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*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 ------------------------------ End of Rollei Users list digest V10 #124 ****************************************

**Follow-Ups**:**Re: [Rollei] Suplementary lenses, compound system***From:*Richard Knoppow <dickburk >

**Re: [Rollei] Suplementary lenses***From:*Jerry Lehrer <jerryleh >

**Re: [Rollei] Suplementary lenses***From:*Richard Knoppow <dickburk >

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