Depth of field and "f-stop" (aperture)

3.5 & 1/250, 24-70mm @ 70mm.  Window light on the left & a little "bounced flash" from the white ceiling.  (This is an "untouched" photo)

Keep it simple definition: 

Depth of Field =   How in focus something is from foreground to background.

There are three things that affect Depth of Field : Your aperture setting (F-stop,) the focal length of your lens, and the distance you are away from your subject.

1. Aperture: the smaller the number (ie: f/2, f/4) the shallower the depth of field (less in focus from foreground to background). The larger the number (ie: f/16, f/22) the wider the depth of field (more in focus from foreground to background).
FYI: The smaller the number = the larger the hole, the larger the number = the smaller the hole...but don't focus on this fact.  It can be confusing at first.
2. Focal Length: A wide angel lens will produce imagery with more in focus from foreground to background such as a 35mm, 14mm, or lower. A telephoto lens will produce imagery with less in focus from foreground to background such as a 200mm, 400mm, or higher.
3. Distance: The closer you are to your subject the shallower your depth of field will be. The farther away you are from your subject the wider your depth of field will be.  In addition, the farther away your background extends from your subject, the more it will be out of focus.

Why is this important? Most point-and-shoot cameras are set to have the most in focus from foreground to background. It is difficult to manipulate a consumer camera to obtain shallow depth of field. It is a dead give-away that something was shot with a higher end camera when things are out of focus in the background or foreground. Shallow depth of field creates a more beautiful look to your imagery and you will feel more professional. 

3.5 & 1/200, 70-200mm lens at 135mm.  Overcast day in the late afternoon.

Want more to think about? More details....

Consider this: Just because you have a small f-stop, doesn't necessarily mean your background will be out of focus.  If your subject and the background are pretty close to being on the same plane, like if your background is right up next to your subject, and if you are not right next to your subject, then the background won't really be out of focus.
Look at these two photos.  They both use the similar f-stops, but the backgrounds differ in proximity to the subjects... leaving one in focus and the other out of focus.

5.6 & 1/60, 24-70mm lens @ 27mm. No direct sunlight, with a small burst of flash.

5.0 & 1/200, 75-300mm lens @ 85mm. No direct sunlight, overcast day, with a small burst of flash.

This last one was taken at dusk with one flash through a white transparent umbrella.  (The umbrella greatly softened the light...which needed to be pretty strong...since it was getting dark outside and the flash was the key light.) We will go more into lighting in another lesson.

3.2 & 1/160, 70-200 lens at 135mm.