testing the focus accuracy of a camera

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Is there an effective way of testing whether the ground glass is exactly at the same position as the film once the holder is inserted? I would like to test this at my camera.

-- Dr. Lukas Werth (teswerthlvk@zedat.fu-berlin.de), February 18, 1998


There is an article in the November/December 1996 issue of View Camera magazine, starting on page 57.

-- Ted Brownlee` (omfbh@aol.com), February 18, 1998.

There is an ansi standard for film holder depth and film thickness. The holder depth is .197" (5mm), and film thickness is .007" (.178mm). This may give some useful info on determining focusing accuracy. This seems to be one of the larger variables in resolution. Maybe some sort of test fixture can be made (without bellows) to allow viewing the film surface from the front after focusing. I think many LF users are curious about this issue.

-- Ron Shaw (shaw9@llnl.gov), February 19, 1998.

The position of the groundglass versus that of the actual film plane does indeed vary quite a bit. A couple of years ago, I took a large format workshop taught by Joe Englander. In connection with a discussion of various factors which can affect sharpness, Joe used a machinists's dial indicator to measure the GG position for all the camera being used in the workshop. It was quite surprising how much variation there was.

Basically, the technique which Joe used to measure this used a flat bar with a hole drilled through it, exactly perpendicular to the flat surface. The dial indicator was mounted on the bar so that its needle protruded through the hole. To zero the indicator, the whole assembly was placed up against a flat surface (such as a pane of window glass); this represented the position at which the needle was flush with the bar, and the dial was adjusted to read zero at this position. Having zeroed the dial indicator, the camera back was removed and the bar was held up against a flat surface of the camera back, with the needle touching the groundglass. The negative reading on the dial indicator thus measured the depth of the groundglass in relation to the reference surface. The same depth measurment was then taken with a film holder inserted. After accounting for the standard thickness of a sheet of film, the two depth measurements should, in theory, be the same--but they frequently were not.

Incidentally, in many cases, measurements also showed variations between the corners and the center of the groundglass.

-- Rob Rothman (rrothman@riag.com), February 19, 1998.

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