Between 1938 and 1941 the legendary photographer Walker Evans used a hidden camera to photograph subway riders in New York City. The images are haunting because these are unguarded moments "and the mask is off," he said.
I've always been intrigued with this idea, and on a recent trip to New York, spent a few hours walking the streets of the city making photographs with my little Pentax GR. The experience was thrilling.
Part of the fun was guessing what the framing was while keeping the camera down at waist level where I could not see the LCD screen to compose. More interesting was building a good composition. Like a quarterback throwing the football to where a receiver would be by the time the ball got to him, I kept practicing exactly when to push the shutter release so my subject would be in the right spot in the picture.
After a short time, I was surprised by how high the percentage of good framing and placement was, and the wonderful juxtaposition of the individual to their surroundings.
Whether this is a style of photography that excites you, the exercise of knowing how big a field of view you have without looking through a viewfinder or looking at an LCD is well worthwhile. Also of great value is anticipating the placement of a moving subject based on their speed and the delay of your shutter release. Both exercises will increase your abiity to respond quickly and make powerful photographs in a fast-paced world.