Results 1 to 8 of 8

Thread: SAG of astigmatic lenses.

  1. #1
    OptiBoard Novice
    Join Date
    Jan 2008
    Location
    Kent
    Occupation
    Optical Retail
    Posts
    5

    SAG of astigmatic lenses.

    Hello everyone.

    I'm taking a day release course in Ophthalmic lenses and I'd appreciate some clarification with a few areas of SAG if anyone can.

    I can calculate edge thickness of round and shaped lenses easily enough but I got a bit lost during the lecture when it came to astigmatic lenses.

    I have a lens (n=1.5) -4.00/-2.00 x90 edged 50x40mm oval with an axial thickness of 2mm. I have to find the vertical and horizontal edge thickness.

    I know my principal powers are -4.00 and -6.00 but that's where I blank out.
    Last edited by Mymyopia; 01-24-2008 at 02:30 PM.

  2. #2
    Rising Star lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    362
    You would use the sag approximtion formula in this case since you were not given a base curve.


    where
    d is the diameter in mm
    D is the power
    n is the refractive index

    Now use each power in the major meridians (ie -4.00 X 90, -6.00 X 180) and use 50mm for your power at 180 and 40mm for your power at 90.







    Now add this to your axial thickness and you have 5.75 @ 180







    Add this to your axial thickness and you have 3.6 @ 90

  3. #3
    OptiBoard Novice
    Join Date
    Jan 2008
    Location
    Kent
    Occupation
    Optical Retail
    Posts
    5
    That's a great help, many thanks.

    If you can clarify one thing though. How do you decide which power to use for each diameter?

  4. #4
    Rising Star lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    362
    Your lens power was -4.00 -2.00 X 90. You will produce all of your cylinder power 90 degrees away from the axis, so at 90 you only have your sphere power and at 180 you have your sphere power plus your cylinder power which is -6.00.
    The measurements were 50X40 50mm horizontally and 40mm vertically. The power horizontally is -6.00, the power vertically is -4.00.

  5. #5
    OptiBoard Novice
    Join Date
    Jan 2008
    Location
    Kent
    Occupation
    Optical Retail
    Posts
    5
    So if I'd said the lens was 40X50mm oval you'd quite simply use -4.00 with 40mm vertical and -6.00 with 50mm horizontal?.

  6. #6
    Rising Star lensgrinder's Avatar
    Join Date
    Jul 2005
    Location
    Raleigh, NC
    Occupation
    Lens Manufacturer
    Posts
    362
    Quote Originally Posted by Mymyopia View Post
    So if I'd said the lens was 40X50mm oval you'd quite simply use -4.00 with 40mm vertical and -6.00 with 50mm horizontal?.
    40mm would be the horizontal measurement with 6.00 D in the 180th meridian.
    50mm would be the vertical measurement with 4.00 D in the 90th meridian

  7. #7
    OptiBoard Novice
    Join Date
    Jan 2008
    Location
    Kent
    Occupation
    Optical Retail
    Posts
    5
    I feel like such a idiot for asking but I need help once again. I've got a grip on the previous SAG type questions but I've got stuck once again.

    We were given the question....A lens +4.00/+2.00 x90 is edged 48X42 oval. If the thin edge thickness is 1.4mm and n=1.498 calculate the thick edge substance if made on a -5.00 sphere curve.

    I've transposed my RX into +9.00dc x180 +11.00dc x90
    -5.00 ds

    I know which powers and diameters give me my thick and thin edge but I can't get the correct answer (3.46mm)

    I've noticed my problem is never how to calculate SAG it's how to apply it. I promice this will be the last time I ask.

  8. #8
    ATO Member OptiBoard Bronze Supporter HarryChiling's Avatar
    Join Date
    Apr 2005
    Location
    Nowhereville
    Occupation
    Other Eyecare-Related Field
    Posts
    7,562
    Blog Entries
    1
    removed

Thread Information

Users Browsing this Thread

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

Similar Threads

  1. Replies: 0
    Last Post: 05-17-2007, 11:52 PM
  2. Semi finished lenses and Polycarbonate lenses for sales
    By godwin in forum Optical Marketplace
    Replies: 1
    Last Post: 10-23-2006, 12:59 PM
  3. Astigmatic pencil rays
    By Maria in forum Ophthalmic Optics
    Replies: 2
    Last Post: 01-04-2001, 06:41 PM
  4. percentage of spectacle lenses with astigmatic correction?
    By gilweber in forum General Optics and Eyecare Discussion Forum
    Replies: 1
    Last Post: 07-22-2000, 02:16 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
  •  
OptiBoard is proudly sponsored by:
FEA Industries, Younger Optics, Carl Zeiss Vision, Vision Systems, Inc. and Chemistrie Eyewear