Retina Forward Design?

[Joann Raytar]

In the past few months we have sold a couple of Pentax AF Mini's. One of my coworkers asked why they included a card with vertometer readings different from what was ordered. The card stated that they were a progressive with "Retina Forward Design." The Rx was for plus power on top with a 2.25 add and I noticed the actual ground powers noted on the card and read off the lens were slightly different from the original job order (One of the Add Powers read almost .25 stronger.)

Does anyone here want to elaborate on how Retina Forward Design compensates for Progressive lens design issues?

[Darryl Meister]

Hi Jo,

I'll go over a couple of things about this kind of lens design strategy if no one from Pentax steps up to do so in the next day or two.

Best regards,
Darryl

[Jackie L]

I thought it was the AF-Mini by Pentax that has the retina forward design?

[Darryl Meister]

Oops, my typo. I meant Pentax, not Optima. It is both the regular AF and the AF mini.

Best regards,
Darryl

[eyeguy2001]

hey Jo, I use some of those lenses on high myopes and hyperopes and have noticed the powers are off .25 pr so. I think it has something to do with the panascopic tilt on the lenses. They work well and the lenses are relatively inexpensive.

eyeguy

[Joann Raytar]

Besides the powers I've noticed the axis also differ by a few degrees on the documentation for Vertometer readings that comes with the lenses.

Darryl,

Its been a day or two ... tell me what's up with these lenses. I always enjoy reading posts with your take on technology so I'm ready for a lesson. :)

[Jim G]

Jo, I noted that you measured one of the add powers as 0.25 strong. Pentax recommends "When measuring Pentax AF Progressives, the lens should be in the concave measurement position (back side placed on the lensmeter mount) for both distance AND NEAR power verification." Aside from the design that Darryl is gonna explain, this could also make a difference.

[Joann Raytar]

Jo, I noted that you measured one of the add powers as 0.25 strong.

This is also the lensometer reading that came with the Pentax documentation. After looking at the lens it appears that the increased add power pushed the actual reading power a little higher; I am wondering if this is the purpose of the add being ground a little strong, almost .25?

[Darryl Meister]

Hi Jo,

Well, rather than get into the specifics of AF's design philosophy, I think I'll just describe some general principles that apply to several modern lenses. (At least until Pentax makes an offer to put me on its payroll! ;) )

Conventional progressive lenses are designed to allow for ease of power verification at the near and distance reference points by common focimeters (like lensometers and vertometers). Among other things, this requires that the progressive surface be relatively spherical within the distance and near checking circles. It also requires that the distance and add powers match the prescribed correction, as measured by a focimeter.

However, a spectacle lens corrects vision very differently than the trial/phoropter lens used to determine the prescription. Your trial lens is thin, flat, straight, centered, and at a fixed distance from your eye. Your spectacle lens generally has a greater center thickness, steeper curves, more tilt, decentration, and a different vertex distance. Even the effective add power determined using trial lenses will perform differently when made into a spectacle lens. Moreover, factors such as oblique refraction and the near working distance can also affect the actual optical powers experienced by the wearer. We call the optical performance of a spectacle lens, as perceived by the wearer, in its intended fitting position the as-worn or position of wear performance.

It is possible to take into account the influence that oblique aberrations, lens tilt, vertex distance, and other variables affecting viewing conditions have on the optical powers perceived by the wearer. Essentially, the progressive lens can be optically optimized using ray tracing for the as-worn or position of wear. By applying asphericity to the distance and/or near zones of a progressive lens, you can expand the areas of clear vision at distance and near by correcting some of the power differences caused by oblique refraction, lens tilt, working distance, etcetera.

How different manufacturers incorporate this kind of as-worn optimization varies. Some take the entire prescription (including the Rx, measurements, and prism), calculate the powers necessary to compensate for the position of wear, and grind a complex atoric surface on the back of the lens. This surface actually incorporates a kind of asphericity. These lenses are often more expensive and take longer to get, since they have to be produced using special machines. You may also make some general assumptions ahead of time about the Rx and measurements, and apply some asphericity to the distance and near zones of the semi-finished blank. This is generally more cost-effective. You can also supplement this approach with a few calculations that compensate for vertex distance, lens tilt, etcetera, which are then used to modify the original prescription.

Generally, the power readings of the lens will differ from the original prescription, when measured using a conventional focimeter. However, the wearer will actually experience the correct powers when the lenses are in their intended fitting positions. In many cases, the laboratory or lens vendor will supply a compensated prescription, intended to replicate how the lenses are measured using a lensometer of vertometer, so that the job can be verified.

Best regards,
Darryl

[Darris Chambless]

Hello Jo,

although Darryl has explained it in scientific terms (and given me another brain cramp. He's good at that you know? :) I'll simplify it just a smidge if Darryl ahs no objection.

First off to simplify Darryls explaination. The manufacturer takes into consideration where the light will converge "at the retina" (via formula) and determine where it fall when refracted through the spectacle lens. In otherwords they take the script and say "Hummmm. You know what? The phoropter has a different vertex distance than the actual glasses will have. So lets take the average phoropter distance and design a formula with which to convert it to the actual vertex distance the wearer will experience along with the base curve diffences." So you will have a difference in the powers because it is a compensated script which isn't a bad idea at all.

To really simplify it, can you say "Good Marketing"? :)

Darris C.

[Joann Raytar]

Thanks guys!!!
:D

[Pete Hanlin]

Thank you for the information! I will have to credit you in my "Progressive Lens Characteristics" course I'm giving next month...

Now, if the manufacturer could only compensate ahead of time for errors in measurement by the optician, we'll have the perfect lens! Just kidding, but I'm wondering if "retina forward" and other designs assume a particular amount of pantoscopic tilt or face form?

Thanks for this and any additional information you can think of as I put the finishing touches on my material presentation!

Pete

[David Wilson]

Pete,
Some of the manufacturers take the 'as worn' idea to its ultimate level. That is they require all of the parameters from the optician, mono PDs, heights, BVD, pantoscopic tilt and , in some cases, the dihedral angle of the facial wrap. They then calculate the equivalent power in this situation to the flat test lens (or photopter lens) at its BVD. Lenses in this category include the Zeiss Individual and the Rodenstock Multigressiv ILT. As Darryl says, they have atoroidal back surfraces and are very expensive. Other manufacturers will make assumptions about the distance to the centre of rotation (usually 27mm) and the pantoscopic tilt. The resulting lenses will normally have excellent optics, including asperical curves to address the aberrations of form. They will, though, leave the power in the checking circle as ordered so that the power checked will be the power ordered; little spherical windows of simplicity to allow for the more technically challenged but still much loved focimeter (vertometer).

Regards
David