Results 1 to 18 of 18

Thread: Fixed vs variable corridors.

  1. #1
    What's up? drk's Avatar
    Join Date
    Mar 2004
    Location
    Ohio
    Occupation
    Optometrist
    Posts
    9,386

    Fixed vs variable corridors.

    I'm currently of the opinion:

    a. specifying "variable" is always better...
    b. until you have a very high fitting height

    My thinking is that you don't want TOO long of a corridor.

    Now, I doubt if you specified "variable" and the fitting height was 30mm, that you'd get a 25mm corridor...I'm sure there's a cut-off somewhere.

    But in (b) above, I usually specify "fixed 18" at 18mm and above.

    What do you think of that approach?

    Thanks!

  2. #2
    Master OptiBoarder lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    504
    A variable corridor is all around better as it allows for wider intermediate and near. As general rule the longest corridor you will receive in a variable lens is 15 mm, this is to the start of the NRP and is equivalent to 30° of eye rotation( depends on certain factors).
    It varies based on certain criteria, the center of rotation distance (center of rotation plus the vertex distance), PoW, frame shape in relation to pupil, being some factors. A myope will require a longer corridor where a hyperope requires a shorter corridor.
    A common misconception is that variable lenses provide less near, but usually the problem is due to the compensation and the type of lens they had previously. For example if a +1.00 has a +1.50 add and they were wearing a front surface molded progressive they effectively were looking through a 1.75 add. When it is time to get a new pair there add is switched to a +1.75 and if they switch to a compensated design the add is compensated so that they look through a +1.75, so the patient feels no change. This, of course, is more problematic for hyperopes
    Since I work for Zeiss I also hear every week that we have a 6 drop as opposed to 4 and this is why they cannot read. This has nothing to do with the rate of change or how quickly they get into the add power.
    Fixed designs are needed in certain situations like the one I described earlier.
    I would put any new presbyopia in a variable design.
    Of course one size does not fit all and most individuals would benefit from office type lenses where the design is much softer than a typical lens.

  3. #3
    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
    Join Date
    Oct 2016
    Location
    Maryland
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    1,198
    Quote Originally Posted by lensgrinder View Post
    A variable corridor is all around better as it allows for wider intermediate and near. As general rule the longest corridor you will receive in a variable lens is 15 mm, this is to the start of the NRP and is equivalent to 30° of eye rotation( depends on certain factors).
    It varies based on certain criteria, the center of rotation distance (center of rotation plus the vertex distance), PoW, frame shape in relation to pupil, being some factors. A myope will require a longer corridor where a hyperope requires a shorter corridor.
    A common misconception is that variable lenses provide less near, but usually the problem is due to the compensation and the type of lens they had previously. For example if a +1.00 has a +1.50 add and they were wearing a front surface molded progressive they effectively were looking through a 1.75 add. When it is time to get a new pair there add is switched to a +1.75 and if they switch to a compensated design the add is compensated so that they look through a +1.75, so the patient feels no change. This, of course, is more problematic for hyperopes
    Since I work for Zeiss I also hear every week that we have a 6 drop as opposed to 4 and this is why they cannot read. This has nothing to do with the rate of change or how quickly they get into the add power.
    Fixed designs are needed in certain situations like the one I described earlier.
    I would put any new presbyopia in a variable design.
    Of course one size does not fit all and most individuals would benefit from office type lenses where the design is much softer than a typical lens.
    +1

  4. #4
    OptiBoardaholic
    Join Date
    Apr 2012
    Location
    USA
    Occupation
    Dispensing Optician
    Posts
    236
    Quote Originally Posted by lensgrinder View Post
    A variable corridor is all around better as it allows for wider intermediate and near. As general rule the longest corridor you will receive in a variable lens is 15 mm, this is to the start of the NRP and is equivalent to 30° of eye rotation( depends on certain factors).
    It varies based on certain criteria, the center of rotation distance (center of rotation plus the vertex distance), PoW, frame shape in relation to pupil, being some factors. A myope will require a longer corridor where a hyperope requires a shorter corridor.
    A common misconception is that variable lenses provide less near, but usually the problem is due to the compensation and the type of lens they had previously. For example if a +1.00 has a +1.50 add and they were wearing a front surface molded progressive they effectively were looking through a 1.75 add. When it is time to get a new pair there add is switched to a +1.75 and if they switch to a compensated design the add is compensated so that they look through a +1.75, so the patient feels no change. This, of course, is more problematic for hyperopes
    Since I work for Zeiss I also hear every week that we have a 6 drop as opposed to 4 and this is why they cannot read. This has nothing to do with the rate of change or how quickly they get into the add power.
    Fixed designs are needed in certain situations like the one I described earlier.
    I would put any new presbyopia in a variable design.
    Of course one size does not fit all and most individuals would benefit from office type lenses where the design is much softer than a typical lens.
    So what is the advantage of the 6mm drop versus 4 , 2 , 0

  5. #5
    OptiBoardaholic
    Join Date
    Apr 2012
    Location
    USA
    Occupation
    Dispensing Optician
    Posts
    236
    I have had more issues with people saying the reading wasn't good in the variable corridor than in fixed designs with the auto III...they had to look too far down is tge biggest complaint

  6. #6
    Master OptiBoarder lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    504
    Quote Originally Posted by golfnut View Post
    So what is the advantage of the 6mm drop versus 4 , 2 , 0
    Not much, when referring to full free form.
    The idea behind raising the fitting cross was to equalize the prism in the corridor when reading.

  7. #7
    Master OptiBoarder
    Join Date
    Apr 2014
    Location
    Central Texas
    Occupation
    Dispensing Optician
    Posts
    552
    Quote Originally Posted by golfnut View Post
    I have had more issues with people saying the reading wasn't good in the variable corridor than in fixed designs with the auto III...they had to look too far down is tge biggest complaint
    Agreed, I get the incentive for smooth progression, but it doesn't warrant exceeding the patient's angular comfort limit. Variables assume there isn't one, or rather assumes that the frame choice was selected around a variable fit in mind.

    Either strategy would work, in my mind...so long as you know which you're adopting beforehand. I prefer not to have to worry about "too much B" during frame selection. (Hard enough already, isn't it?) Terrific satisfaction rates with fixed here. I know how much intermediate and how much near we're ending up with, and I suspect it's better to force myself into the habit of thinking about it beforehand.

    Using the A3 example, that lens already has so many vertical computations going on that it's better to keep hold of the 'fixed' constants. Your really need a good reason to get off a 15.
    Last edited by Hayde; 05-25-2017 at 12:11 PM. Reason: post was too short. I have a reputation to keep, you know...

  8. #8
    OptiBoard Professional
    Join Date
    Aug 2016
    Location
    USA
    Occupation
    Dispensing Optician
    Posts
    102
    Quote Originally Posted by Hayde View Post
    Terrific satisfaction rates with fixed here.
    +1

    Always like having control over my corridors and not having to worry about frame selection. Plus, you can accommodate those hyperopes that just eat up plus and want very aggressive power profiles in their corridors.
    Last edited by bretk0923; 05-25-2017 at 12:54 PM. Reason: changed my language a bit

  9. #9
    What's up? drk's Avatar
    Join Date
    Mar 2004
    Location
    Ohio
    Occupation
    Optometrist
    Posts
    9,386
    Thanks for the replies.

    I guess we'll have to monkey with it.

  10. #10
    Master OptiBoarder
    Join Date
    Feb 2008
    Location
    Jacksonville, Florida
    Occupation
    Optical Laboratory Technician
    Posts
    1,012
    Using fixed (speaking Auto III) lets us "engineer" the fit of the lense, we know the patients needs better than software.
    Clinton Tower

    The intellect to live free is in short supply
    ALT248=°

  11. #11
    One eye sees, the other feels OptiBoard Silver Supporter
    Join Date
    Jul 2002
    Location
    Wauwatosa Wi
    Occupation
    Dispensing Optician
    Posts
    5,458
    Quote Originally Posted by lensgrinder View Post
    A myope will require a longer corridor where a hyperope requires a shorter corridor.
    That's true if we only consider axial length/stop distance. I believe that object displacement due to minification and prism influences the near performance to a greater degree (see the image below).

    WRT variable and fixed: I use both, but I almost always fit no less than 19mm high, which means I'm getting a corridor no longer than what is used for their fixed 19mm if I'm using Zeiss Individual, or about 13mm (fixed 18mm) if Shamir Auto.

    The key is to make sure the near vision zone is high enough for your client's needs, but not so high that the distance performance is negatively affected.

    Best regards,

    Robert Martellaro
    Attached Thumbnails Attached Thumbnails shamir-natural-posture-2-autograph-3.jpg  
    Last edited by Robert Martellaro; 05-25-2017 at 03:43 PM.
    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.



  12. #12
    Master OptiBoarder lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    504
    Quote Originally Posted by Hayde View Post
    Agreed, I get the incentive for smooth progression, but it doesn't warrant exceeding the patient's angular comfort limit. Variables assume there isn't one, or rather assumes that the frame choice was selected around a variable fit in mind.

    Either strategy would work, in my mind...so long as you know which you're adopting beforehand. I prefer not to have to worry about "too much B" during frame selection. (Hard enough already, isn't it?) Terrific satisfaction rates with fixed here. I know how much intermediate and how much near we're ending up with, and I suspect it's better to force myself into the habit of thinking about it beforehand.

    Using the A3 example, that lens already has so many vertical computations going on that it's better to keep hold of the 'fixed' constants. Your really need a good reason to get off a 15.
    I can only speak to Zeiss lenses as I have seen the technical data. Frame shape and where the eye sits in the frame is one, among many, factors that are calculated before the design is made. We do not have a specific number that says if the B and fitting height are X then we use a corridor length of Y. They will all vary dependent on many factors, some were listed above. So for the new presbyopes a variable corridor is the better option, of course this is my opinion. If you have patients who are wearing fixed and/or some sort of front surface design then they have a stronger effective power and will not do well with the compensation, not necessarily with the corridor length.



    Quote Originally Posted by Robert Martellaro View Post
    That's true if we only consider axial length/stop distance. I believe that object displacement due to minification and prism influences the near performance to a greater degree (see the image below).

    WRT variable and fixed: I use both, but I almost always fit no less than 19mm high, which means I'm getting a corridor no longer than what is used for their fixed 19mm if I'm using Zeiss Individual, or about 13mm (fixed 18mm) if Shamir Auto.

    The key is to make sure the near vision zone is high enough for your client's needs, but not so high that the distance performance is negatively affected.

    Best regards,

    Robert Martellaro
    In order to determine true object displacement you need to have the center or rotation distance along with eye rotation, pantoscopic tilt, wrap, prism, thickness, etc. You also have to consider flatter back surfaces vs steeper surfaces as this determines corridor length. Another factor is digital devices, when we read on one of these we look less down, more in and closer. Because of this we take these factors into the calculation. I did not mean to imply that we use stop distance only to determine the length needed.

  13. #13
    One eye sees, the other feels OptiBoard Silver Supporter
    Join Date
    Jul 2002
    Location
    Wauwatosa Wi
    Occupation
    Dispensing Optician
    Posts
    5,458
    Lensgrinder,

    I understand what you’re saying. Here's my experience.

    I’ve noticed that myopes, myself included, when fit with longer corridor designs, had more symptoms at near, primarily the dreaded chin-lift, needed to acquire the full add power. Emmetropes, and especially hyperopes, rarely experienced poor near vision when the same less design was dispensed. (I must note that I try to select frames for my clients that have the shortest vertex distance possible, resulting in a visual axis at near that encounters the lens surface somewhat higher in the lens than longer vertex distances.)

    This led me to research what could be happening to cause such an effect, learning that prism and minification raises the apparent object position for myopes, and lowers it for hyperopes. Having sorted this out, I started to select lens designs for myopes that had faster power profiles and shorter corridors, decreasing the declination angle needed to acquire the full add. Although this resulted in slightly narrower zone widths for intermediate and near, the minification effect of the minus power lens made that essentially a wash (and inevitable- there’s no cake and eat it too in PAL design!).

    To be sure, a larger value for the distance from the cornea to the center of rotation, typical for myopes, will cancel some of the effects of object displacement (disregarding freakishly short or long axial lengths), I believe that its contribution is minor, at least WRT corridor length and power profile.

    Considering the above, I believe it would be more accurate to say that myopes would benefit from the use of shorter, faster, and narrower corridors, using a slightly harder design, with hyperopes benefitting from an opposite approach, that is, wider, longer corridors with softer power gradients.

    Best regards,

    Roert Martellaro
    Last edited by Robert Martellaro; 05-26-2017 at 05:34 PM.
    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.



  14. #14
    Master OptiBoarder lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    504
    Quote Originally Posted by Robert Martellaro View Post
    Lensgrinder,

    I understand what you’re saying. Here's my experience.

    I’ve noticed that myopes, myself included, when fit with longer corridor designs, had more symptoms at near, primarily the dreaded chin-lift, needed to acquire the full add power. Emmetropes, and especially hyperopes, rarely experienced poor near vision when the same less design was dispensed. (I must note that I try to select frames for my clients that have the shortest vertex distance possible, resulting in a visual axis at near that encounters the lens surface somewhat higher in the lens than longer vertex distances.)

    This led me to research what could be happening to cause such an effect, learning that prism and minification raises the apparent object position for myopes, and lowers it for hyperopes. Having sorted this out, I started to select lens designs for myopes that had faster power profiles and shorter corridors, decreasing the declination angle needed to acquire the full add. Although this resulted in slightly narrower zone widths for intermediate and near, the minification effect of the minus power lens made that essentially a wash (and inevitable- there’s no cake and eat it too in PAL design!).

    To be sure, a larger value for the distance from the cornea to the center of rotation, typical for myopes, will cancel some of the effects of object displacement (disregarding freakishly short or long axial lengths), I believe that its contribution is minor, at least WRT corridor length and power profile.

    Considering the above, I believe it would be more accurate to say that myopes would benefit from the use of shorter, faster, and narrower corridors, using a slightly harder design, with hyperopes benefitting from an opposite approach, that is, wider, longer corridors with softer power gradients.

    Best regards,

    Roert Martellaro
    Call me Brent.

    Myopes do not want to be told where to read. Most myopes, at the end of the day or when longer reading is required, take there glasses off. They also prefer wider intermediate as they get older, this requires longer corridors. Hyperopes cannot take there glasses off so they have to deal with what they are given. With a variable corridor lenses they will not receive the same design nor the same corridor, each will be different, at least a Zeiss variable lens. The chin lift you refer to essentially is the myope giving himself more plus power. As a myope looks away from center plus power increases, along with the fact that the add will break down at a certain point and this can provide some needed plus power as well.
    If you take two new presbyopes, one myopic and one hyperopic and place them in to a variable lens they will most likely enjoy them, assuming of course a near point refraction was conducted. You will have varied results when they have worn something different due to the effective increase in add power from some FF designs and traditional designs.
    Keep in mind in order to determine the full prism at the desired reading point you would also take into account the front curve, thickness at that point, prism thinning, PoW, etc. This is conducted using the center of rotation distance as one of your first variables.
    I totally agree with the soft/hard argument. Myopes prefer a harder design and hyperopes a softer design.

  15. #15
    What's up? drk's Avatar
    Join Date
    Mar 2004
    Location
    Ohio
    Occupation
    Optometrist
    Posts
    9,386
    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.

    Click image for larger version. 

Name:	target_compound.jpg 
Views:	20 
Size:	23.6 KB 
ID:	13328

    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?

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

    Shamir's information has it both ways!

    It would be nice to know the answer.


    *In a plano-powered prism, everything is deflected the same amount. Powered-lenses have VARIABLE prism...increasing from center to edge.

  16. #16
    One eye sees, the other feels OptiBoard Silver Supporter
    Join Date
    Jul 2002
    Location
    Wauwatosa Wi
    Occupation
    Dispensing Optician
    Posts
    5,458
    Quote Originally Posted by lensgrinder View Post
    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?...iii&Itemid=249

    Note the "viewing fields" diagram.

    Hyperopes and myopes have narrower and wider fields view respectively, due to magnification for hyperopes, and minification for myopes. We can see how this works by simply zooming in or out on our monitor that is displaying columns of text.

    WRT corridor length…

    • 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.


    http://essilorpalspro.com/Ovation_ECP.pdf


    Quote Originally Posted by drk View Post
    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.

    Click image for larger version. 

Name:	target_compound.jpg 
Views:	20 
Size:	23.6 KB 
ID:	13328

    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
    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.



  17. #17
    What's up? drk's Avatar
    Join Date
    Mar 2004
    Location
    Ohio
    Occupation
    Optometrist
    Posts
    9,386
    We agree on our diagnosis, but not the treatment.

    This has bugged me forever.

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

  18. #18
    Master OptiBoarder lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    504
    Quote Originally Posted by Robert Martellaro View Post
    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.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Trouble switching Varilux short-corridors
    By boredoptician in forum Progressive Lens Discussion Forum
    Replies: 6
    Last Post: 10-25-2012, 02:51 PM
  2. autograph 2 fixed and variable
    By Dave E in forum General Optics and Eyecare Discussion Forum
    Replies: 10
    Last Post: 03-16-2012, 06:14 PM
  3. There I Fixed It.
    By uniqueu in forum General Optics and Eyecare Discussion Forum
    Replies: 7
    Last Post: 11-05-2010, 08:51 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •