Yes, on the surface we do understand what depth of field is. We are even happy when we achieve that blurred effect we were so longing for. The trick is to be consistent with it and understand why we have that consistency. Here is a little exercise to help you understand how depth of field works and how to achieve more consistent results.
In a recent workshop I explained the definition of depth of field, explained focal planes and hammered the need to play with lenses to find their strengths and limitations. We then went through a series of exercises to illustrate how the participants could better understand depth of field, focal planes and understand their lens' capabilities.
First we began indoors with a simple depth of field scale I had printed out for each of the attendees. I had all the members attach their close up or macro lens to their cameras. They then placed the scales on a flat surface and had them take an image of the scale from a 45° angle using a wide open aperture setting. They then repeated the procedure with a narrower aperture setting. This allows them to compare the difference in depth of field at a close rage to their subject.
This simple scale serves a dual purpose as it not only allows you to play with depth of field but to also help calibrate your lens' focal plane (for cameras that allow adjustment). By checking the sharpness of focus in front of and behind the focal plane it helps determine if a camera has front or rear focus. While not the most technical method of calibration, it does introduce the beginner to the process.
I then placed three targets labeled 'A', 'B' and 'C' on three light stands and placed them at various distances. I had each photographer do a series of three exposures at various aperture settings so they could physically see the results of their settings. By varying the distances of the targets to each other and to the camera, the photographer is able to determine the capabilities and limitations of each of their lenses.
This exercise can be recreated by you at home for your own experimentation. Download and print the two files listed below. Use heavy paper or card stock for durability. Mount the lettered targets on cardboard for stiffness. Instructions on their use are included with the files.
For the depth of field focusing targets, I used light stands as they are height adjustable, sturdy and light weight. Offset each from each other so you are able to see all three targets when you photograph them. Shoot in series of three shots beginning with 'A'. Vary the distances of the targets to each other and to the camera. Keep a notebook handy for recording target distances, lens focal lengths, settings and results. This will help in understanding each of your lens' abilities and limitations. It will also help in setting up your scene when working with your subjects or clients.
Download the Depth of Field Scale and the Depth of Field Focus Targets from the 'files' section of the meetup.com site.