Darryl, as I gather from some of your previous posts on progressives, there is a finite amount of dist, int and near governed by a mathematical formula. Thus if you reduce one zone in size , it allows the other 2 to be increased, right? So then, wouldn't a really short corridor allow the corridor to be wider? I'm just wondering if we will really be able to customize these free-form progressives for the individual patient's needs.
OK , question for anyone who can answer the question.
The length of the umbilic is inversely proportionate to the amount of distortion that is induced, so no, a very short corridor wouldn't produce a great result. In any case, what exactly would be the point of a progressive with no intermediate?
short corridors do seem to work better for absolute presbyopes.
Yes, I agree. I fit short corridor lenses more often than not. I didn't think that was what was being asked.
Originally Posted by Robert_S
Robert, I didn't mean no int but shorter corridor. Wouldn't that allow wider int since the total area stays the same? I agree, I like short corridors better also.
Lookup up minkwitz therom for a relationship between coridor length and astigmatism in the corridor. Intuitivly if you think about the higher the rate of change from the distance to near zone the more extreme the blend must be to achieve a surface without lines. This blending is done to the sides of the corridor so the intermediate would be narrow. Now the minkwitz therom gives a relationship of:
change in cylinder per mm = 2 * change in add power.
With the change in add per mm approximately = add power / corridor length
Using a 16mm corridor compared to a 12mm corridor in a 2.50 add
teh add would change 0.16D/mm in the 16mm corridor length and 0.21D/mm for the 12mm corridor, the corresponding change in cylider would be, 0.32DC/mm compared to 0.42DC/mm so the short corridor exhibits a narrower area of intermediate along the corridor. Now this is assuming a linear relationship with would apply to older design progressive lenses, newer lenses can vary the rate of change along the umbilic allowing for a wider zones in certain areas.
From Darryl's tour de force course in Opticampus, "Optics of a Progressive"
Note the statements at the end---
The Progressive Corridor
Recall that the umbilic represents the centerline of the progressive corridor, which is the channel of relatively clear vision connecting the distance and near viewing zones. It was also demonstrated mathematically that the width and intermediate utility of the progressive corridor depend upon the corridor length of the lens for a given Add power. The lens designer is primarily concerned with three features related to the geometry of the umbilic:
- Total length of the umbilic
- Horizontal placement of the umbilic
- Management of power along the umbilic
Earlier, we defined corridor length as the vertical distance between a point along the umbilic producing the lowest mean power (in the distance zone) to a point producing the highest mean power (in the near zone, where the maximum Add power occurs). The point producing the lowest surface power at the start of the umbilic is generally located at the distance reference point (DRP) of the lens, while the point producing the highest surface power at the end of the umbilic is generally located at the near reference point (NRP).
In most cases, the umbilic is rotated so that it is aligned with the natural movement of the eyes as they converge to see nearby objects through the intermediate and near zones. This ensures that the eyepath, or the path the line of sight travels over the lens surface as it approaches the near zone, remains properly centered within the progressive corridor. As with lined multifocals, the lateral placement of the near zone is referred to as the inset. The total rotation of the umbilic will depend upon the inset. The eyepath of more advanced lens designs is precisely computed by calculating the theoretical reading distance that corresponds to the mean power at each point along the umbilic.
The change in mean power (or Add power) along the umbilic is referred to as the power profile of the lens design. The power management geometry of the umbilic is primarily driven by the corridor length of the lens design, since the length of the corridor will dictate how rapidly the mean power must change in order to reach the full Add power. The average rate of change in mean power along the umbilic is inversely proportional to the corridor length:Rate of Change in Power = Add Power ÷ Corridor LengthHowever, it is actually possible to vary the mean power along the umbilic in a non-linear fashion in order to achieve a degree of flexibility in the optical performance of the lens design. For instance, the power profile often ramps up in mean power rather slowly at first to minimize the excess plus power in the distance zone, which could otherwise blur vision. The power profile may then ramp up more quickly in the progressive corridor to deliver a sufficient amount of mean (Add) power for reading vision well above the near reference point, so that the wearer may realize some degree of near utility higher in the lens. Careful control of the power profile is also necessary to ensure that the change in Add power reflects the natural inclination of reading materials.
In practice, corridor length is measured from the fitting point, not the distance reference point, since the fitting point is aligned with the optical system of the eye. (The location of the distance reference point is somewhat arbitrary; it is generally located to ensure error-free verification of the prescription.) Furthermore, it is common to specify the length to a percentage of the specified Add power, such as 85% or 95%. Consequently, the advertised corridor length is often shorter than the distance to the near reference point.
Although in theory a progressive lens could have a corridor length that approaches zero, there are certain optical and visual considerations that place practical limitations on the range of acceptable corridor lengths. Several factors must be taken into account when choosing the corridor length for a progressive lens design. For example, shorter corridor lengths offer the following advantages:
- More near vision utility in smaller frames
- Reduced eye declination during near vision
Every additional 1 mm of corridor length requires roughly 2° of additional ocular rotation to reach the near zone. If the corridor length is too long, the wearer may not be able to reach the full Add power of the near zone without awkward postural adjustments. However, shorter corridor lengths offer the following disadvantages:
- Less intermediate vision and mid-range utility
- More rapidly increasing power and astigmatism
The shorter the progressive corridor, the more the optics of the lens design must be effectively "squeezed" together. This results in higher levels of peripheral blur, reduced intermediate utility, or narrower viewing zones. Consequently, the length of the corridor should be carefully chosen in order to offer the most utility with the least amount of compromise.
Bookmark the site!
http://www.opticampus.com/tools/thickness.php
Yeah, your math says one thing but how many +2.50 add progressive wearers prefer short corridor to long ?I say most. I'll start a poll
2.50 add here. Far prefer long over short.
Same here, except for a +2.25 add.
It's probably important to note that although I use PALs that might at first glance fall into the long corridor category, some are really medium in length, i.e. Auto 2 fixed 18. Same for some "short" designs i.e. Physio Short. The days of really long corridors and high adds (Gradal Top/AO Pro15 or Omni) are pretty much over because they require too much ocular rotation to be of much use at near, especially for moderate to high myopes.
The corridor lengths and power rates that I use for adds over +2.00 might be more relaxed for less near tasks, or more aggressive for an avid reader of newspapers. I also make sure that those with intermediate tasks have the proper eyewear for such use, resulting in more latitude in optimizing the general purpose eyeglasses. By avoiding the shorties, my clients experience (1) better distance vision, especially at night, and (2) more relaxed and less dynamic visual performance across both principle meridians.
Moreover, I see a greater need, from my client's perspective, for outstanding distance peripheral vision, with a reading zone that is high enough to keep posturing to a minimum, chosen for the type and frequency of near tasks.
Per the Moody Blues, and Bary Santini, It is A Question of Balance.
WRT PAL zone widths and interaction with other design factors see...
http://legacy.revoptom.com/index.asp?page=2_13059.htm
http://www.opticampus.com/cecourse.p...essive_lenses/
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Sorry Barry, but I much prefer the Moody Blues. I haven't heard you sing yet, though. Maybe a concert like Moody blues in Red Rock would do it. Come to think of it, the Moody Blues probably cannot even take a PD.
Instead,short corridor design will increase the distortion of the mid area. That's for sure. I have seen some design from Japnese companies like Hoya or Nikon have very wide corridor and max distortion less than 85% of add but the corridor is quite long and farvision area is narrowed.
I referenced to Barry for two reasons: his ABOM paper had "balance" as somewhat of a theme, and because he's as good of a musician has any of the Moody Blues boys.
Science is a way of trying not to fool yourself. - Richard P. Feynman
Experience is the hardest teacher. She gives the test before the lesson.
Wait a minute, Barry is as good a musician as the MOODY BLUES ? Let's get him to Red Rock!
There are two seperate, but related issues involved. A shorter corridor in a higher add has three negative results. The first is that each portion of the intermediate shrinks, so with a 2.50 add, in a short corridor, each step becomes shorter and steeper. A 2.50 in an 12 seg would effectively have zero usefull intermediate vision. Think of it as steps, if add more steps but don't increase lenght, each step must be smaller. much smaller.
The second factor is that under Minkwitz, the higher add, the shorter corridor, both increase distortion geometrically, which inherently will also narrow the corridor in many adaptive designs.
I can widen the corridor by using a harder design, even with a short seg. but again, distortion increases geometrically in addition to the above as the lens gets harder, the concave front side transitional curves get steeper, diverging from the RX curves at a higher rate. but because the distortion is working against the corridor, I have to make the lens disproportionally harder to increase width.
The result for some patients is often unpleasant. They end up with a lot of total distortion, and a very small intermediate increments.
In general, higher adds will benefit in many ways from longer corridors, proportional to their RX. High Myopes esp will want their add powers to come on quickly, and some other patients can tolerate the high distortion, so there always exceptions.
Sorry that I'm late to this conversation, but I rarely check the posts in the Progressive Lens Forum, which I'm afraid frequently caters to brand-specific marketing propaganda precipitated by a great deal of personal bias, rather than objective fact. However, there have been a lot of very good posts already. That said...
The fact is, advanced or absolute presbyopes face a paradox with progressive lenses: A shorter corridor length allows the wearer to more easily reach the full add power necessary for comfortable near vision and provides a wider field of reading utility for a given distance from the fitting point, whereas a longer corridor length provides more intermediate utility and lower levels of peripheral distortion, which become particularly important as the add power increases (due to the absence of adequate accommodative amplitude for mid-range viewing distances and to the fact that peripheral distortion is proportional to add power).
This is, in fact, why there are actually two diametrically opposed philosophies used by lens designers when designing "multi-design" progressive lenses that vary by add power: The first philosophy seeks to make the design harder as a function of add power in order to provide a shorter corridor with a larger zone of stabilized add power, whereas the second philosophy seeks to make the design softer in order to provide a longer corridor with more intermediate utility and less peripheral distortion.
As to which design philosophy is better is primarily a matter of personal preference for the wearer, based upon his or her specific visual requirements, previous lens design, et cetera. Otherwise, a strong argument can be made for either philosophy or even not varying the corridor length as a function of add power at all.
Best regards,
Darryl
Darryl J. Meister, ABOM
One of those so called marketing propaganda companies signs your paycheck and sponsors this website, an objective fact. Isn't an online forum a place where one can air their opinions about a particular brand biased or not?
Since when did posting become based on factual events? We are not researchers nor scientists.
Last edited by HindSight2020; 03-26-2013 at 06:15 AM.
And this bit of anonymous snark is why so many industry professionals and acknowledged experts choose to walk away from this site. We are poorer for it.
There was nothing wrong with hindsight's post. He has just as much right to express his opinion as Darryl.
Except that the thread was posed as a question for Darryl.
Daryl, Thanks for the feedback, it is always good instruction when you enter the conversation.
Judy, spot on, those that knocked on this probably have likely not read many of Daryl's posts which are as non-corporate as any we ever read. Just straight physics, math and customer focused without the marketing, no matter who he has worked for...
Not only is this not objective fact, it is not correct at all. I challenge you to find one example of me or any other Carl Zeiss Vision employee promoting or advocating a CZV product on OptiBoard over the 17 years that I've been involved with this forum. Nevertheless, you are correct in observing that Carl Zeiss Vision continues to sponsor this site (unlike many of the brands routinely discussed in the Progressive Lens Forum).
Not that I agree with this statement, but I never suggested that an online forum wasn't a place to air opinions. On the contrary, I think the Internet is absolutely full of people airing their opinions, whether misguided and ill-informed or not. That and people posting pictures of cats in various amusing situations.
My point was that I, personally, don't typically engage in product-specific or self-promotional conversations, and have come to spend less time in the Progressive Lens Forum as a result, which is why I didn't respond sooner to a thread that was originally directed at me. Unfortunately, these threads more often than not drift into the realm of statements like, "I really like Brand X, so you should use it, too."
You, on the other hand, are certainly welcome to "air opinions" or to post and discuss whatever you'd like on OptiBoard, at least within the bounds of the user guidelines.
Best regards,
Darryl
Darryl J. Meister, ABOM
Snark? Really? That's hardly the case and I didn't realize I needed 'special' opinion to chime in. And for the record, I am both an industry professional and acknowedged expert.
My post wasn't meant to ruffle feathers or advocate a specific brand or supplier, but rather to state an obvious fact that the mass influx of confusion and self claim marketing propaganda is caused by 'ALL' of the manufacturers. Actually, I admire Darryl's intelligent posts and views on specific topics.
On the other side of the same coin, I can also say that most companies have excellent products and do thier due diligence with clinical trials prior to launching to ECP's. However, they have all created this monster to try and put space between them and their competitors and its time they should all band together and use common marketing jargon.
That's all I meant, so please consider this an apology and lighten up.
Last edited by HindSight2020; 03-26-2013 at 01:02 PM.
Thanks Robert.
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