semi-stupid question

[hawaiioptician]

So heres a quickie for you gurus:


Was at work checking power in a PAL lens, and saw about a diopter, diopter and a half of horizontal prism in the lens. Power, right on. I ask the other guy im working with to take a look, he raises the frame to look the MRP, and says:

"you always check for prism on a progressive at the prism point"

me: "but isn't that only to verify prescribed prism, or the thinning prism"

"no, all prism"

So heres my (semi) stupid question: how does this make sense? My understanding of this is as follows: we verify prism at the MRP because thats the location that is free-est from thinning prism, so this location will show any prescribed prism the most effectively.

Any prescribed prism would be more difficult to check anywhere else in the lens, because of the effect of the thinning prism, which may compound or cancel any prescribed prism in full or in part. I assume some of you could probably verify the prism AND the rx in the distance zone, based on experience in gaging the amount of thinning prism used per the manufacturer and in relation to the RX itself.

What i dont get (in this case, and i suppose in general) is why i would verify power where the eye sits, but not prism in the same location. this seems to fly in the face of all i thought i knew about lenses. Prism is the same thing as power, its just in one direction rather than multiple ones.... so if theres a D and a half of prism where the PT actually looks through the lens, but not 3-4 mm below that, we pass the lens as good?

somebody help me here.

thanks

Z

[Robert Martellaro]

So heres my (semi) stupid question: how does this make sense? My understanding of this is as follows: we verify prism at the MRP because thats the location that is free-est from thinning prism...

It (MRP or PRP) is the optical center, where light isn't deviated by lens power, and if there's no thinning or prescribed prism. Use the term "PRP" (Prism Reference Point) instead of "MRP" (Main Reference Point). We measure for prism here (wanted or unwanted) so that any dioptric power in the meridian you're measuring does not influence (induced prism by decentration- see Prentice's Rule) the result.

Any prescribed prism would be more difficult to check anywhere else in the lens, because of the effect of the thinning prism, which may compound or cancel any prescribed prism in full or in part.

It would be difficult to check elsewhere if there is dioptric power (see above). Thinning prism would not generally interfere with prescribe horizontal prism, as well as vertical imbalance, although there will be some degree (the thinning prism value) of compounding or canceling in absolute terms for each individual lens.

For example, Rx is OD 2^ base up OS 2^ base down, with 1^ of base down prism thinning. The OD will read, at the PRP, 1^ base up, the OS 3^ base down. The prismatic effect is essentially the same- 4 prism diopters of imbalance, regardless of the degree of prism thinning.

I assume some of you could probably verify the prism AND the rx in the distance zone...

For PALs, we should verify the distance Rx at the Distance Reference Point, which is somewhat higher than the fitting point. Prism is measured at the PRP.

Hope this helps,