Nice article, Barry

[drk]

http://www.2020mag.com/l-and-t/64819/

[Chris Ryser]

Nice article, Barry


http://www.2020mag.com/l-and-t/64819/

I have to second DRK.............................

What a job to write all this up, and well done.

[Robert Martellaro]

Thanks Barry. Good stuff.

We were just talking about high index and base curves a week ago!

I consider the "match base curve/IPD" instructions as a caution flag, a heads-up so to speak, that there might be, or has been issues with their eyeglass wearing history and health, that might increase the chance of poor adaptation and/or falling. I perform a very thorough health and lifestyle interview with all of my clients, but it's always nice to see the docs paying attention to these things for those of us on the dispensing and lens designing end who are in too much of a hurry to get to our next client.

Too that end, that is, keeping our clients upright and not falling, I wouldn't work the base curves too much (maybe a ±1 D tweak) for cosmetics or frame matching for our increasing number of eyeglass wearing older folks. I would recommend selecting a frame that is a good match for the optimal lens configuration, one that provides for the best possible visual acuity, comfort, and safety.

Best regards,

Robert Martellaro

[Barry Santini]

Thank you drk, Chris and Robert. This was a lot of work to distill what I wanted to cover down into a primer format.

and Robert, your suggestions are good advice!

Barry

[Hayde]

Well done, Barry.

I'm curious if you have citations handy for your Center of Rotation paragraph: studies of axial length by ethnicity, their degree of statistical validity and how those distribution curves contrast against "the norm" CR that peaks at 27mm. (Or does it still..post-Myopia Boom?)

[Barry Santini]

Well done, Barry.

I'm curious if you have citations handy for your Center of Rotation paragraph: studies of axial length by ethnicity, their degree of statistical validity and how those distribution curves contrast against "the norm" CR that peaks at 27mm. (Or does it still..post-Myopia Boom?)

Both Essilor and The Optical Heritage website have lots of info on CR distance

B

[lensgrinder]

Well done, Barry.

I'm curious if you have citations handy for your Center of Rotation paragraph: studies of axial length by ethnicity, their degree of statistical validity and how those distribution curves contrast against "the norm" CR that peaks at 27mm. (Or does it still..post-Myopia Boom?)

It's important to note that some manufactures account for the CoR in their FF designs. Obviously you would need to take a VD as well to have the full affect.
Using a lens design that takes CoR in to account no only reduces OAE but also creates the needed corridor length in a variable design.

http://www.pointsdevue.com/article/r...on-lens-design

[Hayde]

It's important to note that some manufactures account for the CoR in their FF designs. Obviously you would need to take a VD as well to have the full affect.
Using a lens design that takes CoR in to account no only reduces OAE but also creates the needed corridor length in a variable design.

http://www.pointsdevue.com/article/r...on-lens-design

Great article, lensgrinder!

The relevance is clear. Still, I like how Barry called it the "known unknown," pinning down the measurement in a retail setting isn't something I've ever seen validated. I'm not unduly skeptical that the high-tech measurement devices do a reasonable job (or at least hopefully 'do no harm,') but none of the manufacturers want to show their math--perhaps for fear of revealing proprietorial secret sauce. In the absence of statistical validation of their accuracy, I find the prospect of using population studies (while not misusing them) to temper our "default 27" math in the clinic attractive--for those powers where it matters.

I agree with you that when vertex matters, giving some thought to penetrating the fog of CoR is a worthy cause.

[lensgrinder]

Great article, lensgrinder!

The relevance is clear. Still, I like how Barry called it the "known unknown," pinning down the measurement in a retail setting isn't something I've ever seen validated. I'm not unduly skeptical that the high-tech measurement devices do a reasonable job (or at least hopefully 'do no harm,') but none of the manufacturers want to show their math--perhaps for fear of revealing proprietorial secret sauce. In the absence of statistical validation of their accuracy, I find the prospect of using population studies (while not misusing them) to temper our "default 27" math in the clinic attractive--for those powers where it matters.

I agree with you that when vertex matters, giving some thought to penetrating the fog of CoR is a worthy cause.

Here is an estimate. Some manufacturers use an iterative approach to get a closer number and some lenses use the Rx and not devices. The one below is a basic average and it does you no good to know the CoR. The manufacturer needs it before the designing of the lens takes place. With Free Form designs you can change this number before it is ground, where molded lenses use an average. The "default 27" is the CoR plus the vertex distance. Usually between 25 and 27 are used to determine oblique astigmatism.
sph eq = sph + (cyl/2)
CoR is 13.5 - (0.2 X sph eq)

[Craig]

Well done as usual. I think you might be on to something with this base-curve stuff.