Benefits of "digital" lenses for spherical patients

[drk]

My optician asked a good question, and I think I gave the right answer. But I thought it would be a good brain exercise for Optiboard.


Q: What is the benefit of "digital" designs for SV spheres? (No cyl, no add. Say, -1.00 DS, +1.00 DS, -6.00 DS, +4.00 DS)

[finefocus]

[QUOTE=drk;554519]My optician asked a good question, and I think I gave the right answer. But I thought it would be a good brain exercise for Optiboard.


Q: What is the benefit of "digital" designs for SV spheres? (No cyl, no add. Say, -1.00 DS, +1.00 DS, -6.00 DS, +4.00 DS)[/QUOTE

As long as you hold your head real still, and look straight ahead through the OC, and don't use your peripheral vision, then no benefit at all.
But then, under those circumstances, a pinhole would work pretty well also.

[Robert Martellaro]

My optician asked a good question, and I think I gave the right answer. But I thought it would be a good brain exercise for Optiboard.

Q: What is the benefit of "digital" designs for SV spheres? (No cyl, no add. Say, -1.00 DS, +1.00 DS, -6.00 DS, +4.00 DS)

[drk]

I think you both are on a different track than I was thinking.

I answered that it's "the same benefit" as we have come to expect from the older aspheric molded front lenses.
a. if it's a low power lens, nada.
b. if it's a higher plus, you get lens flattening.
c. if it's a higher minus, you get minimal edge thinning (and we never cared much about that).

I'm operating under the principle that aspheric lenses just correct the slop induced by using flatter front curves (for plus lenses).

You guys are operating under the principle that off-axis viewing can be improved upon, compared to best-form spherical lenses.

Both are right, I believe. Which do you think is a more fundamental consideration (i.e. the one that would compel you to use a good digital design for a high power sphere)?

[Robert Martellaro]

I think you both are on a different track than I was thinking.

I answered that it's "the same benefit" as we have come to expect from the older aspheric molded front lenses.

Not the same.

a. if it's a low power lens, nada.

Yup.

b. if it's a higher plus, you get lens flattening.

All powers would have flatter BCs. We'll probably have better results if we aphericize the meatiest curve, that is, the ocular curve for minus, and the BC for plus. But it's cost less to use spherical BCs working that back only, with a few exceptions.

c. if it's a higher minus, you get minimal edge thinning (and we never cared much about that).

A free-form generator can produce the asphericity on the back of a minus lens, with more effect than using a front apheric, where the BC is already pretty darn flat (+.50 to +1.00).

I'm operating under the principle that aspheric lenses just correct the slop induced by using flatter front curves (for plus lenses).

It also decreases magnification and minification, sometimes significantly. Maybe a decrease in vertex distance, depending.

You guys are operating under the principle that off-axis viewing can be improved upon, compared to best-form spherical lenses.

It can be close if the lens comes in narrowly spaced BCs, 1 D or slightly less, which is expensive to stock, and is unnecessary now that we can custom design a lens on a free-form platform. And if we need a slightly steeper or flatter BC, FF says yes, can do (software dependent).

Note: Best form for hi-index lenses, without aspherisizing, results in very steep curves. For example, a +3.50 DS 1.67 requires a +10.25 BC, 1.74 index best form would be +12.25 BC. I doubt that most people on optiboard have seen a +12 base.

Both are right, I believe. Which do you think is a more fundamental consideration (i.e. the one that would compel you to use a good digital design for a high power sphere)?

Another example of FF superiority is with the aspheric pole/oc position. Semi-finished/finished requires the pupil heights to be lowered to align the optical axis with the center of rotation, not necessary with FF, minimizing vertical prism imbalance (when present due to dissimilar powers at ninety) with slightly less chromatic aberration to boot.

If the lens has optimizations beyond aspheric's/atoric surface designs, we should expect slightly better visual performance at all angles of gaze, depending on the degree of power and tilt. For example, -5.00 DS wrap 10˚ panto 10˚, -4.62 -0.25 x 46 .25∆ BI needs to be surfaced to match the prescribers intent.

Best regards,

Riobert Martellaro

[drk]

Hmm...a lot to digest.

I will be back for mansplainin' sometime soon! :)

[Barry Santini]

...to match the *prescribers intent.*”

WOW! Now there’s a loaded and obfucacious term.

If anyone can, with confidence, ascertain this by looking at a typical Rx, more power to you.

B

[Robert Martellaro]

I've been listening to politicians so much lately that I'm beginning talk like them.

[drk]

A free-form generator can produce the asphericity on the back of a minus lens, with more effect than using a front apheric, where the BC is already pretty darn flat (+.50 to +1.00).

I didn't consider that. So, free form SV for minus can have a bit more of an edge thinning effect than the old molded ones. Huh. Generally FFSV is not expensive, but I do wonder what kind of numbers we're talking. 0.5mm? (I know, dependent on the variables.) But are we talking 1 mm in many cases, or "chump change"?

It can be close if the lens comes in narrowly spaced BCs, 1 D or slightly less, which is expensive to stock, and is unnecessary now that we can custom design a lens on a free-form platform. And if we need a slightly steeper or flatter BC, FF says yes, can do (software dependent).

So...when we order, say, Walman's house FFSV (or even VSP's Unity), are we going to get dual-digitally-surfaced-from-puck, or are we going to get it from a semi-finished blank? What goes on, out there?

Another example of FF superiority is with the aspheric pole/oc position. Semi-finished/finished requires the pupil heights to be lowered to align the optical axis with the center of rotation, not necessary with FF, minimizing vertical prism imbalance (when present due to dissimilar powers at ninety) with slightly less chromatic aberration to boot.

This is news to me, as well. I am probably guilty of ordering FFSV with a 3mm drop-below-pupil-center MRP, instead of a "fitting height". Dang. As a flaming myope with 1.67 PALs, I don't like looking 4mm above the PRP due to standard PAL fitting procedure. I get aberration, and find myself tilting my head back with distance viewing for better optics. (I fit myself high to help offset this.) But SV patients (and anisometropes) would greatly benefit.

But what about this whole "optic axis intersecting the center of rotation of the eye" thing? Is that just for a sphero/cylinder lens and it merely "averages" the worse optics between straight-ahead gaze and downgaze? Does this mean that FFSV can compensate for optics on downgaze without compromising the straight ahead gaze?

If the lens has optimizations beyond aspheric's/atoric surface designs, we should expect slightly better visual performance at all angles of gaze, depending on the degree of power and tilt. For example, -5.00 DS wrap 10˚ panto 10˚, -4.62 -0.25 x 46 .25∆ BI needs to be surfaced to match the prescribers intent.

IOW, you're saying that one of the nice things about FFSV is automatic POW compensation. Yeah.

[drk]

WOW! Now there’s a loaded and obfucacious term.

If anyone can, with confidence, ascertain this by looking at a typical Rx, more power to you.

B

Typical Barry.

Simply put (and you know this), the numbers on the Rx pad is the "prescriber's intent". No more, no less. It's not mysticism.

[Robert Martellaro]

I didn't consider that. So, free form SV for minus can have a bit more of an edge thinning effect than the old molded ones. Huh. Generally FFSV is not expensive, but I do wonder what kind of numbers we're talking. 0.5mm? (I know, dependent on the variables.) But are we talking 1 mm in many cases, or "chump change"?

About 3% to 5% more (compared to front aspherics) according to D. Meister's Optical Analysis program. A tad more for high minus (>8 D), without lenticulation.

So...when we order, say, Walman's house FFSV (or even VSP's Unity), are we going to get dual-digitally-surfaced-from-puck, or are we going to get it from a semi-finished blank? What goes on, out there?

Puck, semi-finished, same animal. Although the blank might have been formed in digitally generated mold, it's a stretch to call it dual FF/digital. That talk is for the marketing dept.

This is news to me, as well. I am probably guilty of ordering FFSV with a 3mm drop-below-pupil-center MRP, instead of a "fitting height".

Dang. As a flaming myope with 1.67 PALs, I don't like looking 4mm above the PRP due to standard PAL fitting procedure. I get aberration, and find myself tilting my head back with distance viewing for better optics. (I fit myself high to help offset this.) But SV patients (and anisometropes) would greatly benefit.

That's not right. 4mm shouldn't generate enough CA at this power. Try a 1.60 refractive index in the same package to rule that out. If you were anyone else I'd suspect overminusing or a hyperopic shift.

But what about this whole "optic axis intersecting the center of rotation of the eye" thing? Is that just for a sphero/cylinder lens and it merely "averages" the worse optics between straight-ahead gaze and downgaze?

Probably better optics at all angles of gaze.

Does this mean that FFSV can compensate for optics on downgaze without compromising the straight ahead gaze?

More better, with the best software, keeping in mind that FF generators can make "dumb" lenses also.

IOW, you're saying that one of the nice things about FFSV is automatic POW compensation. Yeah.

It places the OC on the primary gaze, without compromise, over a wide range of tilt and wrap values. Si.

Best regards,

Robert Martellaro

[drk]

About 3% to 5% more (compared to front aspherics) according to D. Meister's Optical Analysis program. A tad more for high minus (>8 D), without lenticulation.

I'm trying to be clear, and make sweeping generalizations.

If you're saying 5% more than a little, then I'll just neglect this.

Puck, semi-finished, same animal. Although the blank might have been formed in digitally generated mold, it's a stretch to call it dual FF/digital. That talk is for the marketing dept.

So if they're going to do a FFSV, you're saying they choose a "puck" with an approximate certain base curve? And then surface the front and the back?



Agree there. Specify for everything to be consistent. But dropping for panto on FFSV not needed?

That's not right. 4mm shouldn't generate enough CA at this power. Try a 1.60 refractive index in the same package to rule that out. If you were anyone else I'd suspect overminusing or a hyperopic shift.

It's true. I now know that I'm too picky for high index. It's mid-index for me.
Thanks!

[Tallboy]

remember molded minus aspherics usually have at least
.5 mm difference in center thickness compared to surfaced lenses, and often 0.8mm less. Unless FFSV is including some lenticularization it generally will not be thinner than radially aspheric molded lenses. But there are so many other peripheral vision benifits. My current main pir is molded aspheric though

[AngeHamm]

Somewhat off topic, but taking a pupil height also reinforces to the patient that they need more than just a generic PD to get properly fitted glasses. I measure OC height on every patient for this reason.

[Don Gilman]

Somewhat off topic, but taking a pupil height also reinforces to the patient that they need more than just a generic PD to get properly fitted glasses. I measure OC height on every patient for this reason.

Help me out here. I do a height with my Ray ban googles polar suns -1.50 ou with a 48 mm vertical, height is +/- 40 mm. When I look down at my golf ball height is now+/- 20 mm. I now have 3.0 diopters base up prism. Now what?