Archived posting to the Leica Users Group, 1997/02/21
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Thomas Myro wrote: >I still think that measuring the diaphragm by the way it looks is just a >coincidence-- otherwise all those lens designers using apertometers all >of these years will commit suicide when they find out all they really >needed was a good starrett. Hey-- If all you need to do is measure by >the apparent opening in the diaphragm I could put a Proxar close-up lens >on my 50f1.5 Sonnar and I'll have a f1.1 for my Contax -heh heh heh . Thomas, if you put a Proxar in front of your Sonnar, you change a number of things, but the main thing of importance is that you change the focal length, making it shorter. However, if you then measure the aperture from the distance that the lens is now focussed on, you will find that the apparent aperture is the same as it was from infinity without the Proxar. So the aperture is the same, but the focal length of the lens is shorter. Should give a higher aperture ratio, right? No. The focal distance used in determining the effective aperture ratio is the distance of the rear nodal point of the lens to the film plane, and that has not changed with the addition of the Proxar. In this case, with the Proxar, to increase your aperture ratio, you would have to push the lens closer to the film plane until your Sonnar plus Proxar focussed at infinity. THEN you might have an effective f1.1. Don't try this at home (especially with expensive equipment :-)). The above stuff is why you don't need to increase exposure with closeup lenses like you do if you extend the lens via helicoid focussing. LF users can use some of the above info. I have, for example, a 135 Fujinon which has 80 degree coverage, ie., ample for 4x5. I have at times put a +1, or even +2 diopter closeup lens on it to get a slightly wider angle of view. Optical aberration abound, but shooting at f/32 or 45 tames most of them. I did get a lens that was faster than the advertised f5.6, but a lot of good that did :-). Henning J. Wulff Wulff Photography & Design