Although I'm not a cinematographer, I've directed two or three projects now for free to learn, gain experience, and to network and form relationships. Two of them were such terrible experiences that I don't even include them on my resume. One of these scenarios involved me offering myself and my camera package (2K Blackmagic w/ battery, shoulder mount, three lenses, and a terabyte and a half's worth of memory cards) for free, with the promise of making great contacts and future paid work. In the end I handed off the two memory cards we used during the shoot to the producer to hand off to their editor - this was in 2014, and I still have never been given back those two cards, and said producer fell off the face of the earth.
Despite the horror stories, though, I have most certainly had past unpaid work turn up future projects, some paid some not, but it's definitely worth doing your research. As a rule of thumb I don't work, paid or unpaid, for a producer/individual who has no previous work to show, or past contacts that I can get in touch with to feel-out the legitimacy of the individual. If you find yourself working for free, don't be afraid to ask and look into their past work experiences and relationships. You are both doing each other a favour, after all - one providing experience and one providing a free service, so there has to be give and take from both sides.
Movies I had seen without any prior knowledge:
The Sound of Insects
I honestly don't think these movies would have been half as enjoyable had I seen even one trailer. Drive and Ex Machina are easily two of my top 10 favourite films.