The three photographs below indicate the effect of lens aperture on depth of field. They were all taken with a Canon EOS 1DS Mark II. This is a full-frame camera; note that the effect is weaker with a smaller sensor. This is how it works: The wider aperture (f2.8 rather than f16) gives a shallower depth, isolating a subject from its background.

As the aperture gets smaller (that is, the number is higher), depth of field increases, making foreground and background sharper. Incidentally, the values are calculated - in simple terms, by (focal length/diameter of glass). The series, from f2 downwards, is 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45. Notice that each number is doubled alternately, so these alternate values give 1/4 the exposure. The intermediate figures (square root of 2) just halve the exposure. 

Obviously, you don't get something for nothing. If you maximise depth of field the shutter will have to be open longer, so you have more chance of subject movement or camera shake. You may also want a shallow depth of field, maybe to isolate the subject from the background. Note that depth of field is not the same as depth of focus, which is the term sometimes used. The latter is to do with the distance from lens to film or sensor, not lens to subject, and is in almost all cases fixed during manufacture.

The pictures - Tamron lens set to 75mm

1) f2.8 - 23K file size

 

2) f8 - 29K file size

3) f32 - 32K file size. Notice particularly the increased detail in the brickwork in the background. To confirm it, the photo file size also increases.

Practical Use

Here a wide aperture has isolated the boys from the background.