Canon EOS 450D - Manual (Page 72)

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To obtain a blurry background or to make everything near and far look
sharp, set the Mode Dial to (Aperture-priority AE) to adjust the
depth of field (range of acceptable focus).
* stands for Aperture value which is the size of the diaphragm hole inside
the lens.
1
Set the Mode Dial to <f>.
2
Set the desired aperture.
The higher the aperture, the sharper
the picture will look with a wider depth
of field.
Turning the <6> dial to the right will
set a higher f-number, and turning it
to the left will set a lower f-number.
3
Take the picture.
Focus and press the shutter button
completely. The picture will be taken
with the selected aperture.
f : Changing the Depth of Field
Blurred background Sharp foreground and background
About the Aperture Display
The larger the f/number, the smaller the aperture opening will be. The
apertures displayed will differ depending on the lens. If no lens is attached to
the camera, “00” will be displayed for the aperture.