So, while it was annoying me that I got something wrong I carried on investigating the Lunar Orbiter photographic subsystem, as it is something that the ADs, and others, do bring up as it is a pressurised film system, and therefore all photography requires pressure to protect the film.
I found this document:
https://ntrs.nasa.gov/api/citations/19680071016/downloads/19680071016.pdfwhich covers every aspect of what is an incredibly complex piece of engineering by anyone's standards.
The key takeaway from the document is: At no point does it claim that the pressure vessel used to maintain between 1.0 and 1.9 psia of 99% nitrogen (with a tiny bit of oxygen thrown in) exists to protect the Kodak film.
It does, however, require pressure in other areas.
During photography, a small bellows draws the film towards the platen (the area where exposure occurs). In order for that to work, there needs to be a pressure differential. If the entire system is in a vacuum, there is nothing to create that differential.
The next step is the development process. During developing, a bimat strip loaded with chemicals is laminated to the film. That bimat film develops the actual photographic film into something that can be scanned. It's that bimat strip that needs maintaining in optimal humidy and temperature conditions. Temperature is largely controlled by radiating and conducting heat away through the orbiter structure. Humidity is regulated by potassum thiocynate crystals, which deal with the moisture produced by the bimat film during development.
This is the entirety of what it says about pressure:
"2.2.9.3 Pressure. The environmental gas in the [photographic subsystem] pressure shell is made up of 99 percent nitrogen and a total of I percent oxygen and other gases. Oxygen must be below 1 percent by volume to prevent loss of potency in the Bimat. Prior to launch, the nitrogen in the pressure shell is maintained at 1.9 psi above atmospheric pressure to prevent air leakage into the shell. This pressure differential is maintained throughout the launch phase by a relief valve which operates as the external pressure decreases.
During the photographic mission, pressure within the shell is maintained above 1.0 psia by nitrogen supplied from a pressurized bottle. This supply will allow for leakage equivalent to three pressure shell volumes of 1.0 psia at 70F. The lover pressure limit of 1.0 psia is required to keep the boiling point temperature of water above the expected temperatures of materials requiring moisture such as the Bimat film and the humidity control pads."So, nothing to do with protecting the film, everything to do with protecting the Bimat strips and the developing of the film. To link back to the original topic, Corona systems that did not develop film did not need such protection.
As for radiation, the document has this to say:
"It is expected that the amounts of radiation received from the Van Allen belts and from the galactlc-cosmlc sources will have little or no effect on the photographic mission." It summarises the characteristics of the film and why it is radiation resistant (I don't pretend to be an expert in that subject, so won't comment on it), and notes that the structure of the oribter also acts as a shield, specifically mentioning the film container. The risk from solar flares is much greater, and it calculates that there is a 43% probablility of a signifcant one happening over the entire program, which is pretty much in the "might happen, might not" area. If one was to occur, the idea was to orient the craft so that the more of the structure would be in the way.
Long story short: if you use the right film, and plan ahead, vacuum and radiation are not issues when using photographic film in space.
