I sporadically do street photography, mostly tending toward head-and-shoulders portraits, and there are a few advantages to TLR cameras:
1. Small, lightweight, easy to travel with
2. People have a natural curiosity about TLR cameras, I guess because they are funky and notably vintage. Nobody much cares about my Canon EOS Rebel 2000 or the Mamiya 645 Pro, but when I'm carrying a Ricoh Diacord or Rolleicord, people always approach and start asking questions. Such people are basically announcing that they're down to have their picture taken, so it takes the social anxiety out of approaching a complete stranger.
3. Subjects seem to find it less intimidating when you're composing with a waist level viewfinder, so you get more naturally relaxed poses and expressions.
Downsides are that it is indeed more challenging to focus (I still miss focus from time to time with my other cameras though) and compositions can get a bit wonky if you're shooting someone taller than you. I can do a quicker point-and-shoot with an SLR, whereas I might have to take a moment to pose someone or find something to climb on to get a natural-looking shot. When you're stopping strangers, you don't want to test their patience; fortunately, most people are willing to take a few seconds to try and get it right.
I shot a roll of Tri-X on Venice Beach last year and I'm including the only shot that came out reasonably well--others were out of focus, poorly composed, or underexposed. The main lesson I keep learning over and over when I get my results back is that I need to slow down and shoot with more intentionality and thoughtfulness.
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