I've tested my theory. I set up a mission with a variety of docked ships (one merchant, and one of each type of warship), and set a photo recon objective. I set the photo value at 60, and made two tests, taking photos starting with the merchant and working my way up from subchaser to battleship:
- With min. threshold of 30: I could take photos of everything, but none was saved (i.e the picture was not available from the objective notebook) until I reached the heavy cruiser. Both the heavy cruiser and the carrier were saved to the notebook, and the mission was completed.
- With min. threshold of 0: All photos were saved, and the objective was complete once I had photos of merchant, subhunter, destroyer and light cruiser.
I think this more or less proves my theory. Someone with more patience than I have can try and determine the formula for photo value. I suspect it's 10% of the renown awarded (the light cruiser had 235 and the heavy cruiser 500, so it makes sense).