Still trying to rationalize the value of the yoked prism in relation with this Rx. The deflection for upward gaze is very modest and would seem to be completely eroded by any off axis viewing ..... ..... what am I missing?
Still trying to rationalize the value of the yoked prism in relation with this Rx. The deflection for upward gaze is very modest and would seem to be completely eroded by any off axis viewing ..... ..... what am I missing?
Trip
Just came to me....
Should I take a height on the frame for vertical decentration?
So no Height?!
For prescribed prism, I normally always take monocular pd's and oc heights.
If you don't, and there's a problem. 9.9/10 the OD/MD will say its not an rx problem.
Originally Posted by LENNY
I would most definitely take a height, but I always do for a SV distance Rx, even without prism. I don't want to go around inducing it if they don't need it!
"Strictly speaking, there are no enlightened beings; only enlightened activity." -Shunryu Suzuki
Can you ask him how to spot the difference? You know, like you believe him and want to learn. I'd love to hear what he says.
I will try!
There are two ways.
one is by comparing edge thickness at opposing ends of the prism in it's direction(the base and the apex)
The other is with the use of a lensometer.
Most lenses with ground prism will never be able to be centered. Even with powers over +/-10D
Think about trying to find the OC of an actual prism.
Well unless you are grinding in excess of 20d of prism at that power, I'm gonna have to disagree.
Trip
Maybe after surfacing, but not after finishing, especially with minus lenses.
It is often very easy to find the OC on either, but I will admit that it can sometimes be harder to find an OC on a lens surfaced for prism precisely because it couldn't be decentered enough to induce the required prism. However, this in no way supports your assertion.
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
ml43, can you elaborate, because comparing thicknesses of one lens in no way explains any difference between ground & decentred.
As for the second point, if you had surfaced 1D of prism into a +/- 10D lens, you could find the OC by decentering by 1mm. I'm not what you meant when you said it 'couldn't be centred'. The only thing limiting that would be if the diameter of the lens was small enough to prevent enough decentration.
Last edited by standarduck; 07-18-2014 at 05:26 PM.
Also, the prisms we are talking about are 'actual prisms'. They just have a shallow apical angle than the type you're thinking of.
sorry guys, thinking too much about surface, rather than finishing/dispensing.
What about power deviation on an aspheric or digital lens away from the GC/OC?
I know we deal with tolerances, but on a purely academic level.
There is a difference on aspherics with ground prism vs decentered, right?
Maybe I'll run a couple lenses tomorrow and see if I or anyone else in the lab can tell/find the difference.
lets use the basic example we've been using in this thread.
-10D, 1D of prism(maybe run another at 5D of prism to account for generator/blocking errors)
I'll run it on poly, matching BC and center thickness
Last edited by ml43; 07-18-2014 at 06:04 PM.
I thought with aspherics, it always had to be ground, else power deviation can cause issues.
Yah .. certainly exposed to power errors. I would always recommend grinding Rx prism when using an aspheric lens, ...... however you can probable get away with inducing the prism through decentration if it can be garnered w/in a couple millimeters of the lens's center (low prism / high pwr).
Trip
Still say I can see better with ground-in prism. At least for me.
Jacqui, Stop! :)
How big is your prism?
Right now it's 2^ BI OU on the read only. Next script probably more.
I've been wearing prismatic lenses for 45 +/- years. I know not only what the theory says, but also what the patients feel which is something most opticians and ODs don't know.
Can you describe the design of your lenses?
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
Spherical with traditional surfacing, assembled as a Franklin.
Did you ever attempted to do same with the stock lenses?
Many times, Lenny. I've also tried sliding D-45 and 35 and Execs, it's the same as stock lenses. I'll just stay with surfacing and making Franklins, they are better.
Do you reckon that would stand up to a blind (excuse the pun) test? Would be interesting to try, because I've not had issues with decentred over ground prism. But knowing you work in the industry, then I would expect you to be more critical that a patient.
Ok guys, been very busy at work. But I managed to surface and edge a pair of lenses.
Specs:
CR-39 spherical
2.50 BC
-5.00/-1.00 cyl 90 axis
OD surfaced with 2.00D prism BU
OS surfaced with no prism
CT after generating was 2.7.
cut the OD with zero decentration,
cut the OS decentered enough to match prism of OD
end results. Almost identical, although I did notice the OS is about 0.12D weak when the prism is 2.00D BU, at the OC it's spot on.
I'd imagine with an aspheric lens, the power variation will be larger.
Will try with an aspheric poly next if I have time
Sorry, but that's simply not true. Almost all traditionally-surfaced multifocals are ground with prism at the surface block center in order to induce decentration of the PRP. And the edge thickness of a decentration-induced prismatic lens will be the same as a ground-prism lens, given the same center thickness.
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