Archived posting to the Leica Users Group, 1999/07/28

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Subject: [Leica] The meaning of F, as in liFe
From: "Kotsinadelis, Peter (Peter)" <peterk@lucent.com>
Date: Wed, 28 Jul 1999 13:28:56 -0700
 
I believe F in f-stop is factor.  It is the ratio of focal length to
aperture diameter.  Many European camera lenses specify openings such as
1:3,5 (for North Americans 1:3.5).  THis is the ratio but in many cases has
been shortened to state the aperture as a fraction of the focal length, for
example F3,5 (or F3.5).  The f-number has the same meaning in both cases.

A T-number is defined as f-number divided by the square root of
transmittance.  According to Kingslake, the need for a T-stop calibration
was more useful for non-coated optics and almost disappeared from use in the
early 50s.

Peter K