prescribing prism by decentration. how to calculate?

**Win C** · 09-23-2009, 01:47 AM

Hi

I have the following prescription :

R-3.00/-1.75x63
L-4.50/-2.00x15
PD63
Prism 2 baseout R and L

can anyone help me with how much to decentre to get the required 2 prism base out?

I used the prism calculator from opticampus for calculation and there is vertical prism of about 0.2 to 0.3 base up after decentration presumably from the cylinder. Do i just ignore the vertical induced prism since they are minimal and about the same in both eyes?

**HarryChiling** · 09-23-2009, 07:20 PM

.........

**Win C** · 09-23-2009, 11:00 PM

Hi Harry,

thanks! I will try to go thru that thread and see if I can work it out. If not I guess I have to send the job to the lens and let them work it out. :o

**Win C** · 09-23-2009, 11:22 PM

There is actually a very quick and easy way to decentre the above lens (single vision stock) to achieve a 2 BO prism without having to do complex calculations (found this out from the lab).

Just simply put your lens algining it to the axis you want into the the digital lens meter until you locate the position where it shows 2 base out in the read out and then mark it.

Why didn't I think of that!:o

**IndianaOD** · 09-24-2009, 07:52 AM

Decentered prism is not equal to ground in prism IMHO

**Jacqui** · 09-24-2009, 09:15 AM

Originally Posted by IndianaOD

Decentered prism is not equal to ground in prism IMHO

I agree, but others will argue the point.

**HarryChiling** · 09-24-2009, 09:18 AM

.............

**Jacqui** · 09-24-2009, 09:57 AM

Originally Posted by HarryChiling

Why not?

I wear prism (at read only) and it doesn't seem the same to me. I know in theory it is the same, but not to me. This is why I'll make Franklins for myself rather than decentering an exec.

**HarryChiling** · 09-24-2009, 05:52 PM

............

**Jacqui** · 09-24-2009, 06:18 PM

Doesn't seem the same. I use prism blocking.

**chip anderson** · 09-24-2009, 07:02 PM

I don't ever think I have ever heard an "optical expert" including Russell Stimson and Ralph Drew who felt there was a bit of difference in decentration prism and ground in prism. At least if one is dealing with spherical lenses.

Time for Der Meister to chime in.

Chip

RT · 09-24-2009, 07:13 PM

You know when it's ground in the generator/software is moving the optical center, unless your still using the old prism rings.

Not true. Having worked on the software for computerized generators, I can absolutely guarantee you that there were separate routines for decentration and prism.

**Jacqui** · 09-24-2009, 07:18 PM

What I am trying to say is: prism by decentration may be the same as surfaced prism in theory, but it don't look like it when I wear them.

**IndianaOD** · 09-24-2009, 07:56 PM

At primary gaze it may be effectively the same. What about when you look out of primary gaze?

**chip anderson** · 09-24-2009, 09:54 PM

If you turn a Baby Ruth upside down, it still tastes the same.
There is no difference. A diamond is a diamond whether made by tectonic pressure or in a monster hydrolic press. Prism is just a decentered lens no matter how created.

Chip

**mshimp** · 09-25-2009, 05:26 AM

ground in or decentered it's all the same! Only exception would be on weak powers. Not enough power to induce the prism you have to grind the prism in. Standard Prentice rule. Of course we are talking about SV.

**obxeyeguy** · 09-25-2009, 05:54 AM

Originally Posted by Jacqui

What I am trying to say is: prism by decentration may be the same as surfaced prism in theory, but it don't look like it when I wear them.

Jacqui, try using a FT-45 and use an inset of say 6 when surfacing. Decenter lens for edging layout using distance OC, voila, with a +2.50 add, 1 diopter base in seg only. You can't do that with an exec, as you only really have one OC.

RT · 09-25-2009, 06:33 AM

Prism is just a decentered lens no matter how created.

Tell me how far you should decenter a plano power lens to get 2 diopters of prism.

**gemstone** · 09-25-2009, 07:08 AM

Originally Posted by chip anderson

I don't ever think I have ever heard an "optical expert" including Russell Stimson and Ralph Drew who felt there was a bit of difference in decentration prism and ground in prism.

Chip

But it's still fun to read the opinions. I can't believe this is still argued.

**mshimp** · 09-25-2009, 08:13 AM

Originally Posted by RT

Tell me how far you should decenter a plano power lens to get 2 diopters of prism.

see my post for the exceptions to the rule.

**Chris Bowers** · 09-25-2009, 01:32 PM

RT that depend on the lens index

**Jacqui** · 09-25-2009, 03:15 PM

Originally Posted by obxeyeguy

You can't do that with an exec, as you only really have one OC.

HORSE HOCKEY !!! I do it quite regularly with execs and it works out rather nicely. And what's this garbage about only one OC?? Mine always have 2 or 3.

**Jacqui** · 09-25-2009, 03:18 PM

Originally Posted by IndianaOD

At primary gaze it may be effectively the same. What about when you look out of primary gaze?

That's what I've been talking about. Out of the primary area things are different. I'm a speed reader and use much more area that the average person.

**optical24/7** · 09-25-2009, 06:16 PM

Jaq, tell me how you could tell a ground from a decentered prism....(if you didn't make them)?

**Jacqui** · 09-25-2009, 07:31 PM

I can't really explain it, but it just seems different. IndianaOD asked if the outer areas were different and I must say yes deffinately. Theory says that it's the same, but I think there is a difference somewhere in the lenses.