I know this is a strange question and I think I pretty much already know the answer. I've consulted with our local lab gurus and every doctor/lab type I know and no one has an answer. So the question I put to you is this, is there any way to get compound prism using fresnells? Specifically about 8in/3down in each eye? Not that I don't trust the answers I've gotten so far I'm just trying to make sure I leave no stone unturned. Thanks in advance.
"Wise men don't need advice. Fools won't take it." - Benjamin Franklin.
Yes it is possible.
wow im interested to know how! Im very skeptical.... I dont think it can be done ;) , so come on prove me wrong!!!! lol
Think!! How would you surface it??Originally Posted by normuck
And no cheating. No splitting it between front and back surfaces. Can you get it all on the back?
"Wise men don't need advice. Fools won't take it." - Benjamin Franklin.
That's the easiest way
And have you done it this way with success?
"Wise men don't need advice. Fools won't take it." - Benjamin Franklin.
A thought just came into my coffee soaked brain, Why use Fresnels?? Yes, after doing lab work for 43+ years I probably can.
im thinking the only way possible is two prism's and imo that would be horrible to try and look and through, why are you using fresnel instead of ground prism anyways?
Why not use the compound prism calculator on Opticampus to determine total prism and direction and then mark the lens with a protractor?
Bingo! How do you think labs grind compound prisms?
I was trying to make him think the same thing
hmmm guess it has been too long since i worked in the lab, I still dont see how your going to induce 8 and 3 with a fresnell prism????
I asked this too, apparently she's not very stable and likely to be changing a bit. I'm going to experiment with the compound prism calculator a bit. Thanks for the tip.
"Wise men don't need advice. Fools won't take it." - Benjamin Franklin.
I am assuming the lens is surfaced edged and glazed already, is that where im messing up?
Calculate resultant prism and axis, then st fresnel that way.
I still would like to know why he wants to do this way. Surfacing sounds much easier and cheaper
Last edited by Jacqui; 06-12-2012 at 03:13 PM. Reason: added, Multiplied, Divided
Dang , Im still lost... (hangs head in shame)
I worked for an MD who specialized in pediatrics and adult strabismus and we did a lot of this kind of work. It usually involved a fair amount of trial and error until we would find a combination that would work. Way too expensive and time consuming to keep surfacing new lenses.
I've pasted part of an artical I wrote for a trade rag a few years ago...
P² = H ² +V ² √
P² is the amount of resultant prism
“H”is the horizontal prism amount
“V”is the vertical amount
√ is square
If you are not familiar with a scientific calculator, I’ll give you a step by step example; O.S. 5 out and 3 up. Use these buttons on your calculator..
5 x² + 3 x² = √
Your calculator should say 5.83095.. This is the amount of resultant prism. Now,along what axis does it go? The formula for the prism meridian (axis) is;
Tan ‾¹ = V/H
Example; (Use these keys in their order)
3 ÷ 5 =Tan‾¹
Your calculator should say 30.963…
So the final Rx checked on the O.S. Should be 5.8 diopters of prism @ axis 31. Keep in mind the direction of the requested prism in relation to which lens you are neutralizing. If the above prism amounts were on a right lens, an axis of 31 would be up and in. ( wrong direction!) In such cases you inverse the axis from 180.
(i.e. 180 – 31 = 149 )
These same formulas and procedures are used when applying Fresnel prisms.
I would agree with that and that's how I presented it to my doc. I think he's just exploring every option.
"Wise men don't need advice. Fools won't take it." - Benjamin Franklin.
We use to grind oblique prism all the time to make custom shooting glasses, moving the OC up and in.
"Coimhéad fearg fhear na foighde"
Would it work if I put a 6^ auxiliary prism set out in my trusty B&L model 70 and rotated the 8^ Fresnel to 2^in and 3^ down because that's how this math challenged mind would cheat to get it.
Thank you for all of the input guys, I got some good feedback and a couple new directions to explore.
"Wise men don't need advice. Fools won't take it." - Benjamin Franklin.
We make a lot of them here, we make hemianoptic and macular degeneration lenses. Also use fresnels a lot for temporary fittings to make sure of placement before spending all day making a pair of lenses.
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