Archived posting to the Leica Users Group, 2000/03/06

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Subject: Re: [Leica] 50mm f1.5 Nokton question
From: "Dan Post" <dpost@triad.rr.com>
Date: Mon, 6 Mar 2000 15:47:51 -0500

The math is simple; if you have a 50mm lens, and the f ratio is 1.4, then
the diameter of the aperture (all figures to two significant decimals) is
35.71mm, and the radius of the aperture is 17.86mm

For a lens with a 50mm, and an f ratio pf 1.5, the aperture is 33.33mm in
diameter, and with a 16.67mm radius.

The absolute area of the aperture for both figures is (using 'pi*r*r') is
therefore:
50/1.4 lenses have an aperture area of 1001.60 square millimeters (plus or
minus 5% would be a range of approx. 50.1mm, i.e.- 951.5-1050.7 square
millimeters)
50/1.5 lenses have an aperture area of 872.58 square millimeters( plus or
minus 5% of this area would be a range of 43.63 square mm- i.e. 828.95 to
916.21 square millimeters.

In absolute terms- if the amount of light hitting the film is a function of
the aperture size, and the illumination hitting both lenses remain the same,
then the 1.4 will admit 1001.60/872.58 more light with respect to a 1.5
lens.... this ratio is 1.15 rounded to 2 significant decimal places.
Looks like about 15% more light from the 1.4!- a significant increase.

Dan, The Abacus, Post