uLearn.Photography - part of FilmPhotoAcademy.com
The best way of really understanding depth of field is to take photographs to explore all the variables.
The easiest way of doing this is to cajole a friend into standing for you as photo subject.   Put them in front of a brick wall or hedge or trees.   Put them about 3 metres (10 feet) away from the background.
Get record shots of what happens when
1) You move different distances from your subject
2) You keep a close distance but change the aperture from small to large
3) You zoom in and out keeping the same distance
4) You zoom in and out keeping the same crop, but changing your distance
Go over the photographs and compare the backgrounds in each shot.