Archived posting to the Leica Users Group, 1997/10/20
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Rob Garbutt wrote: >Can you please explain retrofocus for me? > . . . and your metered/meterless body comment? Wide angle lenses today are of two basic designs. One is the symmetrical, classic design as exemplified by the Super Angulon (both for Leica and for large format cameras) where the lens elements on one side of the diaphragm look very similar to the elements on the other side. This type of design is easier to calculate, has _no_ distortion by definition if it is truly symmetrical, and usually offers both very high resolution anc contrast when done properly. Also, it follows the cos fourth law to the letter, which means that the amount of light reaching a given point on the image is proportional to the fourth power of the cosine of the angle that the light is off axis. For a 50mm lens, the far corners are 23 degrees off the axis, so cosine of 23 degrees to the fourth power is 0.71. So the corners receive 71 percent of the light, or about 1/2 stop less than the center in a true symmetrical lens. For a 21mm lens, the far corners are about 45 degrees off the axis, and the cos of 45 to the fourth power is less than 0.24, so the corners receive 2 stops less. A little bit of optical fiddling is done to compensate, and the Super Angulon (as well as all other 'classical' designs) use this to fix things a little. The only real fix is a graduated ND center filter, which large format users (and Hologon users) are quite familiar with. In a symmetrical design the rear elements are inevitebly a lot closer to the film plane than the focal length. The other type of wide angle lens is the 'retrofocus' or 'inverse telephoto' type. Whereas in a telephoto, the object is to shorten the lens so that the focal length is less than the length of the lens + body, in the retrofocus design the point is to get the back elements far enough from the film plane to avoid interfering with the SLR mirror or with the light path in the M6 that is required for metering. The back elements of the SA are just too close to the shutter to allow the meter to see the white patch on the curtain. This design tends to be decidedly non-symmetrical, and leads to all sorts of design problems, which affect resolution in the corners, contrast (usually there are a lot more elements involved) and distortion. However, the cone of light from the back of the lens is a lot narrower, as the diaphragm is a lot further away from the film, so the cos fourth falloff is not as severe. Actually, this is an oversimplification, but this is part of what happens. There are a lot of other optical tricks used to even up illumination, but this is the one area where retrofocus designs are superior to symmetrical designs. This light falloff thing discussed above (optical vignetting) has nothing to do with the other type of vignetting (physical, or just 'vignetting'). Physical vignetting occurs when some part of the lens barrel, lens hood or filter (or finger) gets in the way and keeps the lens opening from being seen as a circular (or six sided, depending on your diaphragm) opening by the film. Retrofocus lenses need larger front elements than symmetrical designs, usually, so to make things practical and to save money the lens barrels are often constricted a bit with the result that retrofocus lens often have to be stopped down more to get rid of vignetting than symmetrical designs. The SA only has to be stopped down to f/5.6 (1 - 1/2 stops down) to get rid of vignetting, whereas the older Elmarit needed to be stopped down to f/8 (3 stops), _and_ the SA takes 48mm filters wheras the Elmarit takes 60mm filters. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com