Prism in progressives

[April_01]

I know this will be hard to explain without demonstration, but is it possible to check the amount of prism in a progressive on a lensometer, and how do you do this? Also, I have heard of checking for 'prism' at the dot below the cross on the progressive marking, is this true?

[CME4SPECS]

Yes, you can and should check the prism at the dot below the cross.

[shanbaum]

Yes, you can and should check the prism at the dot below the cross.

Also known as the Prism Reference Point - which is where you "check the prism" on any lens.

[spazz]

This is the only way to get a true prism reading in a pal. :p

[April_01]

I'm sorry but checking prisms really confuses me. So if there is prescribed prism in the progressive lens, you center it as best you can and if it has 2D of BU prism, then the prism should be in the second reticle circle(ontop) and if you move the prism dial on the lensometer 2D in the opposite direction it should center it in the middle? Am I completely off or anywhere near the correct answer.

[shanbaum]

I'm sorry but checking prisms really confuses me. So if there is prescribed prism in the progressive lens, you center it as best you can and if it has 2D of BU prism, then the prism should be in the second reticle circle(ontop) and if you move the prism dial on the lensometer 2D in the opposite direction it should center it in the middle? Am I completely off or anywhere near the correct answer.

You have a lensometer with a prism dial?

[edKENdance]

If you are using a prism ring, take it out when you are checking for prism at the PRP.

[Blake]

Seems like the Marco (?) lensometers I used a few years ago had a prism dial near the eyepiece. It would only measure up to 5 or 6 diopters though.

[Darryl Meister]

To make a long story short, center the Prism Reference Point (that dot under the fitting cross) of the first lens over the lens stop of the instrument. Note the amount of prism. Next, center the Prism Reference Point of the second lens over the lens stop. Note the amount of prism. The difference between the two prism readings is essentially the amount of prism, whether prescribed or unwanted.

Remember that, for vertical prism, like prism base directions cancel (e.g., OD 1.0 base up and OS 2.0 base up = OS 1.0 base up). For horizontal prism, like prism base directions add (e.g., OD 1.0 base in and OS 2.0 base in = OU 3.0 base in).

Best regards,
Darryl

[Darryl Meister]

Seems like the Marco (?) lensometers I used a few years ago had a prism dial near the eyepiece. It would only measure up to 5 or 6 diopters though

That was probably a built-in compensatory prism meant to allow you to measure greater amounts of prism by neutralizing some of it for you.

Best regards,
Darryl

[drk]

I have said Marco lensometer, and it has a risley-style variable prism that allows neutralization of prism in whatever direction. Goes to 15 p.d. Nice.

[Robert Martellaro]

April,

Another interesting anomaly concerning prism and the OC_fitting cross position is the a tendency for the eye to rotate towards the apex of the prism by about .3mm per prism diopter. For instance if there is 5^ BI OU one might want to widen the PD 1.5mm per eye.


Robert

[mc]

April, maybe no one on here really knows the answer to your question.......yes, you are correct.:)