Archived posting to the Leica Users Group, 1999/09/23
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Anthony Atkielski asks: > I once heard that someone (Canon) had once marketed a lens with a maximum > aperture of 0.7. Is this true? Another book I read says that the theoretical > limit is 0.5. Is this correct? If so, what imposes that limit? Canon made a 50mm 1:0.95 lens for its Canon 7 screw mount rangefinder camera. The actual aperture was supposedly 0.99, but JIS (Japan Industry Standards) specifies a error margin of 5%, so its claim for 0.95 was legal. There are other lenses which are brighter. I recall that Zeiss marketed one at 1:0.85, and Wray Company designed a 1:0.71 lens for reproduction of X-ray images at a demagnification of 1:16. The 1:0.5 limit is shown as follows: The numberical aperture designation is by convention given by the ratio of the focal length to the diameter of the entrance pupil of the lens. However, this convention is merely approximate, and does not accurately express the amount of light that actually reaches the focal plane. Let U = the slope angle of the steepest ray emerging from the lens to form the axial image, The amount of light that reaches the focal plane is proportional to sin^2 U. It is defined that Numerical Aperture = sin U. The F number(N) is defined as N = 1 / 2NA = 1 / 2 sin U. At U=90 degrees, the F number is 0.5. An 50mm lens with an F number of 1 (Noctilux!) is supposed to have a entrance pupil diameter of 52.36 mm.