### Bellows factor question

greenspun.com : LUSENET : Large format photography : One Thread

I'm stumped. I'm trying to compute the bellows factor for a 12" Dagor shooting close-ups and I'm using one of the usual formulas: x = bellows extension/focal length. I know I measure from the lens to the film plane to get the bellows extension. But from what point on the lens do I measure to get the bellows extension? The back of the lens? The middle? The front?

-- Richard Lingg (r_lingg@hotmail.com), May 22, 2000

Theoretically you should measure from the nodal point of the lens. Given that god knows where the nodal point of a 6 element plasmat lies, just measure from the centre of the lens to the film plane for the bellows extension (minor errors will not matter much). An alternative is to measure the object and its size on the ground glass and divide one by the other, which is what tools like the Quick Disc do (http://www.salzgeber.at/disc/). Good luck. DJ

-- N Dhananjay (ndhanu@umich.edu), May 22, 2000.

Hi Richard. In addition to the first answer, another method you might use is to first focus your lens at infinity. At that time, measure off one focal length from the film plane (and mark the lens at that place). This will serve as your measurement point in the future.

Alternatively, you might focus at infinity and put a special reference mark somewhere (maybe on a focus rail, etc.) and, for close-ups, measure offsets from that mark. PS; that mark would be considered as one focal length from the image plane.

-- Bill C (bcarriel@cpicorp.com), May 22, 2000.

There is a nifty device available for free on the web called a quick disc. It consists of 2 parts: a target that you position in the field of view and a measuring "ruler" that you use to determine the size of the target on the gg. The increments are exposure factors! Works great. Just download, print and cut out with a scissor. phillipp.satzgeber@vienna.at should get you there. Good luck.

-- Robert A. Zeichner (razeichner@ameritech.net), May 22, 2000.

Don't worry where you take the reading from as long as it's somewhere in the vicinity of the lens. It doesn't have to be that accurate. Plus or minus 10% will get you within 1/5 stop, and I'll bet your shutter and aperture index aren't settable to that precision. Near enough is good enough in this instance.

-- Pete Andrews (p.l.andrews@bham.ac.uk), May 23, 2000.

This technique works, according to John Sexton. Attach a tape measure to your darkcloth. Focus on your closeup subject. Then measure the distance from the film to the lens with the tape meaure. Suppose it is 16 inches. You are using a 210mm lens, which is about 8.5 inches. How many fstops are there between f 8.5 and f 16. About 1.5. Your exposure increase due to bellows extension is 1.5 stops. I have used this for 8-10 years with b/w, chrome, and neg film. It works. bob

-- Bob Moulton (bobmargaretm@home.com), May 24, 2000.

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