Dimensions of sheet film: Is 5x7 film actually smaller than 5x7?

[dcy]

Please bear with me. I don't shoot large format and I am not going to buy a pack of film just to measure its dimensions. I figured that people in this forum would know the answer to my question:

I have a DIY pinhole camera that I looks like a box and takes 5x7 paper. I had the idea of buying 5x7 sheet film holders and adapting the camera to use them. I ordered a pair of vintage film holders on eBay and I am sad to report that the paper doesn't fit. The space in the film holder is too small for 5x7 paper. After a bit of searching, I found a comment in a forum that said that sheet film is actually slightly smaller than their nominal size because sheet film was made to fit inside a kind of adapter for glass plate holders called a "septum".

Did I get the story more or less right?

If so, my next question is: Why is it that the ortho litho film that I have is exactly 5x7 and not this supposedly smaller size of sheet film? I imagine that they must have a different history that somehow exempts them the shrunken nature of their panchromatic brethren.

Finally, does this mean that anyone who uses paper or ortho litho film in a large format camera must use a paper trimmer to cut them down to size?

 

[Rick A]

Yes, large format film (all sizes) is smaller than the listed size by 1/10 of an inch in two directions. This started back when holders were used for glass or tin sheets, which were dimension. Then when film came around they used a film sheath to hold film in the plate holders. Film manufacturers continue to make film in this reduced size. You can use 13x18cm film holders for 5x7 paper without trimming it down, 5x7 film also fits though a bit loose.

 

[djdister]

If so, my next question is: Why is it that the ortho litho film that I have is exactly 5x7 and not this supposedly smaller size of sheet film? I imagine that they must have a different history that somehow exempts them the shrunken nature of their panchromatic brethren.

Yes, lithography sheet film has a very different history (and purpose) in the graphic arts world. Litho film was not designed to fit into septums or film holders, but rather to be held in place by a vacuum frame or contact frame.

 

[dcy]

Thanks!

I just looked up "vacuum frame" ---- Basically, giant photocopiers with a vacuum pump to hold the film flat against the glass. You can buy one on eBay today for the low price of $4,5000.00 https://www.ebay.com/itm/134776675155

 

[MattKing]

Yes, lithography sheet film has a very different history (and purpose) in the graphic arts world. Litho film was not designed to fit into septums or film holders, but rather to be held in place by a vacuum frame or contact frame.

Lithography film was designed to fit into some film holders at the time of exposure, but those film holders weren't the type designed to fit into portable cameras.
They were designed to fit into the big process cameras you would find taking up a room at your local graphic arts shop.
But yes, many/most of those cameras used a vacuum frame instead.
Here is an example of a fairly small version of one of those cameras:
https://www.photrio.com/forum/threads/22x28-enlarger-from-lithographic-process-camera.205233/

The one I worked with was about 2-3 times that size.

 

[abruzzi]

If so, my next question is: Why is it that the ortho litho film that I have is exactly 5x7 and not this supposedly smaller size of sheet film? I imagine that they must have a different history that somehow exempts them the shrunken nature of their panchromatic brethren.

Why its setup that way I don't know, but you should see something like this in the item description of Ortho Litho:

that screenshot is from Freestyle. B&H isn't as helpful:

 

[koraks]

Why is it that the ortho litho film that I have is exactly 5x7 and not this supposedly smaller size of sheet film?

Because it's not intended to be used in a camera.