The Chase

The Chase, originally uploaded by Cocoabiscuit.

Balancing aperture (f stop) and shutter speed can be tricky. Shutter speed stops the action (or conversely slows water down to make it look more velvety). Aperture determines how much of the scene is in focus-- narrow depth of field will have sharp focus on one plane of the scene with the rest blurry while a wide depth of field keeps everything in focus. F stop is represented as a reciprocal, so a high number (16) is 1/16th of the shutter open and provides everything in focus, while a low number (4.5) is 1/4th of the shutter open and therefore spreads the image of a wider area of the lens and makes only a small amount in focus. The more light available, the higher number f stop you can use and therefore have more options for choosing depth of field.
In this shot, "The Chase", I wanted to stop the action of this Ecuadoran boy reaching for the birds in the Plaza San Francisco. I also wanted to keep him, the birds, and the wall behind him in focus. Because it was fairly sunny, I had good light I chose depth of field f 13 and sacrificed a little bit of motion with a shutter speed of only 1/200th of a second. I stopped the boy's motion, but not the pigeons'.