Here's a silly "DERP" question

[drk]

In a spherocylinder, the most plus meridian defines the base curve, ergo sets the center thickness. (True?)

Therefore, in a +4.00-4.00 SC lens, it will be advantageous to use an asphere. (True?)

And even better would be FFSV. (True?)

[Paul Smith LDO]

Good morning, yes and I would put the patient into a FFSV.

[Tallboy]

I mean Any RX approaching +4.00 deseves an aspheric design, and any RX with a -4.00 cyl absolutely deserves a FFSV design, hopefully with POW measurements, but definitely FFSV backside aspheric. I don't run to FFSV as fast as some but this is definitely someone that deserves it.

[Paul Smith LDO]

I mean Any RX approaching +4.00 deseves an aspheric design, and any RX with a -4.00 cyl absolutely deserves a FFSV design, hopefully with POW measurements, but definitely FFSV backside aspheric. I don't run to FFSV as fast as some but this is definitely someone that deserves it.

With that amount of cyl. I personally would not use POW and the resulting compensated formula.

[Tallboy]

With that amount of cyl. I personally would not use POW and the resulting compensated formula.

Honestly I go back and forth on how I feel about this also. I'm still not sure and use both.... If its been the same steady Cyl for years I would probably try POW, otherwise shamir spectrum sv

[drk]

Thank you, brilliant ones.

[Barry Santini]

With that amount of cyl. I personally would not use POW and the resulting compensated formula.

I never, EVER have an issue with a compensated FF in this cyl power.

Barry

[Robert Martellaro]

In a spherocylinder, the most plus meridian defines the base curve, ergo sets the center thickness. (True?)

I'd usually look at the spherical equivalent. But with the Rx below, with one principal meridian plano, I'd use a +8 BC if spherical CR39. I can't help with the CT.

Therefore, in a +4.00-4.00 SC lens, it will be advantageous to use an asphere. (True?)

The problem is which meridian do we optimize, the SE or the +4. Probably the SE on a +6 BC. However, with this particular Rx, there is probably less aberration if we use a spherical blank because while the asphericity/p-value might correct a too flat BC in the strong meridian, it might also introduce more error on the ±45 degree meridian, more than a simple spherical on a +8 BC. That's the beauty of...

And even better would be FFSV. (True?)

An atoric surface design for the above Rx will reduce oblique astigmatism and off-axis power error, at all meridians, maybe over a variety of BCs if the software is capable. POW optimization will reduce astigmatic and power error on and off-axis if there is sufficient wrap and or panto tilt.

[MakeOptics]

Preference is going to be FFSV first with that much cyl, the lens is just going to be better in too many ways. (just assume I mean fully compensated, atoric)

If I had to go surfaced non-freeform, then the choice of base curve suprisingly I would pick the BC that closely matches the power along the 180 for a panoramic design.

[Paul Smith LDO]

I never, EVER have an issue with a compensated FF in this cyl power.

Barry

I have had several patients who rejected the compensated Rx in a SV format, the issues resolved with the non compensated digital design. Whether it is an issue with compensated cyl and it's axis combined with the patients inability to wear the cyl at the designs discretion. It happens or more importantly it happened to them.

[Robert Martellaro]

If I had to go surfaced non-freeform, then the choice of base curve suprisingly I would pick the BC that closely matches the power along the 180 for a panoramic design.

Right! If we ever have to compromise the off-axis optics, it would be sensible to do so on the meridian that experiences the least ocular rotation.

[MakeOptics]

Right! If we ever have to compromise the off-axis optics, it would be sensible to do so on the meridian that experiences the least ocular rotation.

You get it. ; )

As a side note I used to try to base curve match for a more orhogonal lens sacrificing a bit of clarity but I found out in time that most doctors cut the cyl back to accomplish this in the refraction so I was double dosing the patient. Now I correct for marginal astigmatism.

[Barry Santini]

I have had several patients who rejected the compensated Rx in a SV format, the issues resolved with the non compensated digital design. Whether it is an issue with compensated cyl and it's axis combined with the patients inability to wear the cyl at the designs discretion. It happens or more importantly it happened to them.

Interesting.....

[harry a saake]

Interesting.....

also keep, in mind aspherics usually work the best when on pd, patient and frame

[Paul Smith LDO]

Interesting.....

Perhaps. I've had a few talks with Scott Pearl at DEL, as I was surprised by the results. The only thing that I could come up with on my own was that the patients had adjusted to the spherical aberration and that their brain (always the brain) could not make the adjustment to the Rx's presentation in the compensated format and wanted to put the aberration where it had been in the past. Scott wasn't able to add much more to that. The similarities that these patients shared were oblique astigmatisms, moderate + powers sph and cyl powers between 3.25 D and 4.50 D. Is it possible that they could have adjusted with more wearing time, sure but they were rejecting them almost immediately and unwilling to put much time into the notion of 1 to 2 weeks of continued wear. When looking at their reasons for an eye exam they all complained of decreased VA in the distance. I dispense compensated SV lenses but I have developed a twitch (Pavlovian response) when faced with said Rx's. I am definitely more willing to put them into digital first and free form later but I may change my opinion with some positive therapeutic reconditioning.

[Robert Martellaro]

Spherical aberration is only a concern with plus powers over about 8 D. High cyls with oblique axes are typically problematic, especially when the habitual eyeglasses had axes closer to 150/30 or 60/120. Could you post an example case with full data? I'm thinking it's more of a software/lens positioning problem, especially if the primary gaze acuity is soft. Consider using a different product and manufacturer. FWIW, the only failures I've had with optimized lenses is with prescribed prism.

[braheem24]

Spherical aberration is only a concern with plus powers over about 8 D.

I'm with you on everything else, Just wondering how you came up with the 8D number? Especially with LCA induced by reading ht and various abbe numbers, Do you find it's a general rule that works or is there a calculation that derives the 8D?

[Robert Martellaro]

https://books.google.com/books?id=Ib...lenses&f=false

Best form breaks down at about +8 D.

• The focal power limit of the ellipse is around +7.50 D; above this power, lens aberrations cannot be eliminated without using special, aspheric lens designs.

http://www.opticampus.com/cecourse.php?url=lens_design/

[drk]

(I like the creative thinking on optimizing the horizontal meridian, if forced to choose. You can always dip your chin to read.)

I like the FFSV solution...it's really not expensive, either.

OK, then, considering FFSV is not too pricey, where do molded aspherics fit in, anymore? High spherical powers, only?

[Tallboy]

(I like the creative thinking on optimizing the horizontal meridian, if forced to choose. You can always dip your chin to read.)

I like the FFSV solution...it's really not expensive, either.

OK, then, considering FFSV is not too pricey, where do molded aspherics fit in, anymore? High spherical powers, only?

I have to charge considerably more for surfaced FFSV than Molded aspherics stock lenses, and most people don't want to pay the extra.


I just had a 14 year old boy who was a +5.00 with 2 cyl, I put him in a surfaced Resolution Poly aspheric, he loved it.

I've had issues with high plus patients (over +8.00) that didn't like the cosmetics and/or vision of FFSV, maybe they were just so used to their aspheric vision? They all wanted back in their aspheric 1.74 purecoat or Avance. Honestly don't discount the importance of lens aesthetics above all else to most people.

[Barry Santini]

Plus and FF are another story....agreed.

B

[MakeOptics]

https://books.google.com/books?id=Ib...lenses&f=false

Best form breaks down at about +8 D.


http://www.opticampus.com/cecourse.php?url=lens_design/

+1, Tschernings Ellipse tops out at an eight with a spherical design.

[drk]

Plus and FF are another story....agreed.

B

Dish it, girl. What do you mean?

I presume that you mean that there are "many" asphericity choices out there for high plus lenses? And there is no "preferred one" except what a patient has adapted to?

Not to digress, but I remember someone (DM?) saying that the main idea with high plus aspherics is 1. flatten the lens, 2. thin the lens, 3. try to make the correct aspheric corrections so the vision doesn't go all to hell.

Is this the issue? Some aspherics are more "aggressive" than others? And FFSVs are just "another" type of aspheric (i.e. GIGO)?

[Paul Smith LDO]

Spherical aberration is only a concern with plus powers over about 8 D. High cyls with oblique axes are typically problematic, especially when the habitual eyeglasses had axes closer to 150/30 or 60/120. Could you post an example case with full data? I'm thinking it's more of a software/lens positioning problem, especially if the primary gaze acuity is soft. Consider using a different product and manufacturer. FWIW, the only failures I've had with optimized lenses is with prescribed prism.

For what it's worth, some of you have me rethinking my current stand. Even the mighty oak can bend like a reed. I understand the theoretical concept to some degree and have been acquiring more detailed information on FF but still in the back of my mind there is this tape that keeps playing, don't forget the human condition. I've got another call into my friends at DEL who have been extremely helpful even though they did not make the lenses for me. Robert, your comment regarding the software/lens positioning issue really got me looking at this from a different perspective. We are all still students. Thank You.

[Robert Martellaro]

For what it's worth, some of you have me rethinking my current stand. Even the mighty oak can bend like a reed. I understand the theoretical concept to some degree and have been acquiring more detailed information on FF but still in the back of my mind there is this tape that keeps playing, don't forget the human condition. I've got another call into my friends at DEL who have been extremely helpful even though they did not make the lenses for me. Robert, your comment regarding the software/lens positioning issue really got me looking at this from a different perspective. We are all still students. Thank You.

Your welcome. I'm learning from you also, although this is probably the first time I've said it.

Okay, enough of this love fest, let's talk optics.

My two cents WRT aspheric/plus compared to aspheric/minus is how much the lens shape and center thickness is affected by a flatter BC, and that the change in magnification/minification is greater with flattened plus because in addition to a BC change, the center thickness is decreased with plus, something that won't happen with minus powers.

I'm not sure how an atoric surface, that might or might not be optimized for position of wear, would add significanty to changes in spatial perception. IMO, changes in the position of wear due to a different frame fit, a change in Rx, and a change in lens form, should be more disruptive to the visual cortex than a small amount of error correction on and off-axis.