[EDIT: Moiisty beat me with his nice and concise answer, but for those interest in a more blabby reply here it is 😃]
Maleficent said:
I am interested in the topic, but unfortunately I know too little about it. What exactly does “35mm scan” mean? I assume that it means that an original camera negative is scanned and not just any copy of a copy of this negative?
Incorrect (99.9% of the time). The following is simplified a bit; research if you want more solid/detailed info…
When ya see fans doing a 35mm scan, they are scanning one of the many prints that were out in the world showing at theaters back when that was the way the industry operated (pre-digital). There’s pretty much no way in h$## a fan is going to get the one-of-a-kind OCN for a film.
However a STUDIO making/releasing a BD quite likely WILL use OCN. DVD was a different ballgame. A good number of DVD’s did not go back to the OCN. They used prints a lot, which is why some people on this site and FanRestore.com cherish their special DVDs of certain films: they have the correct THEATRICAL color/contrast look of the film as seen in theaters. (Again, these are all broad strokes of example and explanation.)
There are some disadvantages to some fans when it comes to watching OCN scans (used for BD/UHD releases). “How could that be possible? That’s THE best image source, right?” The answer is that color (correction) work was done for releases after the OCN was put together. So… Well, my fave example is The Road Warrior/Mad Max 2. In theaters the (color-timed/changed) release prints had the post-apocalyptic setting looking dingy, dusty, brown-ish. However, the OCN — what the cameras captured — did NOT have brownish sky. The cameras captured nice blue sky (and sometimes nicely green plants), looked much more like a nice postcard than the release in theaters…because the filmmakers knew they didn’t have to spend all the time to shower every location with brown dust while filming. In post they could just brown-it-out a bit and wallah (and it DID work, the recoloring looks much more post-apocalyptic in the theater release prints than the scan of the OCN you can get on BD).
SO, a fan who scans a mere print of TRW/MM2 that showed in theaters will have a movie that reflects the post-apocalyptic haze that the filmmaker wanted, a re-coloring that helps the setting, helps the vision…whereas if you get a BD with the straight OCN scan, you’re getting something a little too pretty looking, which would also be the case if a fan somehow got the one-of-a-kind camera negative to scan. By getting/using a 35mm release print instead, they rather automatically get the color work that we all saw in theaters and don’t have to start from scratch re-doing the color of every shot in the film (to match an old LD or DVD they’d use as a reference).