pal pt w/ 15Δ BO

[optical24/7]

Well, I supposed it would depend on what kind of blocker you have. Using my old ICE-1000 in passive mode, or using my new-ish EZfit I don't compensate anything to get the PDs to come out correctly. I can't speak for other blockers though. But that's not the point. The point is, the lab is going to send you what you ask for. How they do it, with their specific equipment is irrelevant. You need to order what you want for a finished product and check said product before dispensing.

There are unfortunately 2 different discussions happening in this thread. One about how to properly block a lens to get the desired results. Another about altering the PDs(or fitting heights) for PALs in order to maintain proper alignment with the corridor and near zone, when prescribed prism is present. If you have a PAL with 30/30 PD and 10 BO in each eye, you need to order with PDs of 27/27.

^^Yup.

[Prentice Pro 9000]

Well, I supposed it would depend on what kind of blocker you have. Using my old ICE-1000 in passive mode, or using my new-ish EZfit I don't compensate anything to get the PDs to come out correctly. I can't speak for other blockers though. But that's not the point. The point is, the lab is going to send you what you ask for. How they do it, with their specific equipment is irrelevant. You need to order what you want for a finished product and check said product before dispensing.

There are unfortunately 2 different discussions happening in this thread. One about how to properly block a lens to get the desired results. Another about altering the PDs(or fitting heights) for PALs in order to maintain proper alignment with the corridor and near zone, when prescribed prism is present. If you have a PAL with 30/30 PD and 10 BO in each eye, you need to order with PDs of 27/27.

yes yes, there are two discussions happening. So yes, it would be MY responsibility to ask for a correct PD based on the rules laid out in Systems. It’s then the lab’s responsibility to make that PD wether using compensation or not to make it come out correctly.

[Kwill212]

yup

[Prentice Pro 9000]

Technically you need to know the stop distance if you want to use the exact number, which is going to vary by patient.

Is the stop distance the length from the lens to the camera?

[Kwill212]

Stop distance is the measurement from the back of the lens to the center of rotation of the eye. In the comment above made by Elvis which claimed .27 is the exact number, that would assume a stop distance of 27.75mm. Which would be quite a precise measurement, but I don't see how they would possible get it. They most likely meant that most ophthalmic texts assume a stop distance of 27mm, which would result in an "exact" compensation of .26mm. Systems assumes a stop distance of 30mm, and rounds the .29 result up to .3 for simplicity. Tscherning's ellipse that we are all familiar with assume ~27mm stop distance. Though you could plug whatever number you wanted into the equation and end up with a slightly different ellipse.