# Thread: I want the TRUTH!

1. ## I want the TRUTH!

We've kicked this around before, and I am the outlier, so I'm asking again.

If someone is hyperopic, they will experience BU effect to a variable degree on downgaze.

Say, +10.00
Say, 1mm downzage = 1^ BU
2 mm = 2 BU
3 mm = 3 BU

So the lens is a gradient prism:

1^
2^
3^
4^
5^
6^
7^
8^
9^
10^ @ 10mm downgaze.

So the perception of objects' location in space is
correct
lower
lower-lower
lower-lower-lower
lower-lower-lower-lower
etc.

This will mean that, standing, one's dance partner's face is in the right position, their waist is rather low for a person whom you are wrapping your arms around, and their feet on the floor are really down there. The person is optically stretched downwards. This is why people get sick with glasses at first.

Now, lets talk about progressive zones. The view of the bottom of reading material through lower part of the progressive is perceived lower than the view of the upper portion of the reading material.
So, it makes sense to me that one would want to accelerate the progressive corridor's rate of change and get to the stabilized near zone more quickly so that the view would become more compacted.

I am a blithering myope so I cannot test this theory, except in reverse.

Any real-world wearing experience, here? Any theoretical answers from ray-tracing savants?

2. This is the same phenomenon that bifocal wearers complained about the biggest culprit was the Ultex style bifocal that was a 38 mm circle cut in half and the oc was at the bottom creating bd effect. The same problem was found with the 22 mm rd bifocal to a lesser extent. The creation of the ft eliminated this by putting the oc at the reading point 5 mm below the top of the seg. The Exec style went a little further and put the oc at the seg line claiming a no jump bifocal.

3. For fun you could fit yourself with CLs and trial lens for the experience

4. Originally Posted by drk
So, it makes sense to me that one would want to accelerate the progressive corridor's rate of change and get to the stabilized near zone more quickly so that the view would become more compacted.
The other way around is true for hyperopes.

Hyperopic and myopic patients need different progression lengths to accommodate the vertical prismatic effect of plus and minus progressive lenses

• With a plus lens, light from a near object is converged or deviated upward toward the base or center of the lens, displacing the object lower in the lens

• With a minus lens, light from a near object is diverged or deviated downward toward the base or edge of the lens, displacing the object higher in the lens

Moreover, the corridor/near point width also needs to be wider for hyperopes due to magnification. Fortunately, the longer corridor spec for hyperopes makes it easier to widen the near zones. The opposite is true for myopes- shorter and narrower corridor (compared to emmtropes) is preferred.

For example, switching to PALs for myopes (me, and you too, right?), let's look at a PAL design for an emmetrope that is 10mm long to 100% power, with a near point of 15mm. A moderate to high myope will see the object about 2mm higher in their vertical field, requiring a 8mm corridor for the same functionality.

Hope this helps,

Robert Martellaro

5. Thanks for the link, Robert, however it only discusses the problem and doesn't give the answer specifically.

I know that you understand it (probably correctly) opposite than I do, therefore you dutifully try to set me straight every time I bring this up.

6. I may have to, Chris.

7. drk,

I think you're yankin' on my trouser leg, that you don't understand the effects of prism on image position.

http://aamerniazi.blogspot.com/2014/...ve-lenses.html

Anyways, I wonder how much the designers lengthen the corridor for hyperopes compared to emmetropes, considering that prism thinning helps by cancelling some of the base up prism induced by lens power and reading depth value.

To be sure, the key is not having too much eye declination at near, with most lens designers depending on vision science and wearer trials to sort this out.

Best regards,

Robert Martellaro

8. Can you appreciate my concept that there is not just 'base up' prism in plus lenses on downgaze, but there is VARIABLE base up prism in downgaze? I'm not talking about the "instantaneous prism at point -10mm" but the slope of the prism as you look down (greater and greater prism).

I believe that a simple analysis at one point along the lens will simply give object displacement that could be offset by a fixed prismatic amount. But not so, variable.

I'm just saying that progressive prism gives a weird effect that is different that plain old yoke prism. I consider it to be a "compressive effect" of the view of the world with minus lenses and an "expansive effect" with plus lenses. I wondered if the expansive plus effect could be offset by a shorter corridor to reduce downgaze amount, and the compressive minus effect could be offset by a longer corridor to increase (normalize/uniform-itize) downgaze amount.

My concept may be too hard to race trace on here, so I'll try a sketch ASAP.

9. The compressive and expansive effects are due to minification (minus power) and magnification (plus power), and contribute somewhat to the vertical position of the object, as perceived by the wearer, as I alluded to in an earlier post regarding field width.

I suspect that image displacement due to yoked prism at the near point plays a larger role in the image's vertical position, depending on the vertex distance at near. I'm sure this can be expressed mathematically to confirm.

Regardless, I think it's important to know that myopic avid readers (redundant?) need the shortest corridors, and a failure to provide this by the dispensing optician will result in remakes and disappointment on both sides of the dispensing table.

Best regards,

Robert Martellaro

10. I know your position and would bet against myself on this.

11. https://www.2020mag.com/ce/coping-with-corridors-2B2BD
Barry wrote about it and sees it the same as you.

12. Shamir's screwed up this diagram more than they've got it right, but this is a corrected version, apparently, so we can analyze it.

"Dynamic positioning of full reading" has to mean corridor length, in so many words.

And yes the pic is showing kind of a ray trace from an "ideal point" on the left (30 degrees below the line of sight) toward the right, through the lens and deviated upwards for minus or downwards for plus. The idea to move up or down the location of the near zone is thusly illustrated.

I still don't think it deals with compression or expansion, but just static prism amounts through a single point. And notice that the red and blue ray are being shown deviating before it even hits the lens surface! Doh!

I give up, folks. Thanks for fueling my pet peeve.

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