Hello folks, just curious if anyone here decenters SV lenses vertically? I'm not talking only about aspherics, but also simple -250 stock poly for instance.
Regards from NY, where its 62 degrees in mid December
Hello folks, just curious if anyone here decenters SV lenses vertically? I'm not talking only about aspherics, but also simple -250 stock poly for instance.
Regards from NY, where its 62 degrees in mid December
I would keep it within 3 or 4mm below the pupil unless it is aspheric. Depends on the pantoscopic tilt.
Always, and forever, especially poly!
Eyes wide open
I'm doing it a lot especially when I have to use poly for strong lenses, depending on what they are used for. If for distance vision, I'll push it up to the pupil centers, if for near, down toward where the pupil ends up with the primary task. Of course the goal is to minimize vertical prism and its dispersive effects.
Didn't they teach you about pantoscopic tilt, Bill?
Guess not.
Rock on, bro.
Of course, and it has nothing to do with prism induced or reduced by vertical decentration or the lack thereof. Panto only affects astigmatic error that comes from viewing through tilted optics. 2 different subjects.
Wrong.Originally Posted by Dr. Bill Stacy
Although pantoscopic tilt does add some change in the Rx, it also can effect the OC of a lens as well. For example, the OC will change roughly by half of the pantoscopic tilt.
The idea is to have the ray end up over the eyes center of rotation. You can derive this by assuming a 13 mm CoR and a 14 mm vertex for a total stop distance of 27 mm.
9of pantoscopic tilt using the rule of thumb would be 4.5 mm OC difference, i.e. You would lower the OC by 4.5 mm.
or
 \times 27\\ \Delta OC = 4.28 )
Well maybe this a new kind of optics that I'm not familiar with, but I don't think the optical center changes much if any at all with lens tilt. To test this I just now took a +3 trial lens and tilted it and cannot see any shift at all in the position of the optical center by lensometry with up to 20 degrees of tilt. And the fact that your formula doesn't address lens power at all suggests to me the new optics might be voodoo.Although pantoscopic tilt does add some change in the Rx, it also can effect the OC of a lens as well. For example, the OC will change roughly by half of the pantoscopic tilt.
The idea is to have the ray end up over the eyes center of rotation. You can derive this by assuming a 13 mm CoR and a 14 mm vertex for a total stop distance of 27 mm.
9
or
On rereading your post, that last sentence suggests you want to move the OC down by decentration as I suggested, not because the OC in the lens changes by that amount.
Bill, for every 1 degree of pantoscopic tilt, you have to lower the OC by 1/2 mm in order to get the ray going through the optical center and the center of rotation of the eye (or the "sighting intersect" or something like that).
Right. Tilting the lens can misalign the optical axis of the lens with the center of rotation of the eye, resulting in an increase in power and introducing cylinder with a sign equal to the sphere power.
Martin's Formula for Tilt
S'= S{1+(sin a)^2/(2n)}
C'= S'(tan a)^2
S sph power
S' new power
a tilt
n index of refraction
C' induced cyl
Axis is 180 and the sign is equal to the sphere power
For example, with an Rx of -10.00DS with a pantoscopic tilt of 15 degrees, Trivex, and the OC is aligned with the primary gaze. This is not uncommon when the frame is narrow, and rests high along with a high value of pantoscopic tilt. Using the above formula, the power that the eye sees is -10.22 -.73 x 180.
To correct for this error, the OC needs to be lowered 7.5mm (1mm per two degrees of tilt). Another solution is to pick a frame that puts the 180 line and OC about 5mm below the pupil, and reduce the panto 5 degrees.
If a high dispersion lens is used, one might want to reduce the panto so that the OC can be closer the pupil, reducing chromatic aberration on the primary gaze.
Optimized lenses manufactured on a free-form platform allows significant latitude in tilt around both the 90 and 180 axis, with the OC positioned on the straight-ahead gaze, with minimal chromatic aberration and on and off-axis cylinder and power error, although careful frame selection is required to minimize excessive vertical decentation that can cause increased weight and thickness.
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 understand my first post was a little misleading. You are correct, in that, the OC does not change. As DRK and Robert pointed out when you tilt a lens the ray does not end up over the CoR, but above it. This is why you need to lower it so that the ray ends up over the CoR. This might be half of the tilt depending upon the vertex distance and CoR. The formula I showed simply shows how much change the OC needs, Robert listed another formula that shows the power change, there are others that show power change with wrap and tilt combined as well.
It is not a new kind of optics or voodoo as you point out. "The Optics of Ophthalmic Lenses" 2nd edition was printed in 1978 which discusses this topic. "System for Ophthalmic Dispensing" 1st Ed printed in 1979 discusses this topic as well.
Your diagram indicates to me that moving the optical center down that much will introduce unwanted vertical prism in the primary position of gaze much the same way "equithing" or thinning prism does, of which I'm very leery. I would prefer to pay attention to how the lenses will be mostly used and in the diagram example would probably raise the OC ht rather than lower it, unless they were primarily for reading or laptop use. I try to minimize unwanted/unneeded prism wherever I can.
Right. The key is what's best for the client- less yoked prism (check the habitual eyeglasses) or less power/astigmatic error.
Vertical imbalance from anisometropia, excessive weight, thickness and minification/magnification due to excessive decentration, vertical or otherwise, and oblique astigmatism from mispositioning aspheric/atorics are a few of many additional considerations when fitting ophthalmic lenses.
As I alluded to in my post, we can have our cake and eat it too, with some of the more advanced lens designs. For example, Shamir SV asks for pupil heights, and as long as we measure the wrap and panto angle, wearer's vertex distance, and pupil height, (opt out of any prism thinning- consult with your lab), we'll have our vertical OCs on the primary gaze without any optical insult (with optimal frame selection of course).
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.
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