Object PAL data

[Les Moss]

First objective data on PALs I have seen:

http://optometry.osu.edu/research/CO...SciArticle.pdf

Any reason that table 6 on page 18 should not be used as the starting point for selecting a new lens?

[QDO1]

Your link doesnt work in my browser, sure you meant this:

http://www.optometry.osu.edu/researc...SciArticle.pdf

Some of us have been using Dr Sheedys work as a reference for a long time, along with data from other sources

[Darryl Meister]

I agree completely with the basic idea of Dr. Sheedy's evaluation, and SOLA employed a similar visual task analysis about three years ago for assessing SOLAOne and its competitors. However, I do believe that there are several areas of his methodology that could be improved upon for future evaluations. Below are a few comments I made last year in another thread on this very same topic:

1. Dr. Sheedy used vertex or surface power measurements, instead of ray tracing for the as-worn position to model the perception of optics by the actual wearer. This may potentially penalize lenses with optimized asphericity, as-worn optimization, or other design choices based upon optical ray tracing, when they may in fact perform better for the wearer. Essentially, surface astigmatism meant to improve vision in some cases could actually count against the lens in Dr. Sheedy's evaluation.

This would be a consideration for SOLA, since all of our new lenses are optimized for the as-worn position and make use of asphericity in the viewing zones. Fortunately, as-worn optical performance is less of an issue for the Base curves he assessed, which are generally optimized for near-Plano prescription ranges. And, in all fairness to Dr. Sheedy, he didn't necessarily have ray-tracing software at his disposal during this evaluation, since these applications are often proprietary -- especially for progressive lenses. The data from the Rotlex would also need to be post-processed appropriately in order for a lens to actually be reconstructed from the measurements for the purposes of ray tracing.

2. His power error thresholds were also relatively tight compared to our own. Dr. Sheedy focused more on the amount of error that the wearer would actually detect (a just noticeable difference threshold), not on how much error the wearer would generally find acceptable, which we feel is much more practical and clinically meaningful when assessing the usable regions of a progressive lens. According to Dr. Sheedy's measurements, many progressive lenses would be nearly unusable for distance or near vision tasks, though experience proves that this is obviously not the case.

Furthermore, Dr. Sheedy's approach might favor older "harder" designs, since they will generally have larger viewing zones at such a low threshold of unwanted cylinder power. However, history has shown that wearers generally do not prefer a strictly "hard" design to designs that offer a sensible balance between "hardness" and "softness." Additionally, his relatively tight sphere power threshold would appear to severely restrict distance vision in many cases. He doesn't sample the distance zone very high above the fitting cross either, which would further limit the size of the distance viewing zone.

Use of lower thresholds would also make the results more sensitive to any noise in the measurements, manufacturing variations, limits in accuracy of the measurements, and so on, which could be more of an issue in Dr. Sheedy's evaluation, since he only used one measurement sample for each lens design.

3. Another big difference in our approaches is in terms of binocular vision. We assess (again, using optical ray tracing) fields of vision through both eyes, simultaneously, in order to determine the binocular performance of a lens. Dr. Sheedy's current approach does not. Designs with poor binocular alignment, including older "rotated" designs, would appear to produce better vision than they would in practice for the wearer.

In reality, older designs produce smaller relative fields of binocular vision and also disrupt binocular vision by producing differences in prism, power, and magnification between corresponding points on the right and left lenses. Consequently, this is another example of an aspect of his methodology that could favor older -- and inferior -- lens designs. However, this type of optical analysis also requires sophisticated ray-tracing software.

4. Lastly, Dr. Sheedy had only one measure, the maximum level of unwanted cylinder power (astigmatism), that was related to dynamic vision performance and comfort. He also ties this measure in only loosely with distance vision, though it is indicative distortion, image swim, and blur under dynamic conditions typical of many distance tasks. SOLA also assess things like image swim and binocular power differences across the lens. These measures are an important consideration for distance vision, since most distance tasks also rely on comfortable and efficient dynamic vision.

As a matter of fact, any task that requires a large distance zone will also require smooth transitions in power and low levels of swim and distortion. (Watching television, for instance, requires neither a wide field of distance vision nor low levels of image swim, since both the field of view and eye movements are limited.) Dr. Sheedy even notes in his paper that large eye movements are immediately followed by a compensatory head movement, resulting in a relatively small final angle of eye movement. This suggests that measures of dynamic vision, which are directly related to eye/head movements, may even be more important than the absolute size of the distance zone. Clinical studies done by our R&D group have confirmed the importance of soft transitions in the distance periphery.

* * *

In general, the results of Dr. Sheedy's evaluation may be less consistent with some of the modern approaches to lens design that many manufacturers are now pursuing. In particular, some of his measurement choices may potentially disadvantage newer design philosophies, while failing to reveal the performance limitations of older philosophies.

Nevertheless, I feel that his idea was quite sound, and this type of objective performance assessment would be extremely valuable to eyecare professionals. However, since this would be perceived as an objective assessment by eyecare professionals, it is important that the methodology and assumptions employed reflect the continuing evolution in progressive lens technology. With the right the tools, he could further improve future evalulations in order to ensure that the results are as accurate and meaningful as possible.

Obviously, Dr. Sheedy has fewer resources at his disposal than a major lens manucturer, like SOLA or Essilor, would have -- and this is certainly a consideration. While SOLA (or any lens manufacturer, no doubt) would obviously feel that our approach to lens assessment is preferable for modern lens designs, we can still appreciate the effort that Dr. Sheedy put into his evaluation. And, while we might not wholeheartedly agree with all of his results, we can also appreciate his ultimate goal of educating eyecare professionals on the value of utilizing certain lens designs for specific visual tasks in order to match the performance characteristics of the lens with the visual requirements of the wearer.

[QDO1]

I know there is a new report in the pipeline. also Dr sheedy does stand out in virtual isolation as an independant who looks at these lens designs, accross the board, from a (certainly singular) analytical standpoint. As a dispenser on the ground, the reports are an invaluable starting point, with info, that to be frank, is unavailable elsewhere

As much as I respect chaps like Pete and Daryl, my postbag, even at home, is crammed full of all the latest claims from all of the manufacturers of lenses claiming XYZ is better than sliced bread or anything else. Dr Sheedys work does help us strip out the hype from the fact

[drk]

Darryl, what's keeping Carl Zeiss Vision from posting the results of the Sola One analysis? Yes, it's "biased", but I'd take that into consideration. Is there too much proprietary information at stake just in releasing a ranking and the parameters measured? Methodology could be kept masked from us.

Yeah, and I want a new bicycle for Christmas, too.

[Darryl Meister]

Darryl, what's keeping Carl Zeiss Vision from posting the results of the Sola One analysis?

Actually, the bottom-line results (by viewing task) appear in the SOLAOne White Paper for a range of lens designs. Maybe someday we can release a public version of our various progressive analyses, though the Marketing guys usually look at me like I'm crazy when I suggest such things.

[QDO1]

better to wait till an independant tester does it. the white paper would be great reading though

[Darryl Meister]

better to wait till an independant tester does it

You have Dr. Sheedy's, measurement issues notwithstanding.

[Les Moss]

Just got an email from Dr. Sheedy. There will be an update to this study with newer lenses coming this month in "Optometry". I don't know if the methodology will be updated as well.