The Program Mode


A correct exposure is determined by two factors, namely a shutter speed and an aperture value. However, there are many different combinations of shutter speed and aperture to achieve the same desired exposure. To help overcome this complexity, the program mode automatically selects proper shutter speed and aperture. The default exposure mode is the program mode. Of course, if the selected shutter speed or aperture is not suitable, you can always overwrite the selection made by the program mode. In this case, you either choose an aperture and let the camera select a shutter speed (the aperture-priority mode), or choose a shutter speed and let the camera select an aperture (the shutter-priority mode). Sometimes, if you are not satisfied with the selections made by the camera (in either the program, aperture-priority or shutter-priority mode), you can increase or decrease exposure values to match your need. This is exposure compensation.

Note that the aperture value and shutter speed are shown on the LCD monitor and the Control Panel.

A Little Bit More About Exposure

It would be very helpful if you know a little bit more about exposure. An aperture is simply an opening in the lens for light to pass through and form an image on the image capture device (e.g., CCD in our case). The shutter is another device in front of the image capture device. When the shutter is tripped/released, the shutter will open for a while, allowing the light to pass through. Note that the aperture always opens wide allowing maximum light to pass through for viewing purpose. When you trip/release the shutter, there are a few things going on in the following order:
  1. the exposure meter determines the required amount of light for a correct exposure;
  2. close the aperture to the desired setting;
  3. open the shutter so that light can pass through and form an image on the image capture device;
  4. close the shutter after the specified amount of time elapsed; and
  5. return the aperture to its maximum setting.
Based on this description, we know that to maintain the same amount of light that can strike on the image capture device, we could use different combinations of aperture and shutter speed. For example, we could make the aperture to open wider and at the same time reduce the shutter speed. This is shown with the diagram below. More precisely, to maintain a constant amount of light, we could use a combination of wider aperture and faster shutter speed, or a narrower aperture and slower shutter speed. This constant amount of light is measured in an unit of EV (i.e., exposure value).

Since different amount of light associates with a different EV value, the exposure meter of a camera determines the EV value to be used and chooses an aperture and shutter speed. Given an EV value, the choice of aperture and shutter speed can vary as shown in the above diagram. Depending on the focusing mode (e.g., Infinity, close-up, and so on) the exposure system chooses a proper setting. This is what the Program Mode means.