NPD determination in FT reading segs

[Hayde]

I need some lab guys to level with me.

Mine is telling me with a straight face to just measure corner-to-corner of the FT reading segments to "verify" NPD values and disregard where I'm dotting up the OC of the reading segment. Is there some obscurity of advanced optics that makes either my lensometer or prentice's rule unreliable in a reading seg, at any power range? Is there some esoteric caveat in ANSI that says FT reading segs are inscrutable for horizontal imbalance?

Assuming a standard line placement at the limbus, am I incorrect that the vertical fix for such a read should be 4-5mm below the line (just in case an oblique axis makes it trickier to lock onto?)


I'm only talking about horizontal prism. I'm not talking about vertical prism, image jump, or which way to trap a sagittal reflection.

[JGor]

Yep, that is how NPD for a FT is checked. The temporal corner of one seg to the nasal corner of the other. Alternatively measure from the geometric center of one seg line to the geometric center of the other.
The optical center of your near portion will be dependent on the OC and Rx of the distance portion as the distance and near OCs are not ground independently of each other.

[Hayde]

Yep, that is how NPD for a FT is checked. The temporal corner of one seg to the nasal corner of the other. Alternatively measure from the geometric center of one seg line to the geometric center of the other.
The optical center of your near portion will be dependent on the OC and Rx of the distance portion as the distance and near OCs are not ground independently of each other.

Hi JGor, and thanks for the response!

So I catch an OC in the reading 6mm off the NPD value requested and also off as much as its counterpart lens that nailed the target. (Similar cyl values and both axes <10 degrees off horizontal meridian.) The Px comes back reporting symptoms consistent with BO prism in same eye. I induce that that's not a coincidence. Since this occasion I've been looking hard at FTs; a majority of jobs I get in the meantime have reading OCs within tolerance of the NPD values requested, and with powers strong enough in those segs to make such a measure meaningful. I don't feel inclined to write off the lenses that don't.

Are you suggesting I'm in error expecting that to be the case for the rest? If so, what's the optical math justifying ignoring the issue altogether? If not, how does one determine what one 'should' see inside the FT seg to ensure proper binocular function--and most importantly the risk of its absence.
,
Thanks again for chiming in and letting me pick your brain.

[Lensman11]

The NPD is checked by measuring the physical location of the seg. There is nothing the lab can do to move the resultant oc location within the seg. The only time the oc will be where you would like it to be is if the lens is Plano in the distance. When you dot the oc in the seg it is the resultant of the the distance rx oc location vs the power of the seg. This posed many problems with exec style lenses when dispenses had no boundary to check and liked to dot the oc. If you have problem with a patient you might have to order different physical seg location of the seg and not place it at the measured near PD

[Hayde]

The NPD is checked by measuring the physical location of the seg. There is nothing the lab can do to move the resultant oc location within the seg. The only time the oc will be where you would like it to be is if the lens is Plano in the distance. When you dot the oc in the seg it is the resultant of the the distance rx oc location vs the power of the seg. This posed many problems with exec style lenses when dispenses had no boundary to check and liked to dot the oc. If you have problem with a patient you might have to order different physical seg location of the seg and not place it at the measured near PD

Thank you! This makes sense. I see my logical error here.

Is there any valid way to anticipate problems based on Rx and measurement info? We have a conservative earmark of 2D/cm for anticipating slab offs with regard to vertical imbalance in multifocals; is there no similar calculation with regard to horizontal imbalance to alert us to the risk in a FT surfacing?

And also, I still can't believe there's no way to determine if there's some kind of surfacing error within a FT seg that measuring corners wouldn't catch? Are we assuming no such error could exist so long as the powers check out? How safe is that assumption, really?

Much appreciated!

[edit to add: JGor also spelled this out in marvelous brevity, but I hadn't have enough coffee yet to absorb it.]

[Lensman11]

Don’t go looking for for problems where they don’t exist. It is extremely rare for a patient to have a problem from this phenomenon. When it happens deal with it on that particular order.

[Hayde]

Don’t go looking for for problems where they don’t exist. It is extremely rare for a patient to have a problem from this phenomenon. When it happens deal with it on that particular order.

Well let's go back to my anecdote--how would I even confirm the problem at all?
His remake came back with the apparent OC looking very differently located inside the seg (with near-enough identically displaced reading segs.) No further complaint. I can accept my diagnosis of the evidence was not logically valid, but I'm still not convinced the conclusion was wrong.
If it exists for him, I'm sure it exists for others.

(As a point of philosophical dispute: I find optics reminds me of an obscure comic book villain called 'The Quiz': she has every super power you hadn't thought of yet. Beating The Quiz requires anticipating every problem you can imagine, because it's the ones you didn't think of that end up asserting themselves...)

[Lensman11]

The problem absolutely exists but rarely requires action if it does just move the sets in or out from their current location to create the necessary prism to solve the problem. You might have an odd looking pair of glasses with the sets located not where you ordered one might be in the other out but that is the only way to counter the induced prism.

[Hayde]

The problem absolutely exists but rarely requires action if it does just move the sets in or out from their current location to create the necessary prism to solve the problem. You might have an odd looking pair of glasses with the sets located not where you ordered one might be in the other out but that is the only way to counter the induced prism.

How would I gauge magnitude of the correction?

[Lensman11]

Put them in the chair and over refract to determine the amount of needed prism.. take the finished specs to the LENSOMTER and how many mm you have to move the seg to create the desired prism and reorder with the new NPD.

[ChrisBowers]

Resultant Prism for Near PD for Optical Purist
I remember doing this once in school from a tutorial, in a B&L Job Coach. Distant power in the 180 times inset /10= Error. Use Prentice prism rule to figure out the near power decentration to eliminate error. Assumed that the MRP is at pupil center, else we get into complex math. With that said I never used this method in 43 years. Nobody died or went blind.

[JGor]

Two other potential issues that may cause problems for NPD in a FT.
1) The lens blank was blocked incorrectly prior to surfacing. As the lens is ground with the distance optical center as the reference point a tilt in the blank may induce prism in the reading portion without being overtly obvious in the distance portion. Again, the magnitude of the issue will be dependent on the Rx involved.

2) This one is rare but with molded blanks there is a chance of the mold being faulty. If you start to get a particular add/base curve combination that keeps presenting with the problem that could be the culprit. As I said this would be rare.

[Hayde]

Thank you all! This was gold.