Canon Rebel T2i Quick Start Manual page 41

Aperture and Focus
• Aperture also affects how much of the image is in focus, this is called
depth of field.
• The larger the lens opening (the smaller number the aperture) the
narrower the depth of field or the less that is in focus.
• Focal length also affects depth of field. Wide angle lenses show a
greater area in focus. Telephoto lenses will create a narrower depth of
field.
• To achieve a very narrow depth of field with the lens supplied with
this camera, you should have an aperture opening of f 5.6 and a focal
length of 55mm.