Results 1 to 4 of 4

Thread: lower OC for angle of side

  1. #1
    OptiBoard Professional yzf-r1's Avatar
    Join Date
    Mar 2002
    Location
    United Kingdom
    Occupation
    Optometrist
    Posts
    111

    lower OC for angle of side

    i know that for asperics and high powered lenses you need to lower the OC by 1mm for every 2degrees of pantescopic angle. (correct me if i'm wrong)

    What problems is the patient likely to experience if this is not done?

    Many thanks

  2. #2
    Bad address email on file
    Join Date
    Jan 2002
    Location
    São Paulo City, Brazil
    Posts
    64

    Thumbs up

    Hi... yzf-r1

    You are righ, Incorrect pantoscopic tilt can cause a cilinder, and then cause blur and so on.
    The optical axis is suposed to pass through the eye's Center of Rotation.
    I am sure Darryl are going to explain it better and give you all the math involved.

    See you

    Edgley

  3. #3
    Master OptiBoarder Jeff Trail's Avatar
    Join Date
    May 2000
    Location
    Chattanooga TN.
    Occupation
    Optical Wholesale Lab (other positions)
    Posts
    973
    yzf-r1

    If you want to really delve into this look up Martins formula for lens tilt. :) Changing the tilt of a lens induces oblique astigmatism (sometimes called marginal astigmatism depending who is doing the talking ;) ) Panto tilt changes the effective sphere power of the lens and induces cylinder power on the 180 merridian. For plus spheres it plus cyl and in a minus it's minus cyl.
    Face form will also change the power of the lens but it induces cyl. in the 90 degree merridian...

    the formula is S'={1+(sina)2/2n}
    C'= S'(tana)2

    Keys are S'= new spherical power
    S= original sphere power
    a= the degree of tilt
    n= index of refrection of the lens material
    C'= the induced cylinder on the axis of rotation

    Hyperops are more of a problem than a myop when working in tilts and the effect on power, What it amounts to once you use this formula you can check to see the resultant power and how the amount of tilt in relation to the optical axes of rotation will neutrilize this unwanted power... (the one you mention oc 1 mm for every 2 degree of tilt

    If you DO not lower and tilt than you induce power (get the answer with the formula on the amount) ....

    Here is an example say we had a lens that is a +10, cr39 with 15 degree's of tilt .... the effective power if the oc was in front of the pupil and not lowered than the power at the oc would be...

    S"= S{1+(sin a)2} = (+10.00)(1+{sin15}2/2(1.498})
    =(10)(1+0.06699/2.996)=(10.22)(0.0718)=+10.22^

    C'=S'(tan a)= (+10.22)(tan15)2= (+10.22)(0.0718)= +.73x180

    The power if the oc was placed in front of the pupil and the frame did have 15 degree of panto tilt when they looked through the lens the effective power would be +10.22+.73x180 !!YIKES


    That is why you lower the oc as you induce tilt ;)

    Maybe not as smart as Darryl but us lab rats do know a thing or two if Darryl wants to check my math more "power" to him.. get it? ..ok it was a corny attempt at optical humor...


    Jeff "grind if ya got em" Trail

  4. #4
    Master OptiBoarder Darryl Meister's Avatar
    Join Date
    May 2000
    Location
    Kansas City, Kansas, United States
    Occupation
    Lens Manufacturer
    Posts
    3,700
    Jeff Trail said: Maybe not as smart as Darryl but us lab rats do know a thing or two if Darryl wants to check my math more "power" to him.
    You have done well, young Jedi. Soon, you too, will knows the ways of the Force... ;)

    ...Though, while I understood exactly what you meant, use should probably use a little carrot ^ or something before your exponents in those formulas, since it just looks like you're mutliplying by 2 otherwise. You might also put parentheses around the 2n, since they are both part of the denominator, not just the 2. Something like,

    S'={1+(sin a)^2/(2n)}

    Best regards,
    Darryl

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Moulin vs. Luxottica (Take a side)
    By fletch in forum General Optics and Eyecare Discussion Forum
    Replies: 48
    Last Post: 08-20-2004, 02:30 PM
  2. rebuilt gerber back side coaters
    By gpw_ww in forum General Optics and Eyecare Discussion Forum
    Replies: 0
    Last Post: 11-12-2003, 09:54 AM
  3. View from the other side....
    By John R in forum Just Conversation
    Replies: 0
    Last Post: 03-21-2003, 04:41 PM
  4. NEW Titanium...Lower Price
    By kjw1231 in forum Optical Marketplace
    Replies: 0
    Last Post: 04-23-2002, 02:08 PM
  5. Optician NYC Upper Wst Side
    By binocular in forum The Job Board
    Replies: 0
    Last Post: 01-18-2002, 12:18 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
  •