Archived posting to the Leica Users Group, 2001/03/21
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Thank you to Doug, Jason and Henning for the explanation of the microprisms in the focussing scren of the Leicaflex SL. Your explanation was clear. Kind Regards, Tony Salce - ----- Original Message ----- From: "Henning Wulff" <henningw@archiphoto.com> To: <leica-users@mejac.palo-alto.ca.us> Sent: Tuesday, March 20, 2001 5:11 AM Subject: Re: [Leica] Leicaflex SL Explanation > >Excuse my naivete but I do not understand what Dick Gilcreast means when he > >comments in Volume 28 No 2 1995 on the SL that "the central microprism is > >cut for the relatively large lens speed of f2.8 ...and is therefore > >relatively more accurate shooting at f2". He also says that the best lenses > >to use with the SL screen are f2, f2.8, f3.4 and f3.5 lenses. > > > >He makes similar comments about the outer prisms being "cut for about f5.6". > > > >Could list members please help me decipher this ? I am new to the SL and > >require some assistance. > > > > > >Kind Regards, > > > > > >Tony Salce > > > >Kind Regards > > Split image or microprism focussing aids for SLR's work best at > certain apertures. What this really is optimised for is the apparent > aperture at the film plane (or focussing screen), so for strong > retrofocus or telephoto designs this isn't exact. Retrofocus lenses > act like slower lenses, and telephoto lenses act like faster lenses. > > In any case.... > > If a prism is cut to work best at f/2, then it will deliver the > optimal accuracy for f/2 lenses, but will be no more accurate for > f/1.2 lenses, so for these lenses the prism might not be quite > accurate enough. If a lens with an aperture smaller than f/2 is used, > part of the prism will go dark, so then the prism is totally useless > as a focussing aid. > > Most SLR focussing aid prisms were cut to be optimal for lenses of > around f/3.5 or so aperture. Cameras with interchangeable screens > often had screens with microprisms or split image prisms that were > optimised for very fast or slow lenses, but fixed screen cameras had > to compromise, and whatever lenses you couldn't focus with the > central focussing aid you focussed on the surrounding ground glass. > Some cameras, like the SL I believe, and then some Minoltas and > others, used microprisms cut for slower lenses in the outer areas to > increase brightness, but if you tried to focus very slow lenses or > stopped down macro shots, most of the screen might go dark. If you > did this a lot, the factory usually had an option of changing the > screen to a ground glass type for a better compromise. > > The central focussing aids were intended to extend the basic ground > glass focussing capabilities of SLR's in to the wideangle and > standard lens areas with a rangefinder (and the prisms act optically > exactly like a split image rangefinder) to be more competitive with > range/viewfinder cameras in practice. > > -- > * Henning J. Wulff > /|\ Wulff Photography & Design > /###\ mailto:henningw@archiphoto.com > |[ ]| http://www.archiphoto.com