# Thread: focal plane of ground glass not in sync with film

1. ## focal plane of ground glass not in sync with film

Doremus,

If you take a 150mm lens and focus at infinity using for example f5.6, the physical aperture diameter is 26.8mm. That physical aperture will, for any point in the subject, throw a cone of light from the rear lens element to the film plane. The distance of the base of the cone to the film plane and the diameter of the cone base will determine the angle of incidence of the outer edges of the cone of light on the film plane. Now extend the bellows to 300mm instead of infinity(150mm). The diameter of the cone base remains the same. But since the distance has changed then the angle of incidence on the film also changes. It gets less and since it becomes less the depth of focus is increased because the film can be further out of position from perfect focus whilst still being within the deired circle of confusion. At infinity the depth of focus is at its smallest so if the film is not in the correct plane the error will be maximised.

2. ## focal plane of ground glass not in sync with film

have given this some more thought and size of base of cone will change at 300mm (1:1) but it would need to double in diameter for depth of focus to remain the same and I don't think it will so my premise will remain true. I could be wrong though.

3. ## focal plane of ground glass not in sync with film

Ok, what am I missing . . . isn't this a straight forward problem of getting the ground glass into the same plane as the film?

Since manufactured film holders have standardized dimensions (0.197 +/- 0.007" for 4x5), it is only necessary to assure that the GG lies at exactly that distance into the GG frame - easy to do with a simple caliper - measure the depth at each of the 4 corners (since glass don't flex with a sh!#). If the GG frame is sitting tight and square to the camera back and this distance is right, it's time to try some different film holders.

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4. ## focal plane of ground glass not in sync with film

Nothing.
The really great thing about standards is that there are so many to choose from!

does the standard say with film in the holder or without film.
anyone who has bought TOYO film holders should have discovered that they use one of the other standards(their own).

As you say, its simple to measure and its simple to correct providing you have a depth micrometer.

5. ## focal plane of ground glass not in sync with film

Film holders are made to the standard so that they can be used interchangably in cameras of all brands. ULF and really old holders may not follow a standard. I have measured 4x5 Fidelity, Lisco and Toyo holders, and they have all been well within the ANSI standard of 0.197 +/- 0.007 inches. 0.197 inches is exactly 5 mm. This is without film, so the depth to the ground glass so be a few thousandths less. The point of measuring the holders with the film in place is to automatically take this into account.

You can find dimensions for other sizes at http://home.earthlink.net/~eahoo/filmhold.html. The depth row probably should be labeled "Depth to septum".

6. ## focal plane of ground glass not in sync with film

all I can say is that the TOYO 5x4 holders I bought were significantly different from the fidelity holders I had. I changed them.

The important thing is not what the "standard" is but that all your holders are the same depth and and that your ground glass is set to the same depth.

7. ## focal plane of ground glass not in sync with film

the standard quoted at the link you gave says 0.197 +/- 0.007 TO FILM SURFACE which only goes to show that quoting standards is fraught with error for one reason or another(typo or not).

8. ## focal plane of ground glass not in sync with film

"providing you have a depth micrometer"

A caliper will easily measure to +/- 0.001" for \$10, a dial caliper to +/- 0.0005 for \$25. I have a depth micrometer (\$150) but have never used it.

It doesn't really matter what "the standard" says unless you want interchangeability of holders - if you do, adjust the GG to the standard.

If you just want to use the holders you have, measure the depth with film in place and set the frosted surface of the GG to the same depth.

9. ## focal plane of ground glass not in sync with film

Calamity Jane A caliper will easily measure to +/- 0.001" for \$10

My Junior Woodchuck Manual shows that a stack of three brand-new US Quarters == 0.197" / +- .002

That's also the depth for a 4x5 holder.

10. ## focal plane of ground glass not in sync with film

To CHECK the focus without having to shoot and process film:
Attach and open a lens. Remove the bellows. Darken the room. Focus on a glowing clear light bulb. Insert a film holder with a sacrificial sheet of film. Pull the dark slide. Peek at the image of the light bulb's filament on the film surface with a loupe turned around, thereby not blocking the light path. Alternate between focusing with the ground glass and focusing on the film surface and determine if there is a difference in focusing between the two.

This method may or may not be as accurate as others but it is quick and very reassuring if successful.

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