Archived posting to the Leica Users Group, 1999/07/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Hi Steve, usually a lens of focal length f should be separated from the Bildebene by f when focusing at infinity. (This is, as you of course know, the definition of focal length. Could anyone tell me what Bildebene is in English?) So, in theory, for f=35mm the lens should be 35mm away from the film plane. In reality, since lenses have to compensate some errors, they are thicker and the rear lens is a bit closer to the shutter than 35mm. This is no problem on a rangefinder - there is no mirror which gets in the lenses way. The 21mm SA is only 8mm or so away from the film! However, in SLR, there is always the mirror, so some additional optics have to be applied to get the rear lens element out of the mirrors way. Cheers, Peter. P.S. If I remember correctly, on of the early 21mm lenses for the early Nikon SLRs required a mirror lock up and an external finder, because this trick was not understood very well.