**Archived posting to the
Leica Users Group, 2008/12/13**

Date: Sat Dec 13 16:18:28 2008

References: <200812130331.mBD3VQoT089665@server1.waverley.reid.org> <A3B2C7B3-4107-4F30-A014-39B9943526A6@optonline.net> <20081213175209.7C8AB71177@barracuda.rutabaga.org> <3B025B86-25C5-4DF6-8656-C7144604BB5C@bex.net>

Howard, Figures can't lie, but----------etc! Jerry Howard Ritter wrote: >No, it actually is MORE than 1/2 stop, not less, and way more than 1/3>stop. Remember, f/4 is already a full stop down from f/2.8, so the>jump from f/2.8 to f3.5, already well over halfway numerically, is way>more geometrically.> >Squaring the reciprocal of the f-ratio gives a number that is>proportional to the aperture's area.>A full stop is an areal ratio of 2, corresponding to a ratio between>f-ratios equal to the square root of 2. A half-stop is an areal ratio>of sqrt (2), about 1.4, corresponding to a ratio between f-ratios>equal to sqrt(sqrt( 2)), about 1.189.> >2.8 x 1.189 = about 3.3.>3.3 x 1.189 = about 3.9.>So 1/2 stop down from f/2.8 is about f/3.3, with another>equal-proportion areal step down to f4.> >One-third of a stop is an areal ratio of the cube root of 2 (for three>equal-proportion areal steps from one full stop to the next),>corresponding to a ratio between f-ratios equal to the cube root of>sqrt(2), or about 1.12.> >2.8 x 1.12 = about 3.1.>3.1 x 1.12 = about 3.47.>3.47 x 1.12 = about 3.9.>So 1/3 of a stop down from 2.8 is about f/3.1, with two more>equal-proportion areal steps down to f4.> >So from f/2.8 to f3.5 is actually 2/3 stop. Unless you were talking>about going from f/4 to f/3.5, which is 1/3 stop. (If you turn it>around and take 4/1.12, you get 3.57. The numbers are rounded because>they're irrational except for the alternate full stops f/1, 2, 4, 8,>16..., as well as approximated in order to give numbers ending in an>even digit or a 5.)> >--howard> >

Message from marcsmall at comcast.net (Marc James Small) ([Leica] Re: Summaron: a very long post about a small lens.)

Message from hlritter at bex.net (Howard Ritter) ([Leica] Re: Summaron: a very long post about a small lens.)