Apple Aperture Digital Photography Fundamentals - Manual (Page 17)

Chapter 1 How Digital Cameras Capture Images 17
Shutter Speed
Shutter speed refers to the amount of time the shutter is open or the digital image
sensor is activated. The exposure of the image is determined by the combination of
shutter speed and the opening of the aperture. Shutter speeds are displayed as
fractions of a second, such as 1/8 or 1/250. Shutter speed increments are similar to
aperture settings, as each incremental setting either halves or doubles the time of the
previous one. For example, 1/60 of a second is half as much exposure time as 1/30 of a
second, but about twice as much as 1/125 of a second.
Photographers often use shutter speeds to convey or freeze motion. A fast-moving
object, such as a car, tends to blur when shot with a slow shutter speed like 1/8. On the
other hand, a fast shutter speed, such as 1/1000, appears to freeze the blades of a
helicopter while it’s flying.
Digital Image Sensor
When the reflective light from the photographed subject passes through the lens and
aperture, the image is captured by the digital image sensor. A digital image sensor is the
computer chip inside the camera that consists of millions of individual elements capable
of capturing light. The light-sensitive elements transform light energy to voltage values
based on the intensity of the light. The voltage values are then converted to digital data
by an analog-to-digital converter (ADC) chip. This process is referred to as analog-to-
digital conversion. The digital numbers corresponding to the voltage values for each
element combine to create the tonal and color values of the image.
Using Reciprocity to Compose Your Image
You can adjust the aperture setting and shutter speed to create several different
correctly exposed images. The relationship between the aperture and shutter is known
as reciprocity. Reciprocity gives the photographer control over the depth of field of the
image, which controls the area of the image that remains in focus. This is the easiest
way to control what part of the image you want the viewer to pay attention to.
For example, opening the lens aperture by one stop and decreasing the shutter
speed by one stop results in the same exposure. Closing the aperture by one stop
and increasing the shutter speed by one stop achieves the same exposure as well.
Therefore, f4 at 1/90 of a second is equal to f5.6 at 1/45 of a second. The reason is that
the camera’s aperture setting and shutter speed combine to create the correct
exposure of an image.