Originally Posted by
drk
Hey, Buckeye.
This is an oddly coincidental thread, as I was here to start something similar.
Instead of linking, I will cut-n-paste a personally saved excerpt of an Optiboard thread, written by a now-unidentified savant poster:
Manufacturer Name Corridor Length Minimum Height
AO/SOLA Compact 13 mm 17 mm
AO/SOLA SOLAMAX 12.4 mm 18 mm
Essilor Varilux Panamic 12.5 mm 18 mm
Essilor
Nikon? Presio? i 13?
13 mm 17 mm
Hoya Summit CD 11 mm 14 mm
Kodak ConciseTM 14 mm 17 mm
Pentax AF mini 14 mm 17 mm
Rodenstock Progressiv® XS 12 mm 16 mm
Seiko Proceed II Short 14 mm 18 mm
Shamir Piccolo 11.5 mm 16 mm
Vision-Ease Outlook 14 mm 18 mm
Younger Image® 13.5 mm 18 mm
Zeiss Gradal Individual 14 mm 18 mm
Zeiss Short i 11 mm 15 mm
The Essilor varilux lens will actually be a min fitting ht of 14mm and a corridor length of 9.5mm. It will be released April 1st and is called Ellipse
Corridor length (sometimes called progression length)--the distance over which the power progression occurs. Since the power typically begins to vary several millimeters below the fitting cross, the corridor length is generally shorter than the minimum fitting height. For general purpose progressives with a min fitting height of 18-22 mm, corridor length would typically be around 14-18 mm.
Regarding intermediate, any truly short corridor lens is going to have a minimal intermediate zone. These lenses are not going to be especially well-suited for intermediate tasks.
The research conducted when developing Varilux Ellipse turned up a couple interesting findings. First, not surprisingly, patients strongly preferred a short progression when wearing a small "B" frame. Secondly, and perhaps more interesting, patients wearing short "B" frames were most sensitive to peripheral restrictions in the distance portion of the lens.
All truly "short" progressions are going to be relatively (relative to general use PAL designs, that is) narrow in the near zone. However, most of the "short" corridor PALs currently available on the market also use the distance periphery to distribute the unwanted cylinder that occurs with a seamless progression of power.
Varilux Ellipse will have 140 degrees of clear distance. This is about 15% more than the closest competitor design, and really seems to improve patient acceptance of the lens. My theory is that this occurs because- in the end- most of us still use our eyewear for distance viewing more than for close tasks.
In my opinion, that is why a lens like Ovation (or any other general use PAL that has a progression short enough to give near function in a small frame) works better than the "short" PALs. Nothing against Shamir Piccolo, but the area of unrestricted distance viewing is only 103 degrees. AO Compact has 109 degrees, and Summit CD has 118 degrees. Compared to most general use PALs, these lenses restrict the area of distance viewing. In a small frame, the result is a very limited distance area.
Of course, there are other considerations as well. For example, AO Compact is an extremely hard design that presents the wearer with a lot of motion in the near periphery. This explains why even general use PALs like Varilux Comfort score higher than AO Compact in wearer tests- even in the areas of near vision (where a design like AO Compact should theoretically excel).
Unfortunately, physics being what they are, you cannot have a generous intermediate while cramming the progression into a tiny area. Therefore, the vast majority of your "short-corridor" offerings are little more than a general type design with a dislocated fitting cross, near verification circle, or both. The ones that are actually short have considerable distortion crammed into the distance periphery- and usually also have poor symmetry between the nasal and temporal periphery of the near zone (causing swim).
Since most "short corridors" are fit to lower add powers, most wearers don't notice they have been fit down into the top of the progression- or that they have compromised the intermediate zone.
Keep in mind that few manufacturers adhere to the same standard of corridor measurement, so if you asked each of them separately for their own figures, they probably wouldn't be very consistent with each other. Some may measure to 85% of the add power (Essilor), some to 95% (SOLA), some to the add power less 0.12 (AO), some to the near circle, some using surface power, some using front vertex power, and so on. Also, the "length of the corridor" is only so meaningful, as I discuss below.
On this point (and many others, I would suspect), we completely agree. The "old" measures of PAL performance (width of zones, softness of design, and more recently corridor length), have been replaced by several considerations which are addressed in modern lenses. Namely, does the design take into account the ametropia and level of presbyopia of the wearer? How symmetrical is the distortion in the lens? How smooth is the peripheral progression in the lens? And so on...
The minimum height you can fit a Varilux Comfort and receive the optimal benefit of the entire reading area is 22mm.
At 18mm, Varilux Comfort provides about 5.5mm of comfortable reading area. So, while an 18mm height isn't optimal, it does permit the design to work as well as- better in most cases, actually- than a lens originally marketed with a shorter fitting height recommendation.
Based on the power progression profile of the design, the Hoya Summit CD probably doesn't perform all that well at 14-15mm. On the other hand, the Rodenstock Life XS is a rather short- if extremely hard- design, so it should work better at lower fitting heights. In fact, wearer studies Essilor has performed comparing the Life XS and Varilux Ellipse show that the Rodenstock product performs pretty well (Varilux Ellipse scored better, but the Rodenstock product wasn't bad).
Progressive lenses must obey certain mathematical principles in order to produce a change in add power with a smooth surface (having smoothly increasing power and magnification). One of the most important of these principles is that the change in surface astigmatism must increase as the rate of change in add power increases. Shorter corridor lengths require more rapid changes in add power and, consequently, more rapid changes in surface astigmatism.
So, progressive lens manufacturers must make certain compromises or trade-offs when using shorter corridors because of the added restrictions imposed by the increasing astigmatism. They must also find an acceptable balance between viewing zone sizes, peripheral blur, swim, and so on. Unfortunately, there is no magic formula here.
You will also find that conventional progressive lenses have a relatively small range of corridor lengths (on the order of only a few millimeters for +2.00 adds), suggesting that manufacturers have found an optimum range of corridor lengths that allows for a good balance of optical performance and utility. Consequently, you have to ask yourself what compromises you might be tolerating by using corridor lengths that are significantly shorter than necessary simply for the sake of unrestrained frame selection.
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