Archived posting to the Leica Users Group, 2000/10/14
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Isaac, If they are 50mm lenses then you're in luck. Although we think of most R/F lenses having complicated cams, translating the distance the lens moves (to achieve focus) to the distance the cam moves (to show focus), the 50mm is the only one which works 1:1. I.e., for every millimetre the optical unit moves forward, the cam is moved by the same amount. So the profile on the rear of the 50mm lens is always flat. Look at any 50mm lens and then measure (incredibly accurately) the distance moved and the focus achieved. (e.g. one millimetre = inf.-25') etc. I measure (very roughly) that there are three millimteres of travel between inf. and 1m focus. If you wish to be more precise in marking focus points, far better to find a true focus screen and critically examine with a 10x magnifier. I've just remembered you mentioned an 80mm(?) as well. Well, you're on your own there! regards, Jem - ----- Original Message ----- From: Isaac Crawford <isaacc7@home.net> > Well, I did a little homework and it turns out that I need 13.2mm and 48.7mm > of extension to achieve infinity focus with my enlarging lenses. Luckilily I > can use some fairly inexpensive extension tubes to get a little beyond that. > The question is how much closer from infinity will they be? Is there an easy > formula to determine the focusing distance based on the amount of extension > past infinity? > > Thanx > Isaac >