophthalmology
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.
First year optometry school.
This was my understanding of it as well, yes.
As Robert (S, not M) has mentioned, during blocking, the view of the blocker mires through the lenses are displaced by the prismatic Rx, and so we need to compensate the centration accordingly.
I'm saying that's what the compensations are for
we settled on the varilux as that is the only pal our lab is willing to attempt.. even then they said they cap at 10 but **may** be able to squeeze a couple extra diopters out (they did with his last pair so..) no fresnel on the lenses, we're customizing a silhouette for him to get a suppper small eye/bridge size
im just worried about the clarity, i know with such a high rx he's gotta give to get, but im unsure if offsetting the PD would give him any sharper vision than what he's got now :/
You offset the pd's/seg so the end product is correct. If you do not adjust them your seg and pd's will not be what you ordered (due to prism moving the image it will be incorrectly blocked). The lab is not going to make the correction for you because they assume you are a functioning optician and already did so.
I could see John from Opticianworks John@OWDC going to the board with a wig on pretending to be Christopher Lloyd in Back to the Future II and saying the prism has SKEWED the image and now it is at a different place in time!
Last edited by NAICITPO; 03-26-2024 at 12:41 PM.
Wait, WHO is supposed to make the compensation?
1. The lab. Send in the raw data.
2. You. The lab only uses your data and won't accidentally re-compensate it.
I'm confused.
2.
The lab won't adjust it for you because they assume you know what you are doing.
So if I'm putting these (and other dotted up prism blanks and assuming I am blocking them) on a project-o-mark I'm decentering an extra 5 or 6mm in, or I state the pd to be this amount more in to a lab?
Labs have always, “Made as ordered”. You order a 60 pd with 10D of prism, you will (should) get 10D at 60mm. Even in the old days, we never differed from order on high Rx’s unless refracted and worn vertex was given. ( then it was assumed we’d compensate it). Even modern FF designs either comp to a standard average “as worn” or info the optical shop specifies. No lab (TMK) auto comps pd/OC for prism prescribed . How do they know you didn’t do the comp before ordering?
the lab is doing it!! so far only issue is they needed us to send them the longer bushings for the silhouette so. fingers crossed he likes these
And it should be compensated using the same formula? 0.33 times the prismatic power in the opposite direction of the base?
Correct. 0.33 mm per prism dioptre, away from the prism base.
The post linked earlier has a detailed explanation by Daryl Meister, and System of Ophthalmic Dispensing has likewise :)
no, the lab should do this because if they don't then it will not pass final inspection since the PDs or segs will be wrong.
Actually, considering that this topic is being debated here (therefore proving that even eye care professionals keen to learn such things don't always know it)... ��
From there I could imagine the labs having a devil of a time trying to persuade clients to accept centration different from that as ordered, so it's probably much easier from their perspective to just fit the lenses as per the order, presuming any required modifications were done by the dispenser.
As for the prism causing the final product to differ from the ordered centration... yes, that would happen in the hands of an incompetent and/or ignorant lab edging staff. But despite prism jobs not being the most common, so far I've seen all my prism jobs come back well within tolerances (having adjusted the centration myself). If the edging staff over in the USA are like what we have here (mostly folks who learned by informal apprenticeships), makes me think they've got their own rules of thumb and reference materials to ensure what reaches final inspection is what's on the order form.
Last edited by AndyOptom; 04-09-2024 at 06:11 PM.
No. The lab makes what you order. That's it. You tell them what you want, they make it.
Technically, it is 0.27 mm. I try to use the exact amount for calculations especially for higher order prism, and then round the PD to the closest 1/2 mm.
Technically you need to know the stop distance if you want to use the exact number, which is going to vary by patient.
yessss...,. but if I order a 30/30 pd with a 10 BO prism on both sides, and the lab DOES NOT do the compensation calculations, then on final inspection the PD will be incorrect. Hence, it is the lab that needs to do the calcs.
If you order a pd of 30/30 and you received something different, then the lab was at error. How does the lab know that your requested pd wasn’t already compensated? Just like if you order a high minus/plus from a lab, they aren’t going to comp for vertex if not given the difference and request it. Once again, how does a lab know you didn’t compensate it already?
I am going to assume you are ordering finished jobs and not uncuts. If you order a 30/30 pd and 10 BO, you are going to get a job that at final inspection is 30/30 with 10 BO. If you measured the patient at 30/30 with 10 BO and ordered 30/30 with 10 BO you did it wrong.
Okay hold on.
So, in the case of a complete job, in order for it to come out with the correct PDs you need to compensate before blocking them, correct?
Well, I supposed it would depend on what kind of blocker you have. Using my old ICE-1000 in passive mode, or using my new-ish EZfit I don't compensate anything to get the PDs to come out correctly. I can't speak for other blockers though. But that's not the point. The point is, the lab is going to send you what you ask for. How they do it, with their specific equipment is irrelevant. You need to order what you want for a finished product and check said product before dispensing.
There are unfortunately 2 different discussions happening in this thread. One about how to properly block a lens to get the desired results. Another about altering the PDs(or fitting heights) for PALs in order to maintain proper alignment with the corridor and near zone, when prescribed prism is present. If you have a PAL with 30/30 PD and 10 BO in each eye, you need to order with PDs of 27/27.
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