Do you ever come back from a trip and wonder why your photos never seem
to capture what you saw? That's because the camera isn't as selective your eye. Your brain directs your eye to focus on a precise spot, but the camera indiscriminately photographs everything that's there. Here's a nifty trick for
composing your pictures from your photographs, so you can isolate the area that attracted you--or find something you like better. Use an empty 35mm slide mount as a viewfinder, sliding it over the scene till you find a composition you like. Tape the viewfinder to the photograph with artists' low-tack tape and make design sketches of the scene; then see how many other scenes you can find in the same photograph. The viewfinder eliminates unnecessary background details and focuses on your center of interest. It also helps you train your eye to recognize good composition. If you have a scanner, you can enlarge the selected area and edit values and colors in a photo-editing program.
From The New Creative Artist. p. 112.