Don't know about the camera on the U2 and Canberra, but as a bit of a photography enthusiast I have great fun when I went on the Tornado camera pod course back in the late 90's. The pod housed two cameras manufactured by a UK company, and they were real pieces of mechanical engineering which had taken the technology of photographic film cameras to the limit. With a stable platform being flown at a set speed the clarity of the images was amazing, and the level of detail you could see from a considerable distance was like something out of science fiction.
The bonus of using the film method rather than optical sensors as we do in digital cameras was that once the film had been exposed and developed, you could go back to it over and over again using different developing techniques to get slightly different images. Plus you could use good old fashioned glass magnifying optics to select specific areas, once a digital sensor has been exposed to the image the only way you can magnify the image beyond the resolution of the sensor is to digitally "guess" what the gaps between the pixels look like.