Physio and lens design

[eyeboy]

I was looking at the literature for physio and noticed that they say if people have to have astigmatic distortion that they normally prefer it to be vertical. Does this mean that in lens design it would better to correct the tangential and leave sagittal oblique astigmatism. I think this is called minimum tangential form?.

[Darryl Meister]

This is certainly an interesting discussion topic. It's probably important to clarify a few terms and concepts here.

Essilor is referring to text legibility when discussing vertically oriented astigmatism. They are not suggesting that people necessarily prefer one form of ocular astigmatism over another, but that uncorrected astigmatism at an oblique axis makes it slightly more difficult to read printed text. An astigmatic focus forms a blur ellipse on the retina instead of a sharp image point. When trying to recognize letters, these blur ellipses are least troublesome when oriented vertically (that is, when the major axis of the ellipse is close to 90, so that the horizontal strokes of the letters are blurred instead of the vertical strokes) and most troublesome when oriented obliquely. Horizontally oriented ellipses fall are in the middle in terms of recognition.

The surface astigmatism in the lateral regions of a progressive lens is generally oriented at an oblique axis. While it is not feasible to completely shift these blur ellipses to an entirely vertical orientation, lens designers can at least minimize the "oblique-ness" of their orientation. (Though, as with anything in modern progressive lens design, this comes at a bit of a cost elsewhere.)

Now, back to your question... At this point is important to distinguish between the surface astigmatism of the progressive lens design, the ocular astigmatism of the wearer, and the oblique astigmatism produced when looking through peripheral regions of any lens. We can ignore the wearer's astigmatism in this discussion, since it must be independent of the lens design for a semi-finished lens.

Tangential and sagittal power errors are generally discussed in the context of oblique astigmatism -- or the lens aberration that lenses suffer from when the correct Base curve isn't used for a given prescription. These power errors will interact with the surface astigmatism of the progressive design, and many modern progressive lens designs account for this. However, the orientation of the astigmatism axis in oblique astigmatism varies depending upon your direction of gaze, so the tangential and sagittal power errors aren't strictly "vertical" or "horizontal." For example, the tangential error may result in a vertically oriented blur ellipse when looking up or down through the lens, but when looking side-to-side the blur ellipse becomes horizontally oriented. In other directions, the blur ellipse would be oriented at any angle in between.

Consequently, minimizing the tangential error will won't necessarily accomplish the design of goal of minimizing unwanted astigmatism at an oblique orientation. Instead, Essilor's progressive lens designers most likely use a merit function that includes a mathematical term specifically associated with astigmatism at an oblique axis. This term -- along with any other terms in the merit function -- is then minimized during the lens design process. Further, if ray-traced astigmatism is evaluated instead of only surface astigmatism, the combined effects of oblique astigmatism and surface astigmatism can be controlled simultaneously.

[Cindy K]

Darryl, thanx for the 'optician's explanation' of this lens design rather than the 'manufacturer's propaganda explanation'. Though I understood the redesign of the surface astigmatism to translate to clearer vision for the wearer, it also provides a perceived widening of the intermediate corridor.

[66Lenses]

This is certainly an interesting discussion topic. It's probably important to clarify a few terms and concepts here....

Great post Darryl. Certainly cleared some things up with me!

[mhboptics]

Best explaination I have heard since this lens wass released.

THANK YOU!

[Darryl Meister]

Glad to help.

[lensguy]

This is certainly an interesting discussion topic. It's probably important to clarify a few terms and concepts here.

Essilor is referring to text legibility when discussing vertically oriented astigmatism. They are not suggesting that people necessarily prefer one form of ocular astigmatism over another, but that uncorrected astigmatism at an oblique axis makes it slightly more difficult to read printed text. An astigmatic focus forms a blur ellipse on the retina instead of a sharp image point. When trying to recognize letters, these blur ellipses are least troublesome when oriented vertically (that is, when the major axis of the ellipse is close to 90, so that the horizontal strokes of the letters are blurred instead of the vertical strokes) and most troublesome when oriented obliquely. Horizontally oriented ellipses fall are in the middle in terms of recognition.

The surface astigmatism in the lateral regions of a progressive lens is generally oriented at an oblique axis. While it is not feasible to completely shift these blur ellipses to an entirely vertical orientation, lens designers can at least minimize the "oblique-ness" of their orientation. (Though, as with anything in modern progressive lens design, this comes at a bit of a cost elsewhere.)

Now, back to your question... At this point is important to distinguish between the surface astigmatism of the progressive lens design, the ocular astigmatism of the wearer, and the oblique astigmatism produced when looking through peripheral regions of any lens. We can ignore the wearer's astigmatism in this discussion, since it must be independent of the lens design for a semi-finished lens.

Tangential and sagittal power errors are generally discussed in the context of oblique astigmatism -- or the lens aberration that lenses suffer from when the correct Base curve isn't used for a given prescription. These power errors will interact with the surface astigmatism of the progressive design, and many modern progressive lens designs account for this. However, the orientation of the astigmatism axis in oblique astigmatism varies depending upon your direction of gaze, so the tangential and sagittal power errors aren't strictly "vertical" or "horizontal." For example, the tangential error may result in a vertically oriented blur ellipse when looking up or down through the lens, but when looking side-to-side the blur ellipse becomes horizontally oriented. In other directions, the blur ellipse would be oriented at any angle in between.

Consequently, minimizing the tangential error will won't necessarily accomplish the design of goal of minimizing unwanted astigmatism at an oblique orientation. Instead, Essilor's progressive lens designers most likely use a merit function that includes a mathematical term specifically associated with astigmatism at an oblique axis. This term -- along with any other terms in the merit function -- is then minimized during the lens design process. Further, if ray-traced astigmatism is evaluated instead of only surface astigmatism, the combined effects of oblique astigmatism and surface astigmatism can be controlled simultaneously.

To dumb this down one step further, is this not the same theory of making design and power adjustments for "in wear position" just like the Individual lens does. Only to a more accurate degree due to the fact we would provide the ACUAL pano and vertex ?


Thanks,

[Darryl Meister]

To dumb this down one step further, is this not the same theory of making design and power adjustments for "in wear position" just like the Individual lens does.

The bit about optimizing for ray-traced astigmatism as opposed to surface astigmatism is an example of as-worn or position of wear optical optimization, Yes. But Gradal Individual (and Varilux Physio 360, for that matter) are optimized for the individual prescription as well, which goes far beyond what it possible in semi-finished progressive lens design.

Only to a more accurate degree due to the fact we would provide the ACUAL pano and vertex ?

Yes, you can supply actual fitting parameters (including vertex distance and pantoscopic tilt) with Gradal Individual.

[hardbox_happy]

what is the different between physio & physio 360?how consumer the different?
the physio is selling a Wave technology only right,not selling their progressive design?
such as Sola one was post the design for us to know ,how bout physio?
if selling the wave technology only,then any design also can use this wave technology?

[66Lenses]

what is the different between physio & physio 360?how consumer the different?
the physio is selling a Wave technology only right,not selling their progressive design?
such as Sola one was post the design for us to know ,how bout physio?
if selling the wave technology only,then any design also can use this wave technology?

What exactly are you asking here?