# [Rollei] Suplementary lenses

• Subject: [Rollei] Suplementary lenses
• From: Richard Knoppow <dickburk >
• Date: Wed, 09 Jan 2002 22:11:50 -0800
• References:

```  I think I accidently deleted a post asking about calculating the focal
length of a lens with a suplimentary lens on it.
Suplementary lenses are usually calibrated in diopters. A diopter is the
reciprocal of the focal length is meters. So, a one diopter lens has a
focal length of one meter or 39.3 inches. + dioper lenses are postitive or
converging, - diopter lenses are negative or diverging.
The formula for the combined focal length of two lenses is:

F = f1*f2 / f1 + f2 - d

Where f1 and f2 are the focal lengths of the two lenses and d is the
distance between them.
The exact focal length requires knowing the location of the principle
points but for practical use the distance can be disregarded and the
formula simplified by leaving out d. For a more exact focal length the
first principle point can be assumed to be in the space in front of the
iris diaphragm but the difference will be negligible. For practical
purposes the diopter value of combined supplimentary lenses adds linearly.

- ----
Richard Knoppow
Los Angeles, CA, USA
dickburk

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End of Rollei Users list digest V10 #124
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