Thanks for the question! No, there wasn’t a grammatical error made here. It’s just a weird phrasing. I’m assuming the part you’re pointing to is “every photograph, because it involves the light rays that SOMETHING emits hitting film, must in some obvious sense be true.” That’s just saying that “something” emits light rays, which hit film, and that’s what makes a photograph. You can replace the word “something” with “a kind of camera” and it should make more sense.
Hope this helps! Feel free to ask any other questions that you might have.
