Thanks for all the support!
Truthfully, I realized (me knowing quite a bit about this) that there is NO WAY in LoTC to recreate a conventional pin hole camera. This is solely because of the tedious task of creating film. This sap acts as the film, as I saw no other way to have a pin hole camera. I'm guessing, though I may be wrong, there is no such thing as photosensitive plant sap (there are other photosensitive materials, though those would be harder to get). Oh and, the way I described using the plant sap is literally as a replacement for film. It serves the same job in basically the same way (though I guess plant sap is renewable and is more convenient to use, making photography available on the large-scale).