Some “wait, but why?” questions about lens tilt and OCs

[drk]

Not exactly. Aligning the optical axis of the lens with the center of rotation of the eye minimizes aberrations at all all angles of gaze. Tilting the lens around the horizontal axis (panto/retro) also increases the sphere power and introduces cylinder with the sign equal to the sphere power, although the errors are small enough to be safely ignored with low to medium powers and low tilt values.

Remember when we were myopic kids and we would tilt the lenses at a crazy steep angle to see the board clearer? We knew it was time for another bump in power- off we went to the refractionist.

Robert

What about, though, the effect of looking 3 mm above the OC in straight-ahead gaze? Isn't that slighly less clear than if you look through the OC?

[Robert Martellaro]

What about, though, the effect of looking 3 mm above the OC in straight-ahead gaze? Isn't that slighly less clear than if you look through the OC?

The only way I could see that happening is if there was a significant wave in the lens. Besides, we don't look through the OC; dumb lenses' OCs are usually about 4mm below the straight gaze on a well fitted frame, smart lenses usually have a diopter or so of BD prism to thin the lens because the fitting point is coincident with the pupil height, with the geometric center typically about 4mm below the pupil height with about 8˚ of panto, signifying an optimally fit frame.

Robert

[drk]

Not trying too hard to beat a dead horse, here, but I haven't grasped your response.

Besides, we don't look through the OC; dumb lenses' OCs are usually about 4mm below the straight gaze on a well fitted frame,

That's stipulated. That's what is being discussed: what do we give up by not looking through the OC in straight-ahead gaze?

smart lenses usually have a diopter or so of BD prism to thin the lens

I did not know that. I heard that they somehow dis-associated the "optical center" (placed downwards towards the datum line to make it thin) but moved the "correct power" up to where the eye needs it. Is it as simple as "grinding" BU prism?

because the fitting point is coincident with the pupil height

We always use the center of the pupil as the reference point or fitting point on a "smart lens". So are you saying that they thin the bottom of the lens because the lens because the lens is thicker because it sits up higher? Because that's what we do with PALs, too, right?

[Robert Martellaro]

<snip> what do we give up by not looking through the OC in straight-ahead gaze?

Blurred vision, if the lens is tilted around the horizontal axis.

I did not know that. I heard that they somehow dis-associated the "optical center" (placed downwards towards the datum line to make it thin) but moved the "correct power" up to where the eye needs it. Is it as simple as "grinding" BU prism?

BD or BU. See "equithin" and below.

We always use the center of the pupil as the reference point or fitting point on a "smart lens". So are you saying that they thin the bottom of the lens because the lens because the lens is thicker because it sits up higher? Because that's what we do with PALs, too, right?

Thin the top on plus with BD prism and bottom on minus with BU prism, both yoked.

[Barry Santini]

Individual optical measurements, such as
axial length
corneal curvature
anterior chamber depth
entrance pupil position
retinal curvature

B

[Uilleann]

Mispositioning the vertical OCs will introduce aberrations both on and off-axis for all types of lenses, and will be significant on medium and high dioptric powers...

Warby and Zenni have built an entire business model doing precisely this! They're so bad, it's laughable.

On average, the frames we see usually fall anything from 8 to as much as 20 (yep, 20!) mm below pupil center, with the average seeming to fall approx 10-12mm off. That kind of induced yoked prism, usually on myopic RXs, isn't doing these patients any favors I'm sure.

[NAICITPO]

Warby and Zenni have built an entire business model doing precisely this! They're so bad, it's laughable.

On average, the frames we see usually fall anything from 8 to as much as 20 (yep, 20!) mm below pupil center, with the average seeming to fall approx 10-12mm off. That kind of induced yoked prism, usually on myopic RXs, isn't doing these patients any favors I'm sure.

I mean if they just did half the B plus X depending on the B measurement of the frames they could probably be much closer. Like maybe half B + 3 on 30 and under B frames, and half the B + 4 on 31-40 B frames, and half the B +6 on 40-44 B frames etc

Though I guess they probably don't surface anything and just use FSV blanks. Which means they probably play fast and loose with the OC--moving it around so everything cuts out.

[Uilleann]

I mean if they just did half the B plus X depending on the B measurement of the frames they could probably be much closer. Like maybe half B + 3 on 30 and under B frames, and half the B + 4 on 31-40 B frames, and half the B +6 on 40-44 B frames etc

Though I guess they probably don't surface anything and just use FSV blanks. Which means they probably play fast and loose with the OC--moving it around so everything cuts out.

Pretty fast and definitely loose with PDs too of course. But yeah, the OC issue is epic with them. Always fun to "fix" the problems they create for patients, and get the "I didn't realize they were soo bad/off/wrong!" exclamations on dispense of even modestly well made glasses with proper centration.

[Prentice Pro 9000]

Hi Prentice Pro 9000.

Go to http://216.144.236.77/tools/spectacle_optics.php

Open program (Windows only, and you might need an older version- I use XP with a virtual machine on macOS) and select "optical analysis". Enter the parameters, run. The picture below shows an example where the optician selected too flat of a base curve for the power needed, resulting in a high degree of off-axis aberrations (marginal astigmatism and power error). Use the asphericity slider or steepen the BC to reduce these errors.

Mispositioning the vertical OCs will introduce aberrations both on and off-axis for all types of lenses, and will be significant on medium and high dioptric powers, especially those with aspheric/atoric surface designs.

If you need to match the vertical OC with the pupil heights to reduce on-axis vertical prism imbalance (VI), use a lens that has the vertical OC at the pupil height, where the software engine can correct for these aberrations. For PALs, you will probably need to use a design with no drop, or use canceling prism in one eye, aligning the pupil height with the prism reference point (PRP). Keep in mind that this will increase VI on the downgaze- sometimes it best to compromise, equalizing the VI somewhat at both angles of gaze, or use slabbed segmented multifocal, SVDO, etc.

Hope this helps,

Robert Martellaro

I need to find an old computer to be able to use this app lol. I remember playing around with this like ten years ago when I did not understand what was going on quite so much.

[Kwill212]

I need to find an old computer to be able to use this app lol

It works on my windows 10 pro PC.