The effect of aperture on depth-of-field

The larger the aperture (smaller f-stop), the smaller the depth-of-field.

Here's a series of bike photos (click on the photo to see larger size):

f/2.8 @ 1/2000 sec

f/22 @ 1/40 sec

Another example with a fence:

f/2.8 @ 1/1600 sec

f/8 @ 1/200 sec

f/16 @ 1/40 sec