Question about prism

**Neophyte Optician** · 11-27-2007, 11:47 AM

When you displace the optical center in relation to the patient's pupil distance to induce prism, the amount of prism that they see through will change when they move their eyes away from the optical center. Is this still true when you grind the prism into the lenses?

**shanbaum** · 11-27-2007, 05:40 PM

Originally Posted by Neophyte Optician

When you displace the optical center in relation to the patient's pupil distance to induce prism, the amount of prism that they see through will change when they move their eyes away from the optical center. Is this still true when you grind the prism into the lenses?

In a finished pair of spectacles, there's no detectable difference between lenses in which prism has been induced by decentration, and lenses in which prism has been ground.

At least, not as regards prism.

**drk** · 11-28-2007, 11:18 AM

Shanbaum, if I may: I think the question is of variable prism power.

Plano prisms have the same deviating "power" (in prism diopters, of course) regardless of whether you look through the dead center or the edge. If you have a plano prism (see the OD's exam room), put it in front of your eye and decenter it. There will be no additional image displacement (movement).

Realize this (and I don't always realize it, myself): a non-plano ophthalmic lens is a variable prism! Since the lens surfaces have curvature (unlike the plano prism slab), there will be increasing or decreasing thickness from center to edge! We're used to thinking about that, but not as variable prism...Take a +2.00 trial lens an look though it, and images will move when you shake it, of course.

At the infinitesimal optical center of any lens, the front and back surfaces are parallel, ergo there is zero prism power. On the other hand, at the edge, there is maximum angle between the front and back surfaces (actually a prism apical angle) and it will be a prism of some magnitude.

So, think of this, and envision a graticule superimposed on a lens front surface:

optical center: no prism power
2 mm up, down, left, right (all points of a circle): 1* prism diopter power
4 mm up, down, left, right (all points of a circle): 2 prism diopters power
8 mm radius points on a circle: 4 prism diopters power.

In fact, if you were to fracture the lens into pieces, certain pieces would have more prism, some less.

*arbitrary value

So, in answer to your question, YES, the lens will give a variable prism amount depending on where the eye looks through the lens, and it's only coming from the ophthalmic prescription component. Whether you decenter to acheive the prism, or grind a "plano prism slab" on top of an ophthalmic prescription component, you'll get the same result, if the ophthalmic prescription component is non-plano.

Shanbaum, how's the new job?

**shanbaum** · 11-28-2007, 12:08 PM

I'm not sure if he was asking the question you answered, or the one I answered, but he should be well-answered now.

The new job is keeping me astonishingly busy.

**drk** · 11-28-2007, 12:27 PM

To be succinct is a good thing, I know.

Good to see you here. Cheers.

**Barry Santini** · 11-28-2007, 03:23 PM

Originally Posted by drk

: a non-plano ophthalmic lens is a variable prism! .

BTW, did you know that a meniscus plano, if you take a cross section, does have a *prism wedge* appearance, since the front and rear surfaces of any *true* plano menicus are *never* the same curve (backcurve is more minus)?

FWIW

Barry

**HarryChiling** · 11-28-2007, 03:25 PM

Originally Posted by Barry Santini

(backcurve is more minus)?

FWIW

Barry

Back surve is more plus, I had egg on my face for that one, and I think it shanbaum that threw the egg.:D

**drk** · 11-28-2007, 03:36 PM

I had an OSU instructor hold up two quarters to demonstrate that the same radius of curvature on a RGP base curve as the corneal meridian is actually too steep...same principle.

Yes, the back surface must be more steep/front surface more flat to maintain parallelism.

What I DON'T know is if it would be prismatic. I'd think not.

**HarryChiling** · 11-28-2007, 05:07 PM

Originally Posted by drk

I had an OSU instructor hold up two quarters to demonstrate that the same radius of curvature on a RGP base curve as the corneal meridian is actually too steep...same principle.

Yes, the back surface must be more steep/front surface more flat to maintain parallelism.

What I DON'T know is if it would be prismatic. I'd think not.

The back surface would be flatter, shanbaum get out your eggs.:D

**HarryChiling** · 11-28-2007, 05:14 PM

Sorry you guy's are right egg me again.

Man I'm too young to start losing my memory.:hammer:

**lensgrinder** · 11-28-2007, 05:16 PM

A plano lens can have prism, because if you take a protractor and draw a plano lens (keeping in mind that the longer radius is the front surface and the shorter one is the back surface)you will see that as it moves away from center it will get thicker creating a prism. If you had a concentric surface then you would have no prism, but the power would not be plano.

**Darryl Meister** · 11-29-2007, 12:30 AM

The following thread covers *almost* everything you ever wanted to know about plano lenses and prism: Is "plano" (0.00) plus or minus...and why?.

**Barry Santini** · 11-29-2007, 07:47 AM

Originally Posted by HarryChiling

Back surve is more plus, I had egg on my face for that one, and I think it shanbaum that threw the egg.:D

Thanks for the correction, again, Harry.

When it comes to plus and minus, I guess I play the 50-50.

Barry

**gemstone** · 11-30-2007, 06:02 AM

Quote:
Originally Posted by Neophyte Optician

When you displace the optical center in relation to the patient's pupil distance to induce prism, the amount of prism that they see through will change when they move their eyes away from the optical center. Is this still true when you grind the prism into the lenses?

Originally Posted by shanbaum

In a finished pair of spectacles, there's no detectable difference between lenses in which prism has been induced by decentration, and lenses in which prism has been ground.

At least, not as regards prism.

I like the answer Shambaum gave you. It was simple and correct. I believe it answered your question. There followed a lot of technical information that will probably only confuse you. The fact is, is you grab a finished lens off the shelf and spot it with the prism needed (provided enough room for cut out) you have the same thing as if you have ground the prism. Exactly. Now here is a warning that is pertinent to your question. Don’t do this with aspheric lenses. You are asking for trouble if you do. Sometimes there will be room to decenter the lens without getting too far into the aspheric area of the lens but just don’t try that. That is a fudge you shouldn’t risk.

**Fleming** · 12-06-2007, 04:13 PM

Originally Posted by Neophyte Optician

When you displace the optical center in relation to the patient's pupil distance to induce prism, the amount of prism that they see through will change when they move their eyes away from the optical center. Is this still true when you grind the prism into the lenses?

Well ... Hows that for your answer. I learned something too .. every body did - or at least refreshed something. Thats what makes life interesting . Learn something new every day