October 2008 HelpLine
What’s Your Priority?
I typically use my camera in automatic mode, but occasionally I step out of my comfort zone and move into some of the other modes. I really understand the full manual mode but, quite honestly, I don't get the whole aperture priority and shutter priority. I know what they are supposed to do and I sort of get when you are supposed to use them, but when I take pictures, I often switch between the two and I really don't see much difference. Is it me or the camera?
Via the Internet
A) I'm glad that you're trying modes other than automatic mode. There's nothing wrong with automatic exposure, but I really feel that the more you know your camera and its different operations, the less it becomes an obstacle to taking great pictures.
When you talk about controlling exposure in a camera, you're really talking about controlling the amount of light that reaches the light-sensitive part of the camera. For a digital camera, this is the image sensor; for a film camera, it's the emulsion of the film. These exposure concepts aren't unique to digital photography. Film photographers have used these same exposure controls for a long time.
There are really only four things you can do to change the amount of light coming into the camera. First, you can put something in front of the lens to reduce the light, such as a neutral-density filter or a polarizing filter. Second, you can change the lens mounted on your camera—if your camera has interchangeable lenses—to one that's more (or less) efficient at passing light.
The third option is to adjust the opening in the lens that allows light to pass. This opening is a diaphragm that works like the iris in your eye-opening wider to let more light in and closing down to limit light. The term "aperture" refers to the measured diameter of the diaphragm opening. The incremental adjustment of aperture is described as ƒ-stops.
The last method for adjusting exposure is controlling how long the light-sensitive device is exposed to light. This is done through the shutter, which is typically a pair of fairly thin pieces of light-blocking material. These shutter "curtains" move in rapid succession to uncover and then recover the sensor. You adjust the timing of this via the appropriately named shutter speed.
The first two options require additional gear and a commitment to stop to reconfigure your camera setup. In other words, they're not achieved by quickly rotating a dial or pressing a button on your camera. But the last two methods for changing exposure are just that—they're quick and easy to adjust, and are directly related to your question.
You can set your camera's exposure system so that when you select the aperture, the camera controls the shutter speed. This is called "aperture priority," and on many cameras it's labeled either Av or A. You also can let the camera control aperture while you control the shutter. "Shutter priority" is the name of this mode, and it's usually labeled Tv or S. The small "v" stands for "value," so the labels technically read "Aperture value" and "Time value."