Measuring film holder accuracygreenspun.com : LUSENET : Large format photography : One Thread
I've posted on this subject before, but now I have the tool to supply some interesting information. A large aerospece engineering firm, who owned me some favors just built me a device to accurately measure film holder flange depth. They took a dial centering gage, mounted it in an 8" milled SS bar, with an adjustment set screw, and a check gage for exactly .197".....the standard for film holder flange to focal plane depth. It is accurate to .0001! I have done prelimary checks on a few of my 52, 4 X 5" holders. I will report to the LF board on my survey. The majority of my holders are Riteway, (new - old style) some Fidelity, and eleven new-in-the-box Linhof. Just a teaser until I complete the survey. Most Riteway check OK +/- within the .007 tolerance, but vary considerably. The Linhofs are within that tolerance of .007" and fluctuate very little, but 'deeper'in overall depth.. I'll get back to you all in a week with the total survey. (:-)
-- Richard Boulware (email@example.com), July 22, 2001
Hi Richard. Any chance of scrounging a few Toyos to check, for completeness of your survey?
-- Pete Andrews (firstname.lastname@example.org), July 23, 2001.
Don't know if your friend is an entrepreneur, but I suspect that (in light of the cost of sheet film, especially color, and the realities of sample variances among cameras and holders) a lot of us LF photographers would pay a fair price to own such a precise measuring device.
-- John (WhitmanDesign@aol.com), July 23, 2001.
If anyone in the Denver area has a Toyo holder...or any others that they would like to check...call me and 'bring-em-over'! No charge, just bring your own cold beer. With regard to making this device available...these guys are machining for Boeing, Martin and other aerospace giants. Making such a divice for sale would be just a 'nat' on an elephants' butt'. Besides, I already had the $100 precision dial indicator..which made the task easier. P.S. I'm in the phone book. Richard Boulware - Denver.
-- Richard Boulware (email@example.com), July 23, 2001.
Just out of curiosity, does anyone know what the deviation range of film is? If people are trying to measure film holders to a tolerance of 3 decimal places and film is tolerant to 2 decimal places it might be overkill to try to tune your system to such a degree of accuracy. Also, what is the depth of field on the film plane when a lens is at f/16 or greater? It may be that film holder depth may be less critical if the lens being used has a depth of field of 1/4" (.25) on the focal plane? I could also be whistling in the dark too. One reason I ask is that I have an older 8x10 where the ground glass frame is close to a 1/16" different from a film holder and before I have it re-machined I want to make sure this work is warranted.
-- Kevin (firstname.lastname@example.org), July 23, 2001.
Kevin: You can buy sheet film on various different bases, but the standard polyester or acetate base is 7 thou, or 0.18mm thick. This is the base thickness, and all the film that I've actually measured with a micrometer is closer to 0.2mm, including the emulsion (obviously). There seems to be very little deviation from this 0.2mm thickness, certainly much less than filmholder or GG tolerances.
The depth-of-focus of a lens at f/16 on a 10x8 camera is only about +/- 2mm (78 thou), even allowing for a quite generous circle of confusion of 0.12mm diameter. BUT this depth-of-focus translates to a focus error of -1.25 m or + 1.7 metres with a 360mm lens and a 10 metre subject distance.
If you're going to put up with that sort of focusing error, then why bother using a GG to focus at all? Peering at the screen with a loupe is a complete waste of time if the filmholder doesn't put the film in exactly the same plane as the ground-glass.
-- Pete Andrews (email@example.com), July 24, 2001.
The real object, I feel, (and this is based on years of experience in testing and adjusting ground glass alignment in professional motion picture equipment) is to get the ground glass in the perfect position or as close to it as possible. By doing this, the greatest number of film holders will be useable. I think it well to remember that a film holder is molded plastic and sheet aluminum. Film holders don't typically employ materials or constuction techniques used to make precision instruments. They were designed for mass production and relatively low cost. The tolerance allowed (+/- .007") is designed to work well when the ground glass (which is where you are doing the focusing and is your reference point) is centered in that range of tolerance. While it might be possible to precisely measure film holder depth and create a "matched set" of film holders that conform nearly perfectly to a particular ground glass placement in a particular camera, by doing so, you create a non-standard, shutting out the possiblity of quickly replacing worn holders or borrowing same in an emergency.
-- Robert A. Zeichner (firstname.lastname@example.org), July 25, 2001.
Camera back registration seems to be a 'non-standard' anyway. A recent small-sample survey by a respected UK photo journal showed that 5x4 camera GG distances varied from 4.67 mm to over 5 mm across makes and models. This represents a rather liberal interpretation of the +/- 7 thou tolerance.
Since the standard for filmholder depth is laid down as 5mm, it doesn't take a genius to work out that the GG ought to be at 4.8mm from the seating of the camera back.
LF cameras are obviously not made by geniuses.
-- Pete Andrews (email@example.com), July 26, 2001.