Depth of field in practice.

I thought I would add a little to the formal exercise on depth of field by showing a couple of examples from my past photographs where I demonstrate the use of depth of field in practice.  These shots were taken whilst walking the streets of Chicago.  The first is a candid shot of a girl in a crowd of pedestrians walking up the magnificent mile.  I took a number of these type of shots and, wanting to ensure that the subject was always the point of interest for the photograph, I kept the aperture as wide as the lens would allow.  Of course most long telephotos don't open up much more than f4 and with the 310mm focal length that I used on this photo the maximum I could obtain was f5.6.  However, since a long lens limits the depth of field anyway1 f5.6 was sufficient for my task.  Even had I wanted to have a larger aperture I wouldn't have set it as I wanted to make sure that all of the subject was in focus and it wasn't easy shooting and keeping the focus moving perfectly as people walked towards me.  However, I achieved my desired shot with the focus and depth of field correctly set.

Girl in a crowd, 1/250, f5.6, 310mm, ISO400.

As the next photo needed a wide depth of field to cover foreground to infinity I chose a wide angle lens and an aperture which was a reasonable compromise from the available light and the necessary aperture setting.  An aperture of f11 with a focal length of 17mm gave me all that I needed to keep the entire picture sharp.

Chicago in water, 1/125, f11, 17mm, ISO400. 

1. The depth of field change that occurs when moving from a wide angle to a long telephoto lens is due to the reduction of viewing angle that a long lens produces.  If the same subject is photographed with differing lenses and is kept the same size by moving closer as the focal length reduces, then the depth of field remains constant.  This concept is now fairly well accepted, certainly by me.