This is a neat demonstration of the effect of camera aperture on picture depth of field. All of these photos were taken at the same time with decreasing aperture (or increasing f-stop if you prefer).
f/5.6
f/6.3
f/7.1
f/8
f/9
f/10
f/11
f/13
f/14
f/16
f/18
f/20
f/22
f/25
f/29
f/36
For those who are curious about such things, the pictures were all taken with a Nikon D50 fitted with a Sigma 18-200mm zoom lens. According to the EXIF information in the original images, the focal length when the pictures were taken was 65mm (and I have no reason to doubt it !!).