Archived posting to the Leica Users Group, 2000/03/06
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Marc, your calculation is fine. I arrived at exactly the same number by a slightly different method: 1.4/1.5 = 0.9333... (ratio of iris diameters) Squaring this (to get ratio of areas) gives 0.8711... or about 13% less area (and therefore 13% less light). This is, as you calculated, about a seventh of a stop. Regards John Bean At Monday 06/03/2000 18:54, Marc James Small wrote: >The surface of a circle is pi x radius squared. I don't have lenses at >hand, but roughly, the putative radius of a 50mm f/1.4 lens is 17.86mm, >that of an f/1.5 lens is 16.66mm. Squaring these, the ratio is 318.9:277.56. > >Reducing this gives 1.14, or 1 1/7. > >Mayhaps my math is way off?