Fixed vs variable corridors.

[Robert Martellaro]

Call me Brent.

Hello Brent. My intent was to respect your anonymity, but finally realized you have your name at the bottom of the page. Doh!

Here's a bit more to chew on.

http://www.shamirlens.com/index.php?option=com_k2&view=item&id=16529:shamir-autographiii&Itemid=249

http://essilorpalspro.com/Ovation_ECP.pdf

I'm going to put in my usual nonsense.

With minus power lenses, they have progressive BD prism. As a visual aid, envision a six-foot-tall target painted on a wall across the room and the myope standing in front of it, looking at the bull's eye.

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The bull's eye will be perceived exactly where it is, in real space, because that's coming through a no-prism section of the lens.

The next ring down will be coming through the lens where there is some BD prism. Let's just say, 1/2^ BD. So the "9 points" ring is going to look higher than it actually is, but only by a small amount.

Light from the "5 points" ring comes through the lens where, let's say, 2^ BD is present. So that ring in the peripheral vision is going to look higher, too, but even more so than just off-center objects, because while the first ring is only slightly displaced, this ring is displaced even more.*

So the net effect is that the inferior field is compressed for a myope.

So what do we want in a progressive lens? Do we want one that is likewise compressed to fit the prism compression, like RM thinks?

If the object is higher in the wearer's apparent field of view, the line of sight at near will intersect the lens higher in the corridor where there is less add power. Solution- ramp up the add power quicker and/or shorten the corridor.

Or, do we want one that is expanded, to offset the prism compression, as seems intuitive to me?

The prism displaces the object, compression or expansion due to minification or magnification respectively has an additive effect.

Shamir's information has it both ways!

I've seen it displayed inaccurately. Shamir's USA page has it correct (see my link above).

It would be nice to know the answer.

See Essilor's approach in the link posted above.

Best regards,

Robert Martellaro

[drk]

We agree on our diagnosis, but not the treatment.

This has bugged me forever.

You're right, though...the Ovation link agrees with you.

[lensgrinder]

Hello Brent. My intent was to respect your anonymity, but finally realized you have your name at the bottom of the page. Doh!

No worries, I created that name years ago.

The viewing fields that are referred to are controlled in most back surface free form lenses. A variable corridor lens can control this more than a fixed corridor as we can better determine where to place the surface astigmatism and how to better control the rate of change. These lenses will also need to employ variable near inset as this will play a role in the field of view.

The amount of prism along the corridor needs to be controlled, Zeiss uses a 6 mm drop for this very reason. We decided to do this when we went to full back surface free form. By moving the DRP up the prism is better controlled at near.

It all needs to be balanced because you want to have a wider intermediate, but if you shrink your corridor you reduce the intermediate portion.
To understand where you want your target you first need to know your stop distance(CoR plus vertex distance) otherwise you are using averages.
As the back curve becomes more minus or more curved the less you look down the corridor or the less you look down to get in to 85% of the add like the chart shows, this does not mean that a myope cannot look further down than a hyperope. The flatter the curve the more you need to look down or the corridor needs to be reduced. This amount will be different as the lens progresses into the near portion. The curve will become flatter as you look down through the progression which requires a change in the length and only a variable will be able to determine these changes. All of this is also dependent on wrap and tilt.
Of course every ECP needs to do what works best for them. I would still suggest that all new presbyopia and most myopes will be happy with a variable lens.