I’m not a huge expert on this area but I think it would have to do with changing the contrast and brightness properties of the film. How this was accomplished in the 1970’s I do not know.
It’s achieved with the exposure of the film. The Director of Photography can select what level of light moves through his lens and hits the film per frame, thus affecting things such as brightness, color, etc. Much easier to do today with video technology where you can see exactly what you are ‘filming’ on the fly. Back in the day you didn’t know what you had shot until you got the film back from the lab, so being Director of Photography became a real important craft.
It’s very easily done nowadays with computer editing programs such as Final Cut Pro or Avid but like Cappy said back in the day it was much more difficult. The DP of a film is almost as important as a director especially when it comes to the technicality of a scene. I still think it’s a very underrated position in the film industry.