prism location progressive lenses

**Scrat** · 02-08-2007, 04:24 AM

Dear all,

Is there a "rule" for the location of the 0 diopter prism point in a progressive lens? we have some lenses here with the 0 diopter prism point in the near-point of the lens. is this always the case or are there also design with the 0 prism point in the far-location of the lens.

thanx in advance for helping me out on this.....!

**shanbaum** · 02-08-2007, 08:07 AM

The Prism Reference Point on a progressive addition lens is the midpoint between the semi-visible engraved marks. It's initially indicated in ink by a dot (at least, that's true of the vast majority of manufacturers).

**Scrat** · 02-08-2007, 08:53 AM

Dear Shanbaum,

thanks for the answer! perhaps i didn't explain myself well enough: I actually meant the point in hte progressive lens where the prismatic power equals zero. the lens I have here has this point somewhere on the near vision area, but i wondered whether all progressives have this point of least prismatic power in the ADD region? does this vary from design to design or is there some rule involved?

thanks again!

**shanbaum** · 02-08-2007, 09:53 AM

There may not be such a point - one where the prism power equals zero. What you really want to know is the prism at the prism reference point, and whether that comports with the amount specified on the Rx (plus any prism applied for thinning).

p.s. I should add - if an Rx specifies zero prism, and no prism thinning is applied, you should theoretically find zero prism at the PRP. You usually won't; barring extraordinary efforts, progressives will tilt slightly in the blocking ring, inducing some amount of base down prism (3/8 diopter is typical). That's usually masked by prism thinning.

**Scrat** · 02-09-2007, 02:43 AM

You talk about base down prism induced by prism thinning. are there also designs around that have a base up prism due to prism thinning or is this a very rare situation. My optician told me that there are cases depending on the grinding phylosophy of the lab and depending on the design of the lens.

what's your opinion on this?

**shanbaum** · 02-09-2007, 08:36 AM

Originally Posted by Scrat

You talk about base down prism induced by prism thinning. are there also designs around that have a base up prism due to prism thinning or is this a very rare situation. My optician told me that there are cases depending on the grinding phylosophy of the lab and depending on the design of the lens.

what's your opinion on this?

For base up prism to afford a thinner progressive lens would require an unusual combination of factors, including a low add power and a very low fitting height in a very tall shape. That's unlikely to be a good idea very often.

**fistkuen** · 02-16-2007, 11:40 PM

:hammer: Hi everyone,

I came across a pair of PAL that the prism reference point showed no vertical prism difference between the right and left lenses. But there showed nearly 2 vertical prism difference at the near rings of the PAL. Is that ok? I heard that it is fine if the vertical prism between both eyes less than 1.5 prism at the prism reference point, is that still ok if we found prism at the near portions?

**shanbaum** · 02-17-2007, 09:41 AM

Originally Posted by fistkuen

:hammer: Hi everyone,

I came across a pair of PAL that the prism reference point showed no vertical prism difference between the right and left lenses. But there showed nearly 2 vertical prism difference at the near rings of the PAL. Is that ok? I heard that it is fine if the vertical prism between both eyes less than 1.5 prism at the prism reference point, is that still ok if we found prism at the near portions?

You are probably confusing the nominal maximum imbalance at near - as a general rule, 1.5 prism diopters imbalance is the amount at which one would consider bicentric grinding - with the maximum imbalance at the PRP, which the ANSI standard limits to 0.33 prism diopters, or the amount of prism induced by 1.0mm off-centration, whichever is greater.