I exposed this 4x5 film on the back side and I got an image

Today I learned something that surpised me. I loaded new Foma 200 4x5 sheets into my wooden Chamonix film holders. And did a test shot. Then after shooting I remembered, that I loaded the film the wrong way, the nodge of the sheet was at the lower right corner (the upper side being the side where the dark slide comes in). This means that the film was facing with its back side to the lens when I shot the image. I still developed the film and to my total suprise there is actually an image on the film:



So I have questions:
1. Why did the image get exposed?
2. Isn't there supposed to be some sort of light blocking layer on the backside of the film?
3. Isn't that layer called anti-halation layer, which is meant to block the light that may be reflected from the camera (or film holder) back?
4. Do only some films have such a layer and Foma does not?

Funny: With some of the holders I even re-shot the the scene after I have flipped the negative in the holder (the nodge now pointing at the lower left). That got me these kind of double exposures.

It depends on the back layer but you can shoot through it. E6 slide film can have a very dense layer that is difficult to shoot through. When I shoot redscale with C41 film I add about 3 extra stops of exposure. Movie film with a rem jet backing would be very hard to shoot through, I’ve never tried it but it is quite opaque. I’m not sure about Foma but there can be quite a variety of backings on black and white film.

Some examples of redscale I’ve shot. https://www.flickr.com/search/?user_id=60348236@N07&sort=date-taken-desc&text=Redscale&view_all=1

It's not an opaque layer.

Otherwise all those data-backs or cameras with integrated data-exposure would be usesless, as they all work with a rear exposure.

bonk said:

3. Isn't that layer called anti-halation layer, which is meant to block the light that may be reflected from the camera (or film holder) back?

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It already starts with the reflection off the film base. Thus Agfa devised a layer between the emulsion and base.

bonk said:

1. Why did the image get exposed?

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You know, open shutter and all that.

2. Isn't there supposed to be some sort of light blocking layer on the backside of the film?

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Foma's anti-halation layer is not quite as effective as that of, say, Kodak film, and even that will give an image if you expose through the back. With TMX you loose something like 2 stops when trying to expose through the back, probably depending on wavelength as well.

3. Isn't that layer called anti-halation layer, which is meant to block the light that may be reflected from the camera (or film holder) back?

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Yes, but it's not 100% effective. Different story with cine film and proper remjet backing btw, but that's a different matter.

4. Do only some films have such a layer and Foma does not?

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All photographc sheet film AFAIK has an antihalation layer, as does all 120 roll film again AFAIK. 35mm is a different story; it generally doesn't have it save for one exception I think (Adox CHS 20 or whatsitcalled)

Practically all camera films have an AH means, typically a layer.

Not only did you get an image but it looks pretty sharp too. I wonder if the normal rules of development go upside down in this situation. Normally the shadow comes up first because it’s shallower in the emulsion, in this case you have exposed the opposite side of the emulsion layer…presumably

Google Redscale film. Essentially, it’s inverted color film.
Can give a very nice effect.

Accidentally did this with Fomapan 100 a few years back, I loaded two sheets of film the wrong way round in the holders.
The images were slighly under exposed from the AH layer, but were still quite useable.

I've done this with 1990s vintage Tri-X 320, around 2005. Research after exposure (problem discovered when unloading the film holders into the developing tank) suggested I'd be about 5 stops underexposed, so I created a super-hyper developer (mainly by throwing together everything I had on hand) and got rather normal-looking images. I later used the same developer to process correctly oriented Tri-X 400 at EI 5000...

First time I loaded film holders, many years ago, I did that with some sort of C41 film and got strange colored images.

dpurdy said:

First time I loaded film holders, many years ago, I did that with some sort of C41 film and got strange colored images.

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Redscale!

Back in 2012 I accidentally reversed paper for a paper negative in one side of a 4x5 holder. Even that got a visible exposure! Many stops under, and a bit soft(er -- it was a pinhole shot) from diffusing through the paper. The paper was Arista EDU RC, grade 2.

You can even slightly make out the high tension lines.
(I have no plans to repeat the process.)