Archived posting to the Leica Users Group, 2000/03/01
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]> John Collier wrote: > > I have just started playing with a visoflex and a Novoflex 400mm f5.6 (old, > good shape and $170US) and the screen brightness is amazing! You can stop > the lens all the way down (yes, I know, not recommended) and still see very > clearly compared to every other SLR I have tried. Why is this? Why has no > other company done this? > <<< > > The Visoflex screen is optimized for longer focal lengths. Most SLR viewscreens are a compromise between requirements for wide, fast normal and long lenses. I couldn't tell ya what design parameters are involved in the viewscreen design. When I used N****, there were some viewscreens available that were optimized for the longer focal lengths. Most focussing screens assume a certain range of the exit pupil position. They incorporate a Fresnel lens, or by change the angle of microprisms to ensure that the light coming from the lens end up towards the viewing eyes. If the focal length of the Fresnel lens does not match the pupil position well, you experience vignetting which makes it difficult to view the image. Visoflex screen, as well as Nikon type-D screen are ground glass without a Fresnel lens. This configuration works, as you found out, with a long lens with the exit pupil quite far away from the focal plane. Nikon provides four different microprism screens (H1 through 4) for its F3 camera for different lenses. The brightness is quite amazing, although the microprisms are not as fine as that of SL, so it is not quite as pleasant.