A Lesson in Depth of Field

This a purely amateur post on the subject.

Depth of field means how much of the picture (foreground through background) is on focus. (what does "in focus mean"? It when the rays of light from an object converge on one point, the "focal point".) This is determined by the aperture setting on the lens. It doesn't make sense (at first), but looking though a big hole (large aperture, small f number on the lens) means a short depth of field. Only things within an inch or so (in the case of my lens) of the focal point will be in focus. Looking through a small hole (small aperture, big f number on the lens) means a long depth of field. Things many feet away from the focal point will be in focus. This is because there is an inverse relationship between the size of the aperture (D for diameter) and focal length (f). The f number is written as f/D.

The following two pictures are taken with the same settings except f stop. The first picture was at f/5.6. Only a few raindrops are really in focus, everything else is slightly out of focus. More importantly, look at the background between the two pictures.

This one was at f/18. Notice how the whole flower is in focus, and the change in the background.

The cartoon below is showing the pathway of light (pink) from an object outside somewhere to the film (or sensor in a digital camera). The aperture is allowing certain amounts of light through, but in this simplistic explaination also bending the light by certain amounts. The larger aperture (small f number) bends the light a lot to get the object in focus on the film (a known focal length and doesn't change). Because it bends the light so much (the sharp X shape it makes at the point where the film is) just a small change in your position when taking the picture will bring the object in and out of focus.
But when the aperture is small (large f number) the light is only bent a little (shallow X shape at the point where the film is). Now the range of focal lengths is big, so you can move around a lot and your object is still in focus. Also meaning things in front of and behind your main object will be in focus.