Ground glass - Sq. vs cut corners?greenspun.com : LUSENET : Large format photography : One Thread
Thinking of buying barely used ground glass with grid and 6x5, 6x7 indicators. Camera currently has glass with square corners, but barely used one has cut corners.
Any problems with that - other than I can't check focus all the way into the corners?
BTW: this forum is incredibly helpful! The fast and kind responses to utter rookies like me is making my 1st week with 4x5 far easier.
-- Douglas Gould (firstname.lastname@example.org), February 01, 2002
6x7, 6x9...sorry 'bout that....
-- Douglas Gould (email@example.com), February 01, 2002.
Chamfered corners should not make a major difference in focusing or other operations. I believe they exist as a method of checking for vignetting by the lens shade (by peeking through the open corners). Chamfered corners also reduce the chance of breakage, making the ground glass a bit more tolerant of the bumps that are bound to happen. (I am a mechanical engineer working on automated handling equipment for large flat panel display glass.)
-- Ted Brownlee (firstname.lastname@example.org), February 01, 2002.
Douglas: The traditional answer for this (never heard the breakage issue, but I suppose that makes sense) is that the corners are cut to make it easy to see if you've exceeded your lens coverage. Peek at the lens from the corner, can you see all of the diaphram? The other reason I've heard is that some camera are surprisingly air tight and the corners cut down on the sucking or pressurization you get when moving the lens board in or out with the lens on it. Personally, I don't think it makes a whole lot of difference. You can always look for the corners from the lens side, or slowly open and close the camera. Stick with it!
-- Kevin Crisp (KRCrisp@aol.com), February 01, 2002.
The cut corners also prevent bellows blowout should you compress or extend the bellow quickly. This is particularly important on large cameras with lots of bellow draw.
-- Robert A. Zeichner (email@example.com), February 02, 2002.
Thankyou Kevin and Robert for giving the correct answer to this question. That is the reason for the corners being cut. To allow air to vent when opening or closing the cameras bellows. The camera is airtight(to keep unwanted light out obviously) and this is the way the Rochester Camera Co. solved the problem in the late 19th century. Yes you can also check on lens vignetting but that isn't why the corners are cut. James
-- james (firstname.lastname@example.org), February 02, 2002.
Sorry James, but the primary reason for chamfered corners in modern LF cameras IS to check for vignetting. Most GG screens these days rest on 4 or 6 little raised lands in the camera back. This allows a small space under and around the edges of the glass, allowing air in and out of the camera. In any case, a camera doesn't have to be airtight to be light-tight once the filmholder's in place.
Collapsing bellows may have been a problem with the old wooden bookform plate backs, but screens with cut corners are still fitted to metal monorail and technical cameras, which have plenty of venting through the GG spacing described.
-- Pete Andrews (email@example.com), February 04, 2002.