Results 1 to 9 of 9

Thread: Trapezoid intermediate to near

  1. #1
    OptiBoard Apprentice
    Join Date
    Feb 2013
    Location
    North Central USA
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    45

    Trapezoid intermediate to near

    I've run across this issue a few times, and it has generally been multiple remakes before resolution is reached. The shape distortion is most evident from intermediate to near, and it has occurred with both traditional, digital,and computer/office style progressive lenses. The most recent occurrence is easily evident with a standard rectangular reading card, the top being wider than the bottom (perceived distortion not actual card).

    The lenses are a plastic full back side progressive, configured for a minimum height of 16. They are mounted in semi-rimless groove frame 53:18 w/ a 33.5 B measurement.
    R) +1.25 -0.25 X 103
    L) +1.25 -0.25 X 081
    monocular pd's 32/35 fit at a seg height of 22

    After Rx verification I typically look to the fitting parameters for cause, ...... although the last time I went through this exercise it was ultimately resolved with a Rx change.

    I would be appreciative to have comments from folks that also experienced this and feedback as to cause ...... thanks.
    Trip

  2. #2
    Master OptiBoarder
    Join Date
    Mar 2010
    Location
    north of 49
    Occupation
    Dispensing Optician
    Posts
    3,002
    The three most possible causes are missing from your informationZ:

    Not necessarily in order of importance....
    1)add power
    2)index of refraction or material used.
    3)base curve chosen(brand and index and base curve dependent)
    Eyes wide open

  3. #3
    OptiBoard Apprentice
    Join Date
    Jul 2013
    Location
    North Central Florida
    Occupation
    Dispensing Optician
    Posts
    20
    I think you did most if not everything just right. This kind of script calls for a lot of Pantascopic tilt, maybe 20 degrees. Don't let the vertex distance get too big. Fine tune it, then require the patient to wear it for one week, not swapping to an old pair. How old is the pt? What's the ADD?

  4. #4
    OptiBoard Apprentice
    Join Date
    Feb 2013
    Location
    North Central USA
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    45
    Not necessarily in order of importance....
    1)add power
    2)index of refraction or material used.
    3)base curve chosen(brand and index and base curve dependent)[/QUOTE]

    ahh .. thanks,
    Add power = 2.50
    material = plastic 1.498
    nominal BC = 6.00 (clocking a bit flatter )
    Trip

  5. #5
    Master OptiBoarder
    Join Date
    Mar 2010
    Location
    north of 49
    Occupation
    Dispensing Optician
    Posts
    3,002
    jonadonis is right about changing pantoscopic tilt to reduce the phenomena.

    1) Add........This add power is a indicator of intermediate usefulness, and when you chose a 16 mm version of the brand, you automatically compressed the digression to fewer powers, therefore exacerbated the distortion values within the narrowest presentation of the power range.
    2) Index..... The index or refraction of cr39 is low....therefore the most forgiving in distortion value. This should not be a contributing factor, unless the lens is exceptionally thick.
    3) Base curve.....The one used should be ok, but might be in the flattest possible range of choice. See brand recommends. It might be the most suitable for the frame, or what the patient is accustomed to.....this may not be the main cause, unless it is much flatter than what they are accustomed to, or if they have become more farsighted than with previous rxs.
    Eyes wide open

  6. #6
    OptiBoard Apprentice
    Join Date
    Feb 2013
    Location
    North Central USA
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    45
    Additional info - when the wearer has one eye closed the shape distortion is reduced, the same for either eye .... although there may be a bit of magnification perceived in the lower nasal quadrant of each lens when viewed monocularly. When he views binocular then the rectangle to trapezoid manifests.
    Trip

  7. #7
    Master OptiBoarder
    Join Date
    Jun 2008
    Location
    Seattle WA
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    3,137
    Its physics. You should have run across this issue every time.

    Its important to note that all Progressives WILL create trapezoidal distortion intermediate to near. Its simply due the power differential from one section to the other. people are looking through one object at 2 different powers, it WILL distort. Its physics.

    For more info, please read this by the legendary Darryl Meither, and the section on Skew Distortion near the bottom http://www.opticampus.com/cecourse.p...essive_lenses/

    The more the gaze (or image) departs from the Umbilic, the more trapezoidal the image will be. The easiest way to understand this is that all Progressives create a MESS of unwanted prism, but in different directions, some up, some down, and that variable prism when it encounters the same object (like a box) will twist the top and side one way, the bottom and other side another.

    If you want to reduce the PERCEPTION of those distortions, there are few lenses that offer different tools. The first tool is to narrow the reading, and widen the intermediate, it changes relative distortion. The second tool is the change the direction of distortion, off traditional axis. The third tool is to apply complex variable surfaces, either cross cylinder or asymmetrical aspherics, really inducing secondary prism to counteract primary prism. The net effect though is that by correcting this we lose something somewhere else in the lens (I don't believe you can correct for higher order abberations without cost some where else in the optics). All manufactures have different philosophy on the best way to reduce geometric distortions, the problem is they don't tell us what theirs is.

    Progressive Lenses that are known to reduce geometric or trapezoidal perceived distortions, in no particular order: Shamir Intouch and Autograph, Zeiss Choice and Individual, Seiko Surmount, Hoya iD/Lifestyle, Essilor S Series.

    All these lenses use different techniques, so I don't know enough about your patients lifestyle to recommend one over the other.

    The good news is that something magical happens no matter what lens you choose. Its called adaptation. Which simply means the brain adapts the trapezoidal distortions that all progressive create, and reprograms the mind to ignore it. That box looks square, no matter what the eye really sees.

    Quote Originally Posted by Trip View Post
    I've run across this issue a few times, and it has generally been multiple remakes before resolution is reached. The shape distortion is most evident from intermediate to near, and it has occurred with both traditional, digital,and computer/office style progressive lenses. The most recent occurrence is easily evident with a standard rectangular reading card, the top being wider than the bottom (perceived distortion not actual card).

    The lenses are a plastic full back side progressive, configured for a minimum height of 16. They are mounted in semi-rimless groove frame 53:18 w/ a 33.5 B measurement.
    R) +1.25 -0.25 X 103
    L) +1.25 -0.25 X 081
    monocular pd's 32/35 fit at a seg height of 22

    After Rx verification I typically look to the fitting parameters for cause, ...... although the last time I went through this exercise it was ultimately resolved with a Rx change.

    I would be appreciative to have comments from folks that also experienced this and feedback as to cause ...... thanks.

  8. #8
    OptiBoard Apprentice
    Join Date
    Feb 2013
    Location
    North Central USA
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    45
    [QUOTE=sharpstick777;468835]Its physics. You should have run across this issue every time.

    Its important to note that all Progressives WILL create trapezoidal distortion intermediate to near. Its simply due the power differential from one section to the other. people are looking through one object at 2 different powers, it WILL distort. Its physics.

    Experienced wearer ..... and not normally observed, or described to me as common at least (not arguing the statement).

    For more info, please read this by the legendary Darryl Meither, and the section on Skew Distortion near the bottom http://www.opticampus.com/cecourse.p...essive_lenses/

    Thank you .. others on other topics have helpfully pointed me in this direction as well .... continue to try to take advantage of this resource

    The more the gaze (or image) departs from the Umbilic, the more trapezoidal the image will be. The easiest way to understand this is that all Progressives create a MESS of unwanted prism, but in different directions, some up, some down, and that variable prism when it encounters the same object (like a box) will twist the top and side one way, the bottom and other side another.

    Yah .. what I found counter intuitive was the viewed object was larger at the top vs the bottom. I would have expected with increasing plus power to find the shape distortion reversed. Your explanation though, helps provides me with a reasonable graphic visual on how this could occur when viewed binocular.

    Appreciate everyone's responses ...... thanks
    ,
    Last edited by Trip; 09-19-2013 at 05:58 PM. Reason: clarity
    Trip

  9. #9
    Master OptiBoarder
    Join Date
    Jun 2008
    Location
    Seattle WA
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    3,137
    Quote Originally Posted by Trip View Post
    and not normally observed, or described to me as common at least (not arguing the statement).
    A person's experience will vary by RX change, The degree and combination of RX distance and add power changes will exaggerate the image differences... for some people.

    If someone is noticing that the TOP is larger, they may have become more hyoperic (or less myopic), as people age, its very common. Since the brain has compensated, they notice the top more, but it will NEVER really be larger. Its just the brain's interpretation, not an actual optical effect.

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Intermediate Question
    By PartTimer in forum Progressive Lens Discussion Forum
    Replies: 14
    Last Post: 07-10-2012, 09:46 PM
  2. Upside down trapezoid @ near point?
    By DC Optix in forum General Optics and Eyecare Discussion Forum
    Replies: 9
    Last Post: 05-30-2012, 03:57 PM
  3. intermediate pal (access) or short corridor with intermediate at top
    By tolpuppy in forum Progressive Lens Discussion Forum
    Replies: 7
    Last Post: 08-27-2010, 12:41 PM
  4. Modify RX for intermediate
    By libellule in forum General Optics and Eyecare Discussion Forum
    Replies: 9
    Last Post: 07-16-2009, 08:33 AM
  5. Help with Intermediate RX
    By misstara007 in forum General Optics and Eyecare Discussion Forum
    Replies: 10
    Last Post: 04-28-2009, 12:29 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
  •