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:
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
- the exposure meter determines the required amount of light for a
- close the aperture to the desired setting;
- open the shutter so that light can pass through and form an image
on the image capture device;
- close the shutter after the specified amount of time elapsed; and
- return the aperture to its maximum setting.
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.,
and so on) the exposure system chooses a proper setting. This is what the
Program Mode means.