Prism in plano lenses

[kisselt]

Could anyone help me understand why plano lenses often have some unintended stationary prism? Even when only one principal meridian is plano in a toric lens there is usually a tolerable amount of prism. I'd think it was a consequence of less than perfect blocking with generated lenses if it wasn't also often present in cast molded FSV lenses. Is it just something that happens because it's difficult and unnecessary to make a lens whose front and back surfaces are perfectly parallel and thus would have no prism? Or is there some optical reason why this prism is inevitable in plano lenses?
Thanks

[drk]

Please help. What's "stationary" prism? Just unwanted prism?

Are you measuring prism through, like, mounted plano wrap sunwear? Because there can be prism induced/compensated in those.

[kisselt]

No not any compensated rx or anything. So there's no reason for there to be any prism. Just a ground lens with no power, or a finished lens with no power in one or both principal meridians. Seems like there's always a quarter or so prism. By stationary prism I just mean uniform throughout the lens so that moving it in the lensometer to try to block the lens where there's no prism proves futile. I'm just wondering why this seems to be present in most plano lenses and sphero cylinder lenses with one plano meridian.

[drk]

Got it.

I don't know the answer.

[uncut]

No not any compensated rx or anything. So there's no reason for there to be any prism. Just a ground lens with no power, or a finished lens with no power in one or both principal meridians. Seems like there's always a quarter or so prism. By stationary prism I just mean uniform throughout the lens so that moving it in the lensometer to try to block the lens where there's no prism proves futile. I'm just wondering why this seems to be present in most plano lenses and sphero cylinder lenses with one plano meridian.

Quality control is simply lacking............prism is prism whether deliberate or accidental. One of the most difficult lenses to create is a plano that is distortion and/or prism free.

Also check your lensometer for accuracy, if the direction of the prism is consistent.

[kisselt]

I can understand that with lenses generated in the lab from semi-finished blanks, because if it's blocked slightly off center or the generator isn't calibrated properly that can lead to things like slight degrees of prism. But why is it also often present in cast molded lenses that are mass produced from injection molds in outside labs from places like essilor and other reputable manufacturers, you'd think they'd be able to make plano lenses consistently without any prism. So it's just a matter of the difficulty of making a plano lens with no prism, and the fact that producers are allowing less than perfect, yet within tolerance, lenses pass their inspection? Oh and I don't think it's the lensometers because I've noticed it in more than one lensometer and the prism isn't consistent, just consistently within tolerance.

[Quince]

The lower the power in the lens, the more it may feel like you need to 'decenter' the lens to get it blocked just perfect. Sometimes, we block them slightly not perfect because it seems it won't make cut-out due to how off-center the area of perfect power is.

I am under the understanding that in finished lenses this is just a matter of dealing with imperfection within tolerance. Obviously, if the strength is out of tolerance the lens should be replaced by the manufacturer with a better lens. I haven't had any issues with planos before- who are you getting them from?

Semi-finished lenses have the same variations though. Even though it is the exact same lens, pulling two and measuring may provide slightly different thicknesses and curves. It is the curse of inspecting lenses to the minute detail we do- we will always find dissimilarities.

[lensmanmd]

Could anyone help me understand why plano lenses often have some unintended stationary prism? Even when only one principal meridian is plano in a toric lens there is usually a tolerable amount of prism. I'd think it was a consequence of less than perfect blocking with generated lenses if it wasn't also often present in cast molded FSV lenses. Is it just something that happens because it's difficult and unnecessary to make a lens whose front and back surfaces are perfectly parallel and thus would have no prism? Or is there some optical reason why this prism is inevitable in plano lenses?
Thanks

Planos and low powers are difficult to produce with perfectly parallel surfaces, even with casting.

Its not just quality control, as there are multiple factors involved. I don’t cast lenses, so I cannot answer that part, but many low power FSV from VE, Zeiss, Essilor, et al, have some induced prism. Off brand lenses have even more issues.

Surfaced lenses can suffer from unwanted prisms from multiple sources. Mostly from calibration, but dirt and swarf can also play a role.

First place is in blocking.
1. The blocker is not properly calibrated for a perfectly flat surface.
2. The block is not properly cleaned, resulting in a slightly raised surface on one or more sides, either on the front or back (or both)
3. The operator does not allow the medium to properly set before removing the lens from the blocker
4. The surface saver has wrinkles
5. The medium is not fully cured prior to generating
*medium is alloy, wax, onyx, etc.

Generator
1. Not calibrated
2. Chuck face is not clean. Dirt/swarf is present
3. Operator does not insure that the block is seated properly in the chuck. This usually creates prisms outside of standards, but not always

Toric surfacers
1. Calibration. Strokes/pattern is off, not properly centered, creating offset stock removal.

These are the most common issues related to unwanted prisms, but not all. Even with strict quality control, most labs will allow a small amount of unwanted prism in uncuts, even with the strictest quality and maintenance controls in place.

Not to mention that many practices use lensmeters that are not properly calibrated, as well.

[kisselt]

Thank you, that's an excellent, comprehensive answer. I'll experiment with our process to see if I can isolate any particular reason why there might be small amounts of unintended prism. It's always within ANSI standard but still bothers me.

[kisselt]

Mostly essilor fsv lenses. But again it's never been out of tolerance. They've always been below .33 prism diopter. Also, I hate poly too, I'm wearing crown glass at the moment.

[Don Gilman]

Mostly essilor fsv lenses. But again it's never been out of tolerance. They've always been below .33 prism diopter. Also, I hate poly too, I'm wearing crown glass at the moment.

In my experience, almost 35 years running a wholesale lab, and 25 years selling our FastGrind in office surfacing system, the most errors, except for human errors ,are unwanted prism over tolerances. Don't loose sleep over it.

[rbaker]

Why are you losing sleep over a variance that is within tolerance?

[Uilleann]

We had some seriously bad Chemistrie "plano" yellow tinted computer clips come in about a month ago. More than a diopter in each lens. They got sent back straight away....

[kisselt]

Thanks for all the input. I'm not bothered by it really; if something is within tolerance I know there's the tolerance limit for a reason. I just want to get a more comprehensive knowledge about optics and I think learning about what causes subtle variations can help understand the big picture better.

[lensmanmd]

We had some seriously bad Chemistrie "plano" yellow tinted computer clips come in about a month ago. More than a diopter in each lens. They got sent back straight away....

Some BI^ is actually good for these, but not a full D!

[Quince]

Thanks for all the input. I'm not bothered by it really; if something is within tolerance I know there's the tolerance limit for a reason. I just want to get a more comprehensive knowledge about optics and I think learning about what causes subtle variations can help understand the big picture better.

I think that is fantastic reasoning. This is exactly the place for those kinds of questions- and as you can tell, there are a lot of really knowledgeable folks on here to chime in (even if they don't always agree!)

[Quince]

Some BI^ is actually good for these, but not a full D!

Like the Gunnar gaming glasses. I think they add about a third of a diopter.

[lensmanmd]

I think that is fantastic reasoning. This is exactly the place for those kinds of questions- and as you can tell, there are a lot of really knowledgeable folks on here to chime in (even if they don't always agree!)

+1

[rbaker]

Like the Gunnar gaming glasses. I think they add about a third of a diopter.

Physiologically undetectable !

[Uilleann]

Some BI^ is actually good for these, but not a full D!

Like the Gunnar gaming glasses. I think they add about a third of a diopter.

They had about a +0.60 diopter of magnification, but no prism that I was ever aware of or told about by their silly marketing people. Perhaps some natural base in for their 9 base wraps just to keep people from vomiting on their shoes...but nothing else.

The Chem clip thing was bizzare, in that not only was there a boatload of it in otherwise plano lenses, but that the directions were all skiwampus too.

[ThatOneGuy]

Why are you losing sleep over a variance that is within tolerance?

I hope that was tongue in cheek.

[Prentice Pro 9000]

Could anyone help me understand why plano lenses often have some unintended stationary prism? Even when only one principal meridian is plano in a toric lens there is usually a tolerable amount of prism. I'd think it was a consequence of less than perfect blocking with generated lenses if it wasn't also often present in cast molded FSV lenses. Is it just something that happens because it's difficult and unnecessary to make a lens whose front and back surfaces are perfectly parallel and thus would have no prism? Or is there some optical reason why this prism is inevitable in plano lenses?
Thanks

Without reading the other comments: I think that, because refraction is occuring(regardless of the lens being plano), then prism will occur as well. I have noticed this with flat pieces of glass as well.