Re: pinhole nearsighted prescriptions

Date view Thread view Subject view Author view

From: Paul Doherty (pauld@exploratorium.edu)
Date: Thu May 31 2001 - 12:01:57 PDT 

Message-Id: <l03110776b73c445d4a2f@[192.174.2.173]>
Date: Thu, 31 May 2001 12:01:57 -0700
From: Paul Doherty <pauld@exploratorium.edu>
Subject: Re: pinhole nearsighted prescriptions

Hi David

Presciptions are given in terms of diopters.
a diopter is 1/(focal length in meters.)
So a +1 diopter reading glasses prescription has a focal length of 1 meter.
If you have a distant street light 9Many 10's of meters away, its image
will be formed on a sheet of paper when the paper is located at the focal
length distance from the center of the lens.

Prescriptions for nearsighted people are negative diopters.
To measure the power of a nearsighted lens, combine a strong reading lens
say +4 diopters with a nearsighted lens. Measure the power of the
combination.

The law for adding lens powers in diopters is D = d1+d2 simply sum the two
lens powers.

so if the measured focal length of the pair of lenses is 0.5 m the combined
power is D = 2 = 4 + d2 where d2 = -2 diopters.

Paul D

Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld

Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Tue Oct 16 2001 - 12:22:23 PDT