Super-8!
I must have shot a dozen or so Super-8 movies, each more elaborate than the last. By the time I finally went to film school, my Super-8 films were pretty sophisticated - I was shooting in b&w reversal, had film noir lighting (using only a few lights), tilted angles, Russsian montage editing style, dolly moves (I built a small dolly), macro photography and time-lapse photography, etc. I was determined to make my little Super-8 movies look like "real" movies.
After about a decade of Super-8, I started to shoot a little 16mm for some cheap Karaoke music videos (someone had seen my Super-8 shorts and wanted some b&w Super-8 footage for a video, then I convinced them to let me shoot in 16mm, so I ran out and learned how to load an Arri-S.)
When I got to film school, people were blown away but what I could do in Super-8 that I got asked to shoot people's 16mm student thesis projects. By the time I left three years later, I must have shot eighteen or so 16mm projects and one short in 35mm. I was mostly using the school's 16mm Eclair NPR's and Arri-S cameras, but the last few were using an Aaton from Panavision's New Filmmakers Program.
Then a fellow student got to make a 35mm short film for Universal Studios as part of a Hispanic Filmmakers Fund, with donated gear and stock, and he asked me to shoot that.
After graduation in 1991, I shot low-budget features, 30 to date, most in 35mm, one in Super-16, and eight in 24P HD. The budgets have been going up lately.
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Besides the shooting experience, I've always been an avid reader. When I was at UCLA as an English Lit major, I would sit in the library and read filmmaking books. I read every issue of American Cinematographer back to the 1920's. And then reread many of them. So even though I was only shooting in Super-8, I was keeping up on 16mm and 35mm shooting techniques.
The truth is that if you want to be a DP, lighting, composition, movement, and editing (i.e. visual storytelling) are the most important skills over camera technology. If you learn to light well, the technical stuff is easy to learn.