cameras, you rarely have to worry about setting shutter speed and aperture size at all for typical photography.
How Cameras Choose Aperture and Shutter Speed
Here’s what usually happens: when you apply pressure to the camera’s shutter release, the
camera’s microcomputer samples the scene in front of the lens and determines how much light is needed to adequately expose the scene. With most digicams, the camera selects a shutter speed
and aperture combination that is sufficient to get the job done. But, you might be wondering,
how does it choose? After all, there are a lot of shutter speed/aperture pairs that will work. To
take the same properly exposed picture at ISO 100, any of these combinations should produce
exactly the same result:
Usually, the camera uses the following logic:
The photographer wants to take a picture using the fastest available shutter speed to
minimize camera shake and motion blur from objects moving inside the picture.
Though there are exceptions, most cameras tend to choose the combination that allows for
the highest available shutter speed, limited only by how small they can make the aperture given
the current lighting conditions and ISO setting.
This isn’t always what you want your camera to do, though, and in fact you might sometimes
want to choose a slower shutter speed, overexpose the image, underexpose it, or perhaps base the exposure on a completely different part of the picture. That’s why you might want to investigate your camera and look for controls that let you tweak the shutter speed and aperture.
DID YOU KNOW:
The Truth About Shutters
Though I talk quite a lot about shutter speed in this book, the reality is some digital cameras
don’t have a real mechanical shutter in the same sense that 35mm cameras have shutters.
35mm cameras usually have a physical barrier that blocks light from entering the chamber
where the film is stored. This mechanism—the shutter blade—moves lightning fast, able to
deliver shutter speeds as fast as 1/8000 of a second. That’s fast.
Some digital cameras, in comparison, don’t have real, physical shutters. You can test
your own camera yourself with a simple experiment: when you press the shutter release
on a 35mm camera, you can hear the quick, metallic click of the shutter blade opening and
closing. Try that with your camera and listen carefully—it might not make any noise at all.
Or it might make an obviously fake “click” sound through the camera’s speaker. When I
got my first digital camera many years ago, I actually had to look at the LCD display on the
back of the camera to see if the picture was captured or if, for some mysterious reason, the
camera was still waiting to grab the shot.
So if there’s no shutter blade, how is the picture actually taken? If there’s no mechanical
shutter, the CCD is simply turned on long enough to expose the picture. Since the CCD is
an electronic component that acts as the camera’s film, it can be controlled electronically for
whatever exposure time is needed. In addition, the camera’s aperture may close completely
to keep light from reaching the CCD—prolonged exposure to sunlight can damage this
sensitive part of the camera; but the aperture needn’t spring open and closed as quickly as
the shutter blade in a 35mm camera, so the sound it makes isn’t as dramatic. You’ll hardly
notice it at all.
Dave Johnson (Fifth edition - Mc Graw Hill)