By James Booth
What is depth of field? I often wondered this myself. Early in my photo "career," I was often told that I needed to work on my depth of field. I didn't even know what it was. I was learning all this photography stuff pretty much on my own. Finally, I figured out what the term means. Put simply, depth of field is the range of sharpness from the nearest object to the farthest.
"Depth of field is the range of sharpness from the nearest object to the farthest."
A closer look
So how do we control depth of field? The aperture, the iris inside the lens, controls depth of field. Just like the iris in our eyes, the aperture controls depth of field by either increasing or decreasing the amount of light allowed into the lens. Each stop on the aperture decreases the amount of light allowed into the lens by half, known as stopping down, or increases it by double. So that from f/5.6 to f/8, the light is cut in half.
With the aperture wide open, the maximum amount of light is allowed into the lens. In most cases this will result in a flat, washed-out image, depending on the lighting circumstances of course. Figure A shows an image with the aperture wide open. Notice the central object is the only thing in focus.FIGURE A
A wide-open aperture yields no depth of field. (click for larger image)
In Figure B, the aperture is set at the halfway mark. Depth of field is increased a bit, sharpening the rear object slightly, but the foreground object is still blurred.FIGURE B
Stopping down the lens increases the depth of field. (click for larger image)
If the aperture is stopped all the way down, letting in a minimum of light, you will in most cases achieve maximum depth of field, but the required shutter speed will be so slow that any movement or jarring of the camera or subject will result in blurring.
This is definitely a consideration with an SLR camera, particularly the larger ones that use medium format. The aperture is stopped down all the way in Figure C, giving a sharp image at all ranges, but I'm amazed something didn't blur just from the mirror in the camera snapping up as the picture was taken.FIGURE C
Minimum aperture yields maximum depth of field. (click for larger image)
Getting it right
The goal is to find the happy medium where you will get the desired depth of field with the shutter speed you need or want to use. Figure D was exposed at the setting recommended by the flash meter, which in this case happened to be one stop from wide-open, giving optimal exposure.