Archived posting to the Leica Users Group, 1997/04/24
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]I made the following chart many years ago. I helps to figure just where a particular lens is in relation to normal f/numbers. For instance, an Argus C-3 f/3.5 is just 1/3 stop faster than f/4. An f/12 large format lens is just 1/4 stop slower than f/11. The Leitz 350mm f4.8 Telyt-R is just slightly slower than f/4 +1/2. It's helped me keep these things in perspective. I printed it and reduced it a couple of times on a copier, then laminated it and put it in my wallet. It's amazing how many times it has been useful. FRACTIONAL F-STOP CHART ----------------------- f/no. +1/4 +1/3 +1/2 +2/3 +3/4 - --------------------------------------- f/2 2.2 2.24 2.3 2.5 2.6 f/2.8 3.1 3.2 3.4 3.5 3.7 f/4 4.4 4.5 4.7 5.0 5.2 f/5.6 6.1 6.3 6.7 7.1 7.3 f/8 8.7 9.0 9.5 10.0 10.4 f/11 12.0 12.6 13.0 14.0 14.6 f/16 17.2 17.6 18.6 19.7 20.2 f/22 24 24.8 26.4 28 29 f/32 34.5 35.6 37.7 40.6 41.2 _______________________________________ For those of you that are interested, the way this works is as follows: to get 1/4 & 1/3 stops between f/16 and f/22 take the LOG of f/16 (=1.2) take the LOG of f/22 (=1.34) subtract 1.34 - 1.2 (=0.14) divide .14 by 4 (=.0350) this is the 1/4 increment divide .14 by 3 (=.0467) this is the 1/3 increment f/16 (1.2) + 1/4 (.035) = (1.235) 10 raised to the 1.235 power = 17.2 therefore: f/16 plus 1/4 stop is f/17.2 (LOG's are base 10) ________________________________________ Jim