Cylinder Power in the 180-degree meridian

[Bob Price]

Darryl,

I brought a couple worktickets home tonight, to work on you're latest post, and stummbled across something I never realized (or maybe still don't). The Rx is a CR-39 ST35 +2.00 -100 Ax 70 OU. The OC needs to be moved in 3.5mm and down 2mm. Right eye calls for prism of .45 @322, the left eye .68 @ 226. If I use the formulas that we have been discussing in this thread, I would come up with the same result. In examining the workticket you can plainly see that the OC is traveling differently thru the cylinder of the right lens compared to the left lens. If the left eye axis was 110, I would imagine that the prism amount would be the same.

Sorry to bring up something else when I don't even have a handle on our main topic, but I wanted to get yours and everyone elses opinion.

[Bob Price]

I must have blown it and you guys are scratching your heads thinking, "This guy is an idiot." In regards to my last post, does the formula I'm using take this into account and I'm not seeing it? Also, I think that as the OC height changes a different angle is created in relation to the cylinder and needs to be taken into account.

Maybe this is what your saying about learning the basics first, because I don't know the inner workings of the formula yet, to know everything it is figuring.

[Darryl Meister]

Hi Bob, I won't have time to go through the math for a bit, but you are flipping the sign of the decentration between the right and left lenses, right?

[Bob Price]

I will be now. Thanks.

[Bob Price]

I spent some time yesterday going over my inconsistent results. It all boiled down to not having the formula plugged into my calculator correctly. I went thru the formulas piece by piece and got much better results. Another missing piece was the prism index, which I'm pretty sure is set to 1.498.

I did notice more inconsistencies in the higher powers and am wondering if this is because of a possible "prism rounding" in the software? Also, when dealing with a plano, my calculations call for no prism when there should be around .05. Any suggestions would be greatly appreciated.

Darryl mentioned using the proper sign convention for the decentration between the right and left eyes. Is this just for the vertical? For instance, if the bifocal is 6mm down and the difference between the seg ht and OC is 3mm, I would use a -.3 for the left eye but not for the right? A more in depth explanation of this would also be greatly appreciated.

Thanks.

[Darryl Meister]

Darryl mentioned using the proper sign convention for the decentration between the right and left eyes. Is this just for the vertical?

No, it's just for horizontal.

[lensgrinder]

Darryl mentioned using the proper sign convention for the decentration between the right and left eyes. Is this just for the vertical? For instance, if the bifocal is 6mm down and the difference between the seg ht and OC is 3mm, I would use a -.3 for the left eye but not for the right? A more in depth explanation of this would also be greatly appreciated.

Bob,
If you think of a coordinate system with 0 degrees to the right 180 degrees to the left 90 degrees at top and 270 degrees at the bottom. Now think of two of them side by side like you were looking at the person. On the left you have your right eye and on the right you have your left eye. When you move the OC in or towards 0 for the right eye you are moving in the positive direction. When you move out or towards 180 you are moving in the negative direction. The opposite applies to the left lens, except when you move toward 90 or up then you are moving in the positive direction and the same applies to the right eye. When you move a left lens in and down (towards 180 and towars 270) you are moving both in the negative direction. I hope this helps.

[Bob Price]

Also, when dealing with a plano, my calculations call for no prism when there should be around .05.

I'm thinking that if I were to figure the power, in these formulas, using the radius of the front and back curves (combined with thickness and index of refraction), then when figuring for prism it would take into account that it is not a true plano and that prism needs to be ground for decentration.

I've been messing around a little bit with this tonight, but would like to be shut down, if I'm way off base again, before I waste a bunch of time.

[lensgrinder]

No, it's just for horizontal.

Why do you not use negative numbers in the vertical?

[Darryl Meister]

Why do you not use negative numbers in the vertical?

You do use positive and negative numbers for the vertical meridian, but this convention doesn't change between the right and left eyes (as you noted in your earlier post). For horizontal decentration, on the other hand, the sign convention is flipped between the two eyes.

[lensgrinder]

You do use positive and negative numbers for the vertical meridian,

OK, good. I just wanted to make sure I was not loosing my mind. I misunderstood when you said it is just for horizontal. Thanks for clearing that up.

[Aarlan]

This is on the ABO, right?


AA

[Darryl Meister]

This is on the ABO, right?

Not this kind of stuff. You may need to know that a lens with cylinder power has no cylinder power along its axis and maximum cylinder power at 90 degrees away from its axis, but that's about it.