? "standardized" distance from the front of a film holder to where the film lies

[Greg]

Does anyone know what the "standardized" distance from the front of a film holder to where the film lies is? My notes have it to be 3/16 of an inch, but I do not remember where I got that measurement from, and I seemed to have misplaced my depth gauge.

thanks

[Kevin Crisp]

http://home.earthlink.net/~eahoo/page8/filmhold.html

[DHodson]

Can you check that link please? It's not working for me.

Thanks
Dave

[Dan Fromm]

I just clicked on the link. It worked for me.

As of a few minutes ago the page has been saved 29 times on archive.org. Their link to the saved pages is http://web.archive.org/web/*/http://.../filmhold.html

[Jim Jones]

It should be noted that some of the information in this often cited illustration may be questionable. The depth to film surface distance (the "T" distance) as shown in my 1951 copy of the ASA standards is to the back, not the front, of the film. The Exposure Height measurement given is apparently the ASA D2, not the appropriate D3, measurement. The Exposure Width measurement is the E1 maximum, not the appropriate E3 measurement. The Retaining Tab Location is a maximum; the minimum is .010" less. The Length diminsion is a minimum: ASA specified no maximum. The Width is a maximum: the minimum is about .031" less. The Distance to Exposure Field is a maximum: the minimum is .020 less for up through 5x7 holders and .030 less for 8x10 holders. I don't have ASA information on holders larger than 8x10. Diminsions in newer versions of the ASA or ANSI standards may differ. I don't have access to my other reference materialo at the moment.

[Leigh]

The depth to film surface distance (the "T" distance) as shown in my 1951 copy of the ASA standards is to the back, not the front, of the film.

That's because the T distance is to the front surface of the film septum, which is part of the holder.

It has nothing to do with the film, which is not mentioned in this standard.

If you choose to criticize, please understand what you're talking about.

- Leigh

[Will Frostmill]

Er, hrm. Well, which surfaces should I be checking with a micrometer* to figure out if my ground glass and film holder are going to agree?

*a lucky find in my dad's desk drawer!

[LabRat]

Er, hrm. Well, which surfaces should I be checking with a micrometer* to figure out if my ground glass and film holder are going to agree?

*a lucky find in my dad's desk drawer!

That micrometer is useful for checking the thickness of film (which can now vary due to new thinner bases than days of old), but what is required is a DEPTH micrometer set that has interchangeable measuring rods for different depth ranges...

Also (for the fanatic types out there), the surface of the film is not the optimum point for focus, but rather slightly in-between the top & bottom of the emulsion where the image forming takes place... (Some Leica fanatics fight this out online...)

Remember, we are using old view cameras with a lot of slop in alignment, and note that ANSI has allowance for tolerance, and even film can pop with it's chest out, or crestfallen inwards due to the environmental factors, or plain old gravity, so happy Ground Hog's Day film, and you will be where you will be during exposure time... (The film plane is no place for compulsive, nervous, sissies to dwell!!!) ;-I

Steve K

[Leigh]

Also (for the fanatic types out there), the surface of the film is not the optimum point for focus, but rather slightly in-between the top & bottom of the emulsion where the image forming takes place...

The emulsion on a film is only 5 to 10 micrometers thick (that's millionths of a meter).
That's 200 to 400 millionths of an inch.

Considering the thickness of the base is several thousandths of an inch, the emulsion thickness is irrelevant.

For positioning a ground glass, you should set the distance from the rear surface of the camera to the lens side of the ground glass equal to the "Depth to film surface" from the spec (0.197" +/- 0.007" for 4x5 film).

- Leigh

[Doremus Scudder]

Er, hrm. Well, which surfaces should I be checking with a micrometer* to figure out if my ground glass and film holder are going to agree?

*a lucky find in my dad's desk drawer!

Just to elaborate on what's been said and assuming you indeed have a depth micrometer: What you measure will depend on what you are doing; checking/setting your camera's focus or constructing filmholders.

To check camera focus:

Remove the back from the camera and place it with the inside-up (outside/gg side down) on your work surface.

Measure from the flat surface where the back meets rear standard/camera body to the ground glass. Note: this measurement is NOT going to the the same as the ANSI standard for filmholders! It is specific to each camera design. Also, be aware that it takes some skill and practice to use a depth micrometer correctly and consistently. You should master this before making your measurements to eliminate operator error.

Now, insert a filmholder in the camera back, remove the darkslide and make the same measurement: from your chosen flat surface to the surface of the septum that the film rests on. This measurement minus the thickness of the film should be the same as the first measurement to the surface of the ground glass.

Note that it's often easier to check camera focus with a focus test: Set up a ruler at an oblique angle to the camera. Focus with a relatively long lens on the middle of the ruler (at say, the 6" mark). Shoot wide open. Develop and inspect the focus on the negative with a loupe. If the focus is correct, you're good to go. If the focus is farther out than that on the ground glass, you need to shim the ground glass out. Cut and install some shims and test again. Add or remove shims until you have the correct focus. If the focus is closer than that on the ground glass (this happens more rarely), the gg is too far away and you need to move it closer by milling or otherwise removing material on the gg seat. Note that if your camera was designed to have a Fresnel/ground glass sandwich and now doesn't, that this will happen. TIn this case, the best solution is to get a proper OEM replacement rather than modifying the camera.


If you're building filmholders:

You should use the standards given by ANSI, and the depth to measure is from the front outside surface of the filmholder to the surface of the septum.

Best,

Doremus