press focus????greenspun.com : LUSENET : Large format photography : One Thread
can anyone supply me with the definition/meaning of the expression "press focus?" i've seen this used to describe lenses listed for sale in Shutterbug, only as a descriptor it is waisted on me - as i don't know what it means. also, some where i read where someone said that one shouldn't use a tessar lens below f 11, why not? does a tessar design become soft focus at aperatures: 8, 5.6, ...? are "press lenses" designed to be sharpest when stopped down to f 8? thanks.
-- david clark (email@example.com), April 02, 1999
The "press focus" is a button or small lever on a LF lens that holds the shutter open for focusing. Most lenses designed for LF have such a button/lever, though a few have to be set at "T" or "B."
As for the second part of your question, yes, most Tessars do perform better when stopped down, though, of course, that is true for virtually all lenses. I would say that perhaps someone else can enswer this better, especially if you give them the name of the perticular Tessar in question, but the only way to be really sure is to test: not only are all lenses different, but there is an awful lot of heresay and unfounded mythology in photography (as everywhere else).
"Press lenses" are designed to be used on press cameras without any swings and tilts, hense may not have much coverage beyond the edges of the negatives they were designed to cover.
-- Peter Hughes (firstname.lastname@example.org), April 02, 1999.
The Wollensak shutters commonly found on Speed Graphics need to be cocked first before the press focus (on the left side of the shutter will work. compur shutters have a button near the cocking lever. cock the shutter, hold the button and fire the shutter -- the cocking lever will stop part way thru its travel with the shutter blades open. copal shutters have a press focus that operates independently of whether the shutter is cocked or not. Kodak shutters have a button that operates like the Compurs. I forget how the Ilex shutters operate, there are a couple of different models. Main thing is not to force anything.
-- tony brent (email@example.com), April 02, 1999.