Archived posting to the Leica Users Group, 2006/08/30
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Fascinating! Thanks. Scott telyt@earthlink.net wrote: >Scott McLoughlin <scott@adrenaline.com> wrote: > > > >>OK, so I give up. Why is it so hard to make a nice SLR VF? >> >> >> > >A good SLR viewfinder delivers a bright contrasty image that can be seen >clearly over the entire area. One of the supposed advantages of the SLR is >being able to focus over >the entire picture area, no? > >First you need to deliver a lot of light to the viewscreen, meaning you >need a fully-reflective front-surface mirror, not the semi-silverd mirrors >that AF systems require. The screen itself needs to transmit a lot of this >light while providing enough of the 'ground glass' effect to focus the >image. The 'ground glass' effect scatters light, so there's a trade-off >between brightness and the 'tooth' of the ground glass (actually matte >plastic). AF cameras compensate for the lower light delivered to the >screen by giving the screen less 'tooth'. You can get a reasonably bright >screen on an AF camera, at the cost of the 'tooth' required for efficient >manual focus. Nikon makes (or, made) G- and H-series viewscreens for the F >cameras that substituted either clear plastic (G) or microprisms (H) to >make a very bright viewing image. The microprisms and fresnel rings on >these screens were optimized for particular focal lengths and aperture >ranges. > >The viewscreen also needs to include some way of delivering even >illumination; this is generally done with fresnel rings etched into the >screen in addition to a condensing lens on top of the screen. > >Next the good viewfinder needs to use a large glass pentaprism, not a >skimpy prism or a pentamirror. Pentamirrors are light weight but they >don't reflect light as efficiently as a pentaprism. > >I'll leave discussion of the viewfinder eyepiece to the optometrists among >us, but here's what Leitz did with the SL and SL2: > >The mirror is fully-reflective except for the central region, where the >meter reads the light. The standard screen in the SL has a coarse >microprism in the central area and a very fine microprism over the balance >of the screen. The fine microprism is small enough that you can't >recognize it as microprisms, it focusses like ground glass yet transmits >light like microprisms. The microprisms preclude the use of etched fresnel >rings so instead the pentaprism has a condensing lens ground into the >bottom. And of course the prism is big to provide good viewing out to the >corners. > >The condenser lens in the prism is more expensive to make, the big prism is >heavy, and the fully-silvered mirror means no AF. > > > >>So what about a a nice ~93% coverage , but otherwise spiffy >>VF like in the Nikon FM series? Why aren't all SLR viewfinders >>at least that good? >> >> > >in a word, Autofocus. Try and SL or SL2 and see if you still think the >FM-series viewfinders are all that spiffy. They're quite good compared >with what most people expect in an SLR viewfinder but compared with the SL >or SL2 there's lot to be desired. > >Doug Herr >Birdman of Sacramento >http://www.wildlightphoto.com > > >-------------------------------------------------------------------- >mail2web - Check your email from the web at >http://mail2web.com/ . > > > > >_______________________________________________ >Leica Users Group. >See http://leica-users.org/mailman/listinfo/lug for more information > > -- Pics @ http://www.adrenaline.com/snaps Leica M6TTL, Bessa R, Nikon FM3a, Nikon D70, Rollei AFM35 (Jihad Sigint NSA FBI Patriot Act)