Archived posting to the Leica Users Group, 1999/12/17
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]The split prism screen is indeed rangefinder: the image you see in these two halves are aerial images from different points of the lens. Typically, the angle of the prisms are so that you see the image at the points at f5.6 or so, or 5 degrees or so off axis. This means that the split prism rangefinder has a base length of (focal length) x 0,18. With a 50mm lens, the base length is 9mm. Given that the viewfinder has a magnification of about 4x, the effective base length is 36mm. Note that with an increased focal length, the effective base length also increases, for example, with a 135mm lens, the effective base length is 97mm, far exceeding that of the M rangefinder. However, with a wide angle lens, say 24mm, the measurement is reduced to 17mm. This explains why it is easier to focus a long lens with and it is more difficult to focus a short lens with an SLR. Because the split prism shows light passing through a very small cone, the brightness of the lens does not alter the brightness in the split prism area. > I often hear that the rangefinder is better than an SLR for focusing in > dim light. Most SLR's have a split-screen rangefinder in the center > of the viewfinder. How is this any better or worse than an M6? > > Granted, the RF lets in almost all the light, while a typical fast lens > on an SLR might be f/1.4, but f/1.4 is pretty bright viewing in an SLR. > So what's the big deal? Is focusing in a rangefinder patch really any > different from the split-screen method in your typical SLR? > > Oh boy. Let the flame wars begin! :)