I guess the acronym ERP didn't please the elves...Originally Posted by MakeOptics
I guess the acronym ERP didn't please the elves...
Best thread I've had the pleasure of reading in a long time. Thanks for everyone's input.
Ok, I like to clarify, but I will muddy.
1. On a SV lens, the MRP (which is defined as the point at which the prescribed vertical prism is measured [and horizontal but let's not go there][and it equals the optical center in jobs without prism) is 3-4 mm below the optical axis in straight-ahead gaze (that is, where you "dot" the pupil). TRUE OR FALSE?
2. The reason we drop it has nothing to do with prism. It's about eye rotation and oblique error and head-splodin' stuff. TRUE OR FALSE? (True)
3. (The big question) (In non-plano lenses or non-plano in a meridian lens) there is a not-intangible amount of "unprescribed/unwanted" prism that is measured at that pupil center. TRUE OR FALSE?
Sheedy's study on Yoked prism remains very important, where he determined that even yoked prism as low as 2D can induce loss of DVA in some patients, but by 4D it reaches 100% of patients experience visual loss. We have assumed for eternity that Yoked prism is benign, and that is not true. (Yoked prism is where both eyes have the same power and direction, say 1D BD OU).
Progressives create their own yoked prism relative the add power. As stated well above this is contradicted in myopes and exaggerated in hyperopes. However, we are adding about 1.5D of yoked prism, to prism relative to the add power, to prism created off center of the lens naturally relative to the distance power. The transition zones that link the distance and reading outside the umbilic also create unwanted prism, non yoked. Add to that cyl off center creates its own prism, and you simply get a prismatic mess... and many more patients, esp in hyperopes, that suffer from loss of both Near and Distance Visual acuity than we realize.
Essentially traditional progressives have 3 key zones, each with their own set of issues. 1) distance reference point near OC that is effected by both prism thinning, corridor yoked prism, and spherical/cyl induces prism, because its not really the optical center of the lens. A reading reference point that is effected even more than all of these. And a Prism reference point that is still effected by corridor prism, but at least offers neutralized prism thinning and spherical prism, however this most often the narrowest part of most lenses.
Add to this the near inset induces lateral prism of its own, that negatively effects the entire corridor.
yea, progressives are a mess.
. Since Prism thining itself can be about 1.5D at Distance Reference Point
1. The PRP for a single vision lens is the essentially the distance IPD. If I need to determine the amount of wanted or unwanted distance prism on a pair of eyeglasses, I'll locate the visual axis in the primary gaze, mark the lens at that point with a dot, cross, or plus sign, and measure the degree of prism with a lensometer. Pupil height will depend on lens design, tilt, Rx, anatomy, etc.
2. True.
3. See http://www.opticampus.com/tools/ansi.php
The concern is not with VA (although chromatic aberration can cause reduced VA if there's enough prism combined with a low Abbe value), instead it's the change in spatial perception, the feeling like we're walking uphill for instance (base down prism), that requires adaptation or results in rejection.
http://www.ncbi.nlm.nih.gov/pubmed/3684280
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
I'll take a shot at these
1. maybe true for a reading rx, false for a distance rx
2. maybe true for some "we"s , not for this "we"
3. true. because "Not-intangible" means tangible, or real, and even zero (0.00) is a real number and is measurable.
I do think that VA drops with prism, entirely dependent upon chromatic aberration/dispersion index. But probably not enough to matter.
I thought, Sharpie, that prism thinning is for just that...thinning. Not to offset some "PAL-induced" prism. Right?
1,2. Dropping OC to 3-4mm below pupil center is for ocular rotation. It may apply somewhat less in reading glasses, but I doubt it.
Robert, I wasn't talking about manufacturing tolerance, but simple Prentice's rule.
e.g. a +10.00 DS SV lens with a level MRP/optical center placed 3mm below the pupil center/optic axis in straight ahead gaze for 6 degrees pantoscopic tilt.
(+10.00 D) x (0.3 cm) = 3 PD BD OD and OS.
My question is, essentially: don't we have a trade off in single vision lenses when we specify the level MRP at 3mm below the pupil? We minimize (vertical) off-axis aberration, but we induce a small degree of yoke prism in doing so. (And of course it may be unequal amounts in unequal lens powers, etc.)
Last edited by drk; 07-31-2015 at 02:32 PM.
Understandable, but I don't care for one size fits all thinking. I mean I've got lots of patients who rotate their eyes superiorly as often or more often than they do inferiorly (thinking about electricians, airline pilots, theater goers, bicyclists, etc) and in strong Rx's this is not a moot point.
Kind sir, pantoscopically tilting the lens mandates that you drop the MRP for all directions of vertical gaze, both up and down.
Think this way: with no tilt, the lens would be close in upgaze and far in downgaze and induce error. Pantoscopically tilting makes the lens equidistant in all directions of vertical gaze. But that tilts upwards the optical axis of the lens, so you have to drop the optical center a few millimeters to compensate.
It's all about looking up and looking down.
(And what's more, even in SVNO you depress your eyes, not your neck. Heck, even in a computer SV lens you have to peek at your keyboard once in awhile.)
Don't fossilize, doc!
[QUOTE=drk;510610]
Think this way: with no tilt, the lens would be close in upgaze and far in downgaze and induce error. Pantoscopically tilting makes the lens equidistant in all directions of vertical gaze. But that tilts upwards the optical axis of the lens, so you have to drop the optical center a few millimeters to compensate.
[REPLY] The first part of this sentence is almost true, the 2nd part is false except in one extreme case, as simple geometry will show. Consider the case of a flat inside curve. The corneal apex will always be farther away from the location on the lens surface the eye is directed on upgaze or downgaze, and panto tilt will have only a minimal impact on that fact.
The one time the lens can be equidistant from all parts of the back lens surface is when that back curve is CONCENTRIC with the diameter of the eyeball. Let's see: if we assume an eye with a 22 mm diameter and a vertex distance of 13 mm, the back curve of the lens would have to have a radius of curvature of about 24 mm, which equates to a curve of what, about 14 D.? A very steep back surface indeed, perhaps an ideal lens shape for what you are claiming to accomplish, but not very practical for most frames.
[REPLY} Hey, I often raise or lower the OCHt by as much as 10 mm for different sv near lenses. For a guy on desktop computer all day, I'll raise it up. For someone on a tablet or cell phone all day, I might drop it 10 mm.
Just so they have a minimum of prism at their working position of gaze.
Fossils have had more time to think this stuff through than have mere whipper snappers.
Last edited by Dr. Bill Stacy; 08-02-2015 at 07:55 AM. Reason: CONGRUENT WAS WRONG, CONCENTRIC IS OK
Sorry, the original question seemed somewhat convoluted and I was in a hurry so I tossed out the kitchen sinked. So, yes, there's either non-zero prism or zero prism during the primary gaze depending on the Rx, lens design, fitting values, etc.
BTW, I don't think of Z80 as just a manufacturing tolerance recommendation; if I order a -5.00 OU 66mm IPD and receive 66mm, but when observed in front of the wearer's eyes the OCs are 4mm wide in each eye, the eyeglasses are rubbish! Another reason why opticians must always fit and dispense face-to-face!
Specifically, it's to align the optical axis of the lens with the center of rotation of the eye. Not critical for low powers, but combined with more complex surface designs, that is aspheric/atoric, there can be an unacceptable amount of aberration, especially off-axis.
WRT reading glasses and OC placement, one could argue that to minimize power error and oblique astigmatism, we should use the distance IPD and drop the OC .5mm per one degree of panto tilt, especially with aspheric/atoric surfaces. Hyperopes will encounter some degree of base out prism though, probably more than their habitual eyeglasses if they're presbyopes, but that could be addressed by ordering base in prism. Anisometropes should have the OCs positioned to minimize vertical prism imbalance.
Good point about not dialing in too much panto tilt for these folks- the electrician doesn't like seeing the top of the lens edge or (non-metal) frame on the upgaze.
For the rest of us though, we want to have a certain amount of pantoscopic tilt for cosmetic and functional reasons: the brow extends out from the inferior orbit- the eyeglasses look better when the frame follows this contour, we tend to look down a lot more than we look up so we should tilt in (and out of the way) the bottommost part of the lens/frame, and to minimize the disparity in vertex distance between the primary and downgaze, primarily for multifocal lenses.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Hey Robert, I kind of chopped up that post and this part may have been missed:
[QUOTE=drk;510610]
Think this way: with no tilt, the lens would be close in upgaze and far in downgaze and induce error. Pantoscopically tilting makes the lens equidistant in all directions of vertical gaze. But that tilts upwards the optical axis of the lens, so you have to drop the optical center a few millimeters to compensate.
[REPLY] The first part of this sentence is almost true, the 2nd part is false except in one extreme case, as simple geometry will show. Consider the case of a flat inside curve. The corneal apex will always be farther away from the location on the lens surface the eye is directed on upgaze or downgaze, and panto tilt will have only a minimal impact on that fact.
The one time the lens can be equidistant from all parts of the back lens surface is when that back curve is concentric with the diameter of the eyeball. Let's see: if we assume an eye with a 22 mm diameter and a vertex distance of 13 mm, the back curve of the lens would have to have a radius of curvature of about 24 mm, which equates to a curve of what, about 14 D.? A very steep back surface indeed, perhaps an ideal lens shape for what you are claiming to accomplish, but not very practical for most frames.
[End]
I'm not yet proficient on quoting/not quoting text protocols here. Was wonding what your thoughts are on the above calcs.
Last edited by Dr. Bill Stacy; 08-02-2015 at 07:53 AM. Reason: typo, my bad
So to take this home, I think the prism reference point in a PAL is the same as the optical center of a SV lens...about 4 mm below the pupil center.
Do you get yoke prism effect in straight-ahead gaze with any lens, then? Yes, you do, amount dependent on lens power.
So to Dr. Stacy, if you read your SV lenses in the lensometer the way you read your PALs (where the pupil center is), you'll experience the same kind of prism amount.
Last edited by drk; 08-01-2015 at 03:57 PM.
http://www.opticampus.com/files/pris...ive_lenses.pdf
He's still the man.
Maybe for some jobs. Most of my SV lenses are finished in my office, and if I don't specify OC Ht, the OC's will automatically get decentered vertically by 0.00. For those that I have to farm out, mostly non stock and higher powers, I usually specify OC Hts which is where I will expect to read 0.00 prism O.U. But I will "keep an eye out" for the outliers and will now specify OC Ht on all those Rxs were it matters (like above +or - 1.00 in the vertical meridian). Thanks again for all the posts.
Prism thinning in Traditional Lenses is purely cosmetic, to account for the backward displacement of the Add and thin at the top. Some prism thinning is kept in most free form lenses for adapation issues, but at about half the amount of Traditional. Its need is much less in Free-form, as the Base Curve choice is different.
Great point's no "WE" don't assume that yolked prism is benign. I assume that any prism prescribed or not has a degradation in visual acuity since it is proven not theory like you have pointed out. That was the reason for my earlier post, prism exists in every progressive, FT, multifocal, etc. You cannot make two points on a lens to view through without creating some prism in one or both points. (at least not yet)
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Great thread. Both for the content and especially the demeanor. All of Optiboard should be so pragmatic. Thank-you.
As an addendum, I had a visit from the president of the VSP lab on Monday. He hand delivered a letter of apology to me about the lab ignoring or missing my "no Equithin" on the order, with a hand written "You were right!" on the back side with his signature. And I hear vsp is sending the patient a couple of hundred dollars for his trouble.
I'll continue my push to make equithin an opt in item, or at least a very easy "opt out' option that their computer system cannot overlook. Maybe even a "max" thinning prism amount to select, which can be anything from say 0.00^ to 10^, so if their system calcs 1.5^ and you set max at 2^, they would do their 1.5^, but if you chose 1^ as your max, they would do the 1^. This max would be a default value you could set up, but over-ride on any Rx.
My personal default would be 1^
And I'm thinking any prescribed prism would have to be adjusted for additional thinning prism that is set up or over-ridden...
Last edited by Dr. Bill Stacy; 08-19-2015 at 08:53 AM.
I can't speak for all DVI labs, but ours have always had service codes that allow you to either opt-out of equithinning, or specify the amount of equithinning you want. If you are ordering a one-eye remake, you should ALWAYS specify the amount of equithinning that you want to avoid imbalance.
RT
Great. Is this something that's easy to select, like a dropdown menu, or does the ordering person have to be pretty knowledgeable? Eyefinity sure doesn't have it.
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