Had a discussion with the doctor today.
He claims that the ground prism is better then made by just de centering the lens!
I disagree especially in this case:
OD -7.50-1.00x90 1BD
OS -5.00-1.00x90 1BU
He also claims that he can see if prism was ground.
The only draw back I see is aspheric design of stock lens, but again not on this amount of prism....
Thoughts
Tell him we will leave the Doctoring to him if he will leave the glasses making to us.
Now go decenter the lens and finish the job. Gezzzzz
He may want to be careful with that rx in a multifocal will create even more prism in the down gaze which would require BU in the more plus eye, the opposite of what he's prescribing.
If it's a multifocal, he should be giving the least possible prism so he does not imbalance the down gaze which already has BD in the OS naturally.
you can even do drawings for him in crayons to explain the reasoning:)
Hold on!
I gets better its a yolk prism!
1BD OU
Kill me now!
And its Poly for a 12 years old child!
And its single vision!
Yah .. 1 BD ou seems lite relative to power.
Trip
Prescribed prism for a minor should be referred to a vision therapist.
Prism is such a small segment of refractions, and most OD's aren't experienced enough with it.
Wow. A lot of off target statements here. If optometrists don't know about prescribing prism, then who does?
Len, you will find people (optometrists and opticians) arguing over whether to decenter for prism or not. Not just your doctor.
The issue of decentering aspheric lenses is real... but at the powers you state, and the amount of prism you want, it shouldn't be a problem. Normally, I would say the direction of the prism doesn't matter, but in the case of yoked prism, at least you are inducing symmetrical "errors" in both eyes in the same direction.
Can someone provide an explanation as to why it would make a difference if the prism is induced by decentration vs. surfacing?
As far as prescribing prism, I almost never do. Generally I look around for a sign, something like a burning bush or a glowing apparition before actually writing such a script. Most of the phorias, tropias we measure do NOT need prism correction and can actually be aggravating to a patient who also wears contacts (hence no prism) or just wears glasses intermittently. Adaptation to prism can be difficult. But once in a while, sure.
Personally, I've never seen the difference between ground-in and decentered prism.
If you take two single vision lenses ground to the exact same Rx, with the exact same amount of prism, one with ground in, one with decentered prism, there is no way on earth you'd be able to tell them apart.
And to be perfectly honest, there is always a certain amount of error in grinding in prism, especially in lower lens powers (near plano). There is almost always a certain amount of decentration to get to the final location MRP point in the lensometer before edging. So, in effect, you have a lens that has both ground-in and decentered prism. Again, I sincerely doubt anyone could tell the difference if they didn't know which lens was which.
It is straight up optical physics, not mumbo-jumbo.
The best are certified Orthoptists, and Optometrists.Originally Posted by ml43
Decentering aspherics for prescribed prism will result in unacceptable levels of power error and oblique astigmatism.
Yup, no difference, although the plus powers will usually be thinner when surfaced.
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.
I understand completely about asphericity! But even if poly in this Rx is aspheric the potion we are working on decentering should be way before asphericity. We are talking about 1.5-2.0 mm!
So can I make this statement?
If the lenses are spherical in this RX and with this prism there is NO way to tell if prism is decentered or ground in!
Yes, that is true. Probably for aspherics also unless you have a lens mapper or some such device.
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.
It depends on where they went to school. Pacific near Portland spends a huge amount of time with prism esp with kids, Berkely not so much.
Yoked prism should always be ground with children. Its done a muscular treatment as part of Vision therapy. Grinding prevents children from tilting or turning their head to compensate. At OC ground in decentered prism is the same, the issue is that many people use other areas of lens, and turning or tilting can neutralize the effective prism RX when its yoked.
There was someone here a few years ago on Optiboard that calculated this issue using math and there were some cases where ground prism makes a difference. If you have the time finding that thread could be a help.
So are you saying that when you grind the prism there will be same amount of prism anywhere person looks off OC?
If that's true I could understand.
Say I had an RX with Yoked prism laterally, so imagine OD 1D BO, OS 1 D BI. with a decentered lens, if look through the lens to the right and I have essentially neutralized that prism. If my power or cyls vary, I will have a different point of neutralizaition off center effectively in each lens. If a child has a dominance issue they could easily find the prism that is most comfortable and turn their head, it will reduce binocular vision and the effect of the prism the opposite of what the Dr. intended. Yoked prism rx's should always be ground esp with children, as they can't selt determine binocularity (only comfort) and will form bad habits quickly, possibly making their condition worse, not better.
The 'tilt' is the same with spherical BCs.
I couldn't find any cases of exception. However, I did find this...
http://www.optiboard.com/forums/show...l=1#post312954
The prismatic effect off-axis is the same (see below).
An Rx with refractive power will produce induced prism when the gaze is eccentric from the OC, and that induced prism is either additive or subtractive to any prescribed prism, with the same prismatic effect regardless of how the prescribed prism is produced (decentered or surfaced).If my power or cyls vary, I will have a different point of neutralizaition off center effectively in each lens. If a child has a dominance issue they could easily find the prism that is most comfortable and turn their head, it will reduce binocular vision and the effect of the prism the opposite of what the Dr. intended. Yoked prism rx's should always be ground esp with children, as they can't selt determine binocularity (only comfort) and will form bad habits quickly, possibly making their condition worse, not better.
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.
And this is the correct answer.
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
Robert States it well.
Did I start a fight again!?!?!?
If Darryl Meister says its the same it must be the same! At least theoretically!
I would love to get Shanbaum's point of view on this!
I'm not saying OD's don't know about prescribing it. But most simply do not have enough experience or training.
Likewise with most MD's and refraction.
Funny you should mention, I know a vision therapist that graduated from Pacific University.
I've also worked with another OD that graduated from Pacific University.
Both handle prism very well and are willing to put in the time to get it right.
I currently work with an Optometrist that graduated from UC Berkley, and she does not like to touch prism with a 10 foot pole.
She actually asked me to neutralize a patients old pair, then remake his new pair to the exact same specs,
she would sign off on it, then send him to a vision therapist of my suggestion.
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