Results 1 to 23 of 23

Thread: Vertical Imbalance and Plano Lenses

  1. #1
    Master OptiBoarder
    Join Date
    Apr 2006
    Location
    NA
    Occupation
    Dispensing Optician
    Posts
    1,255

    Vertical Imbalance and Plano Lenses

    Perhaps this is a silly question, but I thought I'd ask anyway.
    Take the following Rx:
    OD +2.00 -2.00 x180
    OS +2.00 sph
    Add +2.50

    So, since the right lens is essentially plano on the 90 meridian, is there still a vertical imbalance? Obviously, there is still an image size difference, but would this situation involve vertical prism that would disturb fusion?

    Thanks!

  2. #2
    What's up? drk's Avatar
    Join Date
    Mar 2004
    Location
    Ohio
    Occupation
    Optometrist
    Posts
    9,385
    Yes, sir, there is.

    Compare OD and OS vertical meridia:
    plano and +2.00

    consider downgaze 10mm: 2 pd BU OS vertical imbalance. Ouch.

    The sneaky thing about anisometropia is that you have to consider each meridian.

    Look at that Rx in plus cylinder notation, BTW. The anisometropia's more obvious:
    plano +2.00 x 90
    +2.00 D.S.

    For a nice practical experience, take a 2 pd prism from the trial lens set and hold it in front of your left eye, base up and hold a plano lens over the right eye (or nothing, of course :)). You'll feel it!

  3. #3
    Master OptiBoarder
    Join Date
    Apr 2006
    Location
    NA
    Occupation
    Dispensing Optician
    Posts
    1,255
    Yes, I agree DRK, but my question is whether or not one lens being essentially plano changes the senario.
    Plano doesn't really have prism away from the "OC" and that's why I was thinking it might not apply here. I do see the difference in powers.
    If you were talking about SV lenses (yes, I know anisometropia doesn't come into play here b/c the wearer looks through the OC) but theoretically,
    SV lenses, one plano, one +2. How much prism is there looking 10mm above the OC? I don't know, because on one hand, there's 2D prism in the left lens. On the other hand, you can look anywhere in a plano lens w/o prism, so that's why I thought this case might not be a fussion issue.

  4. #4
    Master OptiBoarder
    Join Date
    Oct 2004
    Location
    Down in a hole!
    Occupation
    Dispensing Optician
    Posts
    13,078
    Quote Originally Posted by EyeFitWell View Post
    Plano doesn't really have prism away from the "OC" and that's why I was thinking it might not apply here. I do see the difference in power.

    But, plano CAN have prism.

  5. #5
    Master OptiBoarder
    Join Date
    Apr 2006
    Location
    NA
    Occupation
    Dispensing Optician
    Posts
    1,255
    Hmm...
    I've seen a plano prism, with no power except prism power, and I guess that's what you're referring to.
    But, if you take a stock SV plano lens, nowhere in that lens is there prism.

  6. #6
    OptiBoardaholic
    Join Date
    May 2000
    Location
    B.C. Canada
    Occupation
    Dispensing Optician
    Posts
    1,189
    Use Prentice's rule to calculate the prism, then compare the imbalance. As drk says, put a prism over one eye, and you can see and feel the imbalance, even if there is no prism in the other eye. to further demonstrate, you could put a plano lens over the other eye, and you will still see and feel the imbalance, and Prentices rule will still indicate a prism imbalance.

  7. #7
    Master OptiBoarder
    Join Date
    Apr 2006
    Location
    NA
    Occupation
    Dispensing Optician
    Posts
    1,255
    I'm sorry, maybe you're answering my question and I don't even realize, but that just doesn't make sense....(I'm following you that there is prism in one eye, even if it's not in both eyes. I also understand the prentice rule, etc.)
    When looking through the bifocal, the left lens has BU prism, moving the image down. But, the right lens has no prism and no power.

    What if you got the RX:
    OD plano
    OS +2.00 2D BD prism
    SV

    what would keep the right eye from turning up with the left?

  8. #8
    OptiBoardaholic
    Join Date
    May 2000
    Location
    B.C. Canada
    Occupation
    Dispensing Optician
    Posts
    1,189
    Wer'e talking about prism imbalance, not prism. When one image is displaced, and one is either not displaced, or is displaced a different amount, prism imbalance results, and results in working the ocular muscles harder to maintain fusion. This can result in eyestrain, and, where the imbalance is greater, a loss of fusion, resulting in double vision. People can generally tolerate far more horizontal prism imbalance than vertical. I hope this answers your question.

  9. #9
    Banned
    Join Date
    Jun 2000
    Location
    Only City in the World built over a Volcano
    Occupation
    Dispensing Optician
    Posts
    12,996
    Unless the patient has a phoria or other muscle problem. Almost zero patients will have any sensitivity to a 2 diopter imballance. At 3 or 4 this becomes a problem.

    Chip

    If it is a problem it can be solved by using a straight top bifocal on the lens with the most minus power (base up prism here) and a round top on the lens with the most minus (base down prism here). Of course you can't do this if you are one of those people that feels cosmetics is more important that vision.
    Last edited by chip anderson; 01-31-2007 at 12:25 PM. Reason: P.S. comment

  10. #10
    Master OptiBoarder
    Join Date
    Oct 2004
    Location
    Down in a hole!
    Occupation
    Dispensing Optician
    Posts
    13,078
    Chip-

    I could maybe agree with 2 horizontal, but vertical? You folks from the south must be waaaay more liad back then us Northerners!

  11. #11
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    I think you guys might be talking about two different situations...

    When verifying the lens, you are not likely to find any vertical imbalance, since you would generally start by neutralizing the lens with the strongest vertical meridian at the optical center, first. When sliding the eyewear over to measure the second lens, which has no power and produces virtually no prism in the vertical meridian, you would therefore not find any significant vertical imbalance.

    However, when wearing the lenses, since the left eye will not necessarily be looking through the optical center of the left lens, you may in fact experience a significant amount of vertical imbalance at any given moment as the eyes traverse the lenses.
    Darryl J. Meister, ABOM

  12. #12
    Master OptiBoarder
    Join Date
    Apr 2006
    Location
    NA
    Occupation
    Dispensing Optician
    Posts
    1,255
    ok, got it. thanks!

  13. #13
    ATO Member HarryChiling's Avatar
    Join Date
    Apr 2005
    Location
    Nowhereville
    Occupation
    Other Eyecare-Related Field
    Posts
    7,765
    I would like to add that when measureing the power why use the (90o meridian)? Don't most here decenter the seg about 1.5 mm per eye? If so then the meridian changes since the eye is not only gazing down but slightly in. So if the 10mm as metioned earlier were to apply the the position we would measure for prism at would be 10 down and 1.5 in or

    tan (ang) = 1.5/10
    tan (ang) = 0.15
    ang = tan-1(0.15)
    ang = 8.5o

    so the meridian you should check for would be 98.5o the power in this meridian being 0.04, not that it matters much but just a side thought. In the given scenario the amount of prism imbalance should not make a difference, but that is up to the wearer to really decide, I bet you if you mention the price of a slab off they may decide that it is an acceptable amount of prism to deal with. :)

    alberto correia just posted a program in the download sectiopn that would be of use to you in this scenario. Also if you go to www.opticiansfriend.com Al Cordova has a calculator that would help you calculate the amount of imbalance.
    1st* HTML5 Tracer Software
    1st Mac Compatible Tracer Software
    1st Linux Compatible Tracer Software

    *Dave at OptiVision has a web based tracer integration package that's awesome.

  14. #14
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    I would like to add that when measureing the power why use the (90o meridian)?
    Keep in mind that since the principal powers are at 180 and 90 in this particular prescription, the vertical prism component will only be due to the power through the 90 deg meridian, regardless of the horizontal decentration. You can verify this by entering this prescription into this induced prism calculator.
    Darryl J. Meister, ABOM

  15. #15
    What's up? drk's Avatar
    Join Date
    Mar 2004
    Location
    Ohio
    Occupation
    Optometrist
    Posts
    9,385
    I think Harry's nitpicking over convergence. Nobody likes a geek, Harry

  16. #16
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    I suspect that every single one of us reading this thread is probably a geek, DRK... ;)
    Darryl J. Meister, ABOM

  17. #17
    ATO Member HarryChiling's Avatar
    Join Date
    Apr 2005
    Location
    Nowhereville
    Occupation
    Other Eyecare-Related Field
    Posts
    7,765
    I think Harry's nitpicking over convergence. Nobody likes a geek, Harry
    :(
    1st* HTML5 Tracer Software
    1st Mac Compatible Tracer Software
    1st Linux Compatible Tracer Software

    *Dave at OptiVision has a web based tracer integration package that's awesome.

  18. #18
    Master OptiBoarder
    Join Date
    Oct 2004
    Location
    Down in a hole!
    Occupation
    Dispensing Optician
    Posts
    13,078
    Hello, hello...I'm right here. Did somebody call for the geek?

    :D:cheers::D

  19. #19
    Master OptiBoarder
    Join Date
    Apr 2006
    Location
    NA
    Occupation
    Dispensing Optician
    Posts
    1,255
    LOL, Harry I completely agree with you, and I thought about that when I was taught how to calculate the vert. imbalance in school. However, this particular question was more about the theory and less about the math. I had a pt. that Doc said "might" need a slab, and I wondered in my head about the one lens being plano making a difference.

    Thanks for the insight!

  20. #20
    Paper Shuffler GOS_Queen's Avatar
    Join Date
    Dec 2004
    Location
    Portland Metro
    Occupation
    Other Eyecare-Related Field
    Posts
    1,533
    Quote Originally Posted by drk View Post
    I think Harry's nitpicking over convergence. Nobody likes a geek, Harry
    I like a geek, Harry. (heck, I already knew I was a nobody)

  21. #21
    Master OptiBoarder LENNY's Avatar
    Join Date
    May 2000
    Location
    BROOKLYNSK, NY USA
    Occupation
    Dispensing Optician
    Posts
    4,351
    Can I trow this in?

    What if the patient is monocular?

  22. #22
    Master OptiBoarder
    Join Date
    Apr 2006
    Location
    NA
    Occupation
    Dispensing Optician
    Posts
    1,255
    then fusion isnt' a concern, right?

  23. #23
    Master OptiBoarder
    Join Date
    Oct 2004
    Location
    Down in a hole!
    Occupation
    Dispensing Optician
    Posts
    13,078
    Quote Originally Posted by LENNY View Post
    Can I trow this in?

    What if the patient is monocular?
    Tell them to buy a blasted patch, throw a parrot on their shoulder, scream AARRGGHH, and get the blue blazes out of your dispensary.



    Monocular patients......why do I always think of Colonel Klink(Hogans Heroes) and his Monocle?


    :D

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Plano Lenses 1.9mm Polycore Clearance
    By LKahn in forum Optical Marketplace
    Replies: 0
    Last Post: 05-09-2006, 03:23 PM
  2. Verification of Progressive Vertical Imbalance
    By kwai Chang in forum Progressive Lens Discussion Forum
    Replies: 1
    Last Post: 05-09-2006, 09:55 AM
  3. Vertical Imbalance questions.
    By Bob Price in forum Ophthalmic Optics
    Replies: 6
    Last Post: 01-15-2006, 11:07 PM
  4. Vertical imbalance and PD problems with zero power???
    By TracyElise in forum General Optics and Eyecare Discussion Forum
    Replies: 14
    Last Post: 10-10-2005, 03:17 PM
  5. vertical imbalance (when dist o.c. and eyes at diff level)
    By eyechicky in forum Ophthalmic Optics
    Replies: 1
    Last Post: 02-22-2005, 12:54 AM

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
  •