Archived posting to the Leica Users Group, 2000/03/06
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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