Archived posting to the Leica Users Group, 1999/09/05
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]> At 3:51 PM +0200 4/9/99, Doug Richardson wrote: > snip > >Perhaps Leitz thought in neccessary to move to retrofocus in order to > >get the speed of 28mm lenses to f2.8. > snip > > Hi, IMHO there seem to be a couple of reasons: > 1) To allow TTL metering with an M6 (and future bodies) > 2) To decrease vignetting. Symmetrical designs are subject to the cos^4 rule. All lenses are subject to the cos^4 rule, but the exit pupil of a retrofocus lens is further away, lessening the effect from it. In addition, due to the strong negative meniscus element up front, the effective entry pupil at an oblique angle is also effectively larger, providing further compensation. You can check this easily, you will see that the pupil opening appears larger off the axis.