PROGRAMMED AUTO, OR P MODECameramakers know that most photographers not only want control of their cameras, but they also would like to control them in the easiest way possible. For this reason, companies include what's usually called the Programmed Auto (or simply Program) mode on even their top-tier professional models. This is one of the most misunderstood, and therefore most underutilized, exposure modes.
Unlike the green camera mode—which chooses all settings, including exposure, flash, ISO and white balance—Program mode controls only the aperture and shutter speed settings. This leaves the rest up to the photographer. When in Program mode, the camera doesn't randomly choose an aperture and shutter speed; it uses parameters that are preprogrammed into the camera's firmware.
The parameters are quite simple. In low light, the aperture is fully opened and the shutter speed is set as needed to get a proper exposure according to the metering method you've selected. As light increases, the camera will shorten the shutter speed until it reaches a speed that's the reciprocal of the focal length of your lens (the handholding limit).
From there, the aperture is stopped down and the shutter speed shortened alternately until one of the settings maxes out. As you may surmise, this isn't necessarily the best mode to use if you want to achieve a specific effect in your photograph, such as controlling depth of field or portraying motion.
The good news is that when using Program mode, you still can easily modify camera settings to suit your needs. You can engage Flexible Program or Program Shift (depending on your camera system), which allows you to adjust the shutter speed and aperture settings as you see fit. Simply rotate the appropriate command dial until you achieve the aperture or shutter setting that you want, and the camera takes care of the corresponding setting.
If you know which aperture setting or shutter speed you want for a particular effect, why not use Shutter or Aperture Priority settings? The answer is that, when using these semiautomatic settings, the camera doesn't necessarily take advantage of all data that's available. For example, when using Aperture Priority, the camera doesn't take into account the focal length of the lens and attempts to keep the shutter speed fast enough to reduce camera shake, as it does in Program mode.
Program mode may not be the best option for all shooting situations, but it's very handy for photographing scenes when you want to focus your attention more on composition and less on settings. Since you can physically adjust the settings on the fly, it's more flexible than most people think.