High index progressives

[drk]

Two questions:

1.) Since peripheral optics with higher-power lenses are lesser, especially with higher indices, are near portions of multifocals (usually progressives) inheirently less clear than the distance portion?

I realize the decrease in power in a minus lens can offset the optics decrease in the periphery.

I don't think there is going to be a quantitative response available, but does the decrease in power sufficiently offset the chromatic abberation increase in the periphery?



2.) I was looking at isoastigmatism plots on Shamir's website and was able to compare a design in different materials.

It showed a better overall lens performance (width of near zone, max unwanted astigmatism, etc.) in 1.67 than in 1.5. Do higher indices allow a progressive design to be more effective, in general?

(Of course, see #1 above to offset any gains in performance with high index progressives, right?)

Muchas gracias.

[Darryl Meister]

In theory, there's really no reason why a higher-index lens can't perform comparably to a lower-index lens. It's generally the other factors associated with the higher index that make the lens more susceptible to oblique aberrations, including the Abbe value (which is generally lower with higher indices) and prescription (which is generally higher). Higher prescriptions are more sensitive to factors such as base curve selection and pantoscopic tilt, and will produce more rapidly increasing aberrations.

However, it is possible to optimize a progressive lens with many of these factors in mind, which will help close the performance gap between the higher-index/higher-Rx lens and the lower-index/lower-Rx lens. Progressive lenses can be optically optimized for the "as-worn" position (including lens tilt) and for the base curve associated with a range of prescriptions, at both distance and near.

As far as differences you noticed between Shamir designs, it could be due to a number of factors. New molds must be made for the high-index version of a lens design. These molds must be adjusted for differences in base curve, refractive index, and even asphericity, and the molds may also need "tweaking" one or more times in order to get the optics exactly right after casting. Consequently, the finished product may look slightly different from the low-index version of the progressive lens, either by design or as a result of subtle, random variations in the tweaking, casting, and measurement processes.

Best regards,
Darryl

[Robert Martellaro]

drk,

About twenty years ago I fit a client with new lenses, and I remember this clearly because I was so arrogant back then (or at least much more so than now) and the client was a whiner. They were 7x28 poly instead of his regular cr39, the goal being reduced weight. Rx was about +4.00 with a 2.50 add. He was not happy when he found out that his near vision was worse with the new glasses, and I had no clue as to what was causing the problem. At this point in my optical career I had fit about a hundred or so aphakics with glasses so I was well aware of Martin's Rule and had the OCs placed optimally. So here I was telling the client that the glasses were fine and he would come back and say he could see better with the old glasses. It wasn't a pretty scene.

So, to answer your question "Since peripheral optics with higher-power lenses are lesser, especially with higher indices, are near portions of multifocals (usually progressives) inherently less clear than the distance portion?" Absolutely, and since the amount of blur is a function of the lenses abbe number and the amount of prism induced when the gaze is off-axis, we will see more of a problem with trifocals and PALs where the reading depth is 13mm to 16 mm instead of the typical 10mm or 11mm with bifocals.

ECPs *should* be trying to tweak the last bit of performance out of the lenses that they dispensing. There is a market for excellence, and it's the pragmatic, knowledgeable and ethical ECPs that will survive as the schlock opticals continue to lower the bar.

Regards

[Darryl Meister]

Absolutely, and since the amount of blur is a function of the lenses abbe number and the amount of prism induced when the gaze is off-axis

If memory serves me, this was used as a selling point for the use of more expensive barium bifocal segments instead of the cheaper flint segments many years ago.

Best regards,
Darryl