Canon EOS 30D Instruction Manual page 278

An aperture diaphragm is a mechanism that controls the
amount of light entering a camera through the lens.
By
adjusting the opening of the aperture, one can vary the depth
of the zone of in-focus elements. if the aperture is wide open
(fj2.8,
for example), the focused area becomes shallow,
resulting in greater blurring of images in front
of
and behind the
subject in focus. In contrast, if the aperture is small
{f/22,
for
exampleL the focused area becomes deep, resulting in greater
clarity of images in front
of
and behind the subject in focus.
In a lens description - such as "EF-S60mm f/2.8
Macro USM - If/2.8" refers to the widest opening
of the lens aperture. When this "f" number is low, the
lens is considered "bright" or "fast." Bright lenses not
only capture beautifully blurred backgrounds but also
let you choose faster shutter speeds even in dark
places, reducing the risk of undesired blurring
through "camera shake."
Automatic exposure is intended to create optimal photo
exposure. However, you can intentionally change the
exposure level through exposure compensation. When
you want to get brighter just set the exposure
level on the "plus" side; when you want to get darker
results, set the exposure level to the "minus" side.
If you use the AEB (automatic exposure bracketing)
function on your camera, one shot generates three
images with different exposure levels. Afterwards,
you can pick the best result from among these three
choices.
Minus
(-I
compensation Neutral Plus (+) compensation
I 34 I