Description

This article is from the Vision and Eye Care FAQ, by grants@research.canon.com.au (Grant Sayer) with numerous contributions by others.

2.0 Terminology of Spectacles

Lenses commonly used for spectacles are either spherical powered or
cylindrical powered, or a combination of the two. A spherical lens is
one that focus a distant object to a point of focus. As in the
'crude' ASCII image below the distant parallel light is focused at F1

                     /\
   .......>..........||.. 
                     ||   .
                     ||     .
                     ||       . F1
                     ||     .
                     ||   .
   .......>..........||.. 
                     \/

A cylindrical lens has 2 focal points corresponding to the two different
powers of the lens. Distant light is focused at two focal planes, labelled
F1 and F2 in the diagram below. The diagram shows the axes of the
cylindrical powers at 90degrees to each other.

                      ^
          ------------|--------------  
                      |     l         -
          ............|...l             -
                      | l   .            -
                      +      .F1          - F2
                   l  |   .              -
   ..............l .  |.               -
               l      |              -
          ------------|-------------
                      V

Other WWW references:
URL: http://www.west.net/~eyecare/optical_lens_prescriptions.html

URL: http://www-sci.lib.uci.edu/~martindale/Medical.html#Oph
Contains references to other web sites on glasses and how they work

URL : http://www.nbn.com/youcan/glasses/eyeglasses.html
(Questions and answers on spectacles and glasses)

 

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• prev: 1.6 How is Visual Acuity Measured
• Index
• next: 2.1 How to read a glasses prescription

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health, vision, eye care, contact lenses, spectacles, eyecare, prescriptions, sunglasses, surgery