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P3D focal length

  • From: Tom Hubin <thubin@xxxxxxxxx>
  • Subject: P3D focal length
  • Date: Thu, 11 Feb 1999 01:03:11 -0800
> Could someone one the list tell me if there is a formula for converting
> close up lenses to focal length. For example, what is the focal legth of
> a +1,+2 etc. close-up lens. I guess these are diopters. Should I be
> asking how to convert diopters into focal length. I want to order some
> lenses from Edmund Scientific. Sorry if this should have gone to T3D.
> Rod Sage

The markings are probably in diopters. A 1.0 diopter lens has a 1 meter
or 1000mm focal length. A 2.0 diopter lens has a 1/2 meter or 500mm
focal length. In general, divide 1000 by the power in diopters to get
the focal length in mm.

The plus or minus sign for the focal length is the same as the sign for
the power in diopters. 

Tom Hubin
thubin@xxxxxxxxx
AO Systems Design



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