Kane was not an impressive movie at all for content. For its lighting, composition, and editing it is amazing. But that does not make up for lousy content. IMHO. One needs to look back at films such as Faust by F.W. Murnau and Cabinet of Dr. Caligari by Robert Wiene.Two great examples of German Expressionism. Pandora's Box by Georg Wilhelm Pabst is amazing. And Fritz Lang's M is amazing as well. But one does not need to go back that far for great b+w films. Jean-Luc Godard's French New Wave film Breathless is excellent. Hitchcock's Strangers on a Train is excellent as well. The list can go on.
Citizen Kane borrowed (or, was influenced by, shall we say) a lot from the German Expressionist lighting evidenced in the films of Wiene and Murnau but at the same time it is distinctly American in the way lighting is definitely non-stagey (a cinematographic problem that most, if not all, European movies suffered from at the time). I personally find the film very important in terms of content and formal structure as well, so I have to disagree with you on that.
In many ways,
Citizen Kane was a happy accident. Welles was fresh out of the theatre and radio but (by his own admission) clueless on moviemaking. As a result he had absolutely no preconceptions about what should be done or could be done in a film. Fortunately for him, he had by his side Gregg Toland, a cinematographer that could deliver whatever Welles requested in his wide-eyed cinematographic innocence. If Welles asked for something that could not be done, Toland wouldn't say anything, he 'd just go away and work on it and come back with some sort of solution the next day. There's no denying that Welles himself was a visual genius and this was brought out with all the cinematographers he worked with later.
The Magnificent Ambersons (a very important film even in its truncated version),
Touch of Evil, and
The Lady from Shanghai all show Welles' visual genius. Incidentally, since the film comes up very often, I don't think he had anything whatsoever to do with the cinematography of
The Third Man, although the film is justly celebrated for it (it was lucky for Reed to have him to film his scenes for the little that Welles, ever busy with numerous other projects, made available himself for filming - although he did pen the well-known 'Cuckoo clock' lines).
As for other films, I also like very much the B&W photography in Bergman's films (the cinematographer was nearly always Sven Nykvist). It's not just the photography (as in camera movement and angle), it's also that they went for a specific kind of light in Faro island where they were filming most often. Hitchcock is another favourite director of mine but I don't think the photography in his B&W period was anything to write home about. The visual genius was there - the angles, the ability to narrate visually - but the lighting was somewhat average, mostly because light tecnicians in the UK, where he shot most of his B&W films, were lagging behind their counterparts in Hollywood. Most of Hitchcock's films that are now recognised as masterpieces were produced/filmed in the US and were in colour (the one exception being
Notorious, which is B&W).
Last, many Soviet films show understanding and appreciation for light never really attained in the West. An off the cuff example is
The Cranes are Flying by (the actually Georgian) Kalatozov - the plot is terribly cumbersome (I should say, for my taste) but the photography is jawdropping. In that respect perhaps even better is (again) Kalatozov's
Soy Cuba. Exactly why people from the ex-Soviet Union are so sensitive to the changing qualities of light is a mystery to me but I kinda feel it is something deeply entrenched in their visual culture as I see it also in photos from FSU RFF and flickr members who do good photography. Or, may be, that's what photography is partly about anyway.
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