kevinwilley.com - Topic 2 - Depth of Field

In addition to varying the amount of light that passes through, aperture size also controls the depth of field. Although a camera can only truly focus, at any given time, on a single, discreet distance in front of the lens, objects that are slightly nearer and also slightly more distant than that focusing point may also appear to be in focus. That is because limitations in the human eye can allow us to perceive this area of sharp focus. The entire area of perceived sharp focus is known as the Depth of Field (DOF). Objects within the DOF will appear to be in focus in the picture, while objects nearer or more distant will look blurry.

The Depth of Field, to an extent, can be controlled by your aperture setting. A smaller aperture (larger f-stop number) will give you a greater DOF than a larger aperture (smaller f-stop number). To put some numbers to it, an aperture setting of f/32 will give you a greater DOF than an aperture of f/4, but will require a slower shutter speed due to the fact that less light is allowed to pass through (64 times longer, to be exact, since you are allowing only 1/64 the amount of light to pass through). See Topic 1 if you don't understand why.

The DOF will certainly vary with your choice of lens (wide angle lens & large DOF vs. telephoto lens & greatly diminished DOF), but the choice in lens will depend on the type of shot you're after. (With a telephoto lens and its shallow DOF, your subject will more clearly stand out in the photograph against a very blurred background.) I won't go into lens selection here, but I do want to demonstrate the effects of f-stop setting and DOF. What better way to do that, than with pictures?

The following photographs are completely identical, with the exception of changing aperture settings (and the corresponding shutter speeds). By varying only the aperture setting, you can directly see the impact this had on the Depth of Field in the picture. In all photographs, the camera was focused on the blue push pin in the center. Each push pin is exactly 1/2 inch away from the next push pin. I used a 200mm macro lens (by its design, a very shallow DOF).

Click any image to enlarge.

While the blue push pin is in focus, the red ones at the far ends are practically unrecognizable, and the line drawn on the paper is blurred to the point where you can't even tell that it's there.		f/4.5 1/2000
		f/5.6 1/1250
The red push pins adjacent to the blue one are beginning to look sharper.		f/8 1/640
The line drawn on the paper is still very blurred at the farthest red pin, but at least you can now tell that it's there.		f/11 1/320
More of the red push pins appear to be coming into focus.		f/16 1/125
		f/22 1/60
While not all red pins appear sharp, it is certainly the clearest of the pictures, and a vast improvement over the first image.		f/32 1/30