Archived posting to the Leica Users Group, 2008/12/13
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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 On Dec 13, 2008, at 12:50 PM, Marc James Small wrote: > At 12:05 PM 12/13/2008, Lawrence Zeitlin wrote: > > > > >I simply quoted Emil Keller's book. He was there. None of use were. > >He specifically mentions the lens as an f3.5 Summaron although he > >does err in stating that the diaphragm modification added a full stop > >instead of half a stop. > > > Larry > > With respect, it is a third of a stop, not a half-stop. The > European apertures are 1/3 of a stop different from the > International scale which has been the norm since the 1950's. > > Marc