Downtown Lens: Depth of Field

DOWNTOWN LOS ANGELES — In photography just as in life, it's important to pick up the signal above the noise. To do this in photography you have to find a way to isolate your subject from the background. In this edition of Downtown Lens, I'll teach you the basics of depth of field and how to manipulate it in camera by adjusting your aperture.

The human eye instinctively latches on to the sharpest part of a blurry photograph. By controlling the plane of focus you can direct your viewer's gaze.

To effectively control depth of field you need an SLR. Compact point-and-shoot cameras have sensors that are too small to give you shallow depth of field. This is also why video looks different than film. Modern video cameras have very small sensors compared to film camera, which use a 35mm negative. This will all change in a month or two when the Canon 5D Mk II comes out, but that's a subject for a different post.

Once you have an SLR you will need the "fastest" lens you can afford. When photographers talk about fast lenses, they don't mean shutter speed. They mean how wide an aperture it has. Generally lenses are considered fast if the are f/2.8 or larger. I have three f/2.8 and an f/1.2. The f/1.2 is a giant hunk of glass, focuses slowly, and is not the most pleasant lens to carry around. One of my favorite all-purpose lenses is the 24-70 f/2.8 L.

Depending on your camera system, you may also be able to find cheap prime lenses that are fast. Primes have a fixed length, which means they don't zoom. A fast 50mm prime used to be the standard lens that would come with old film cameras.

So now that you have your fast lens and an SLR the rest is (relatively) easy. Start by putting your camera on aperture priority mode. This will allow you to set your aperture and the meter will set your shutter speed to capture what it thinks is a properly exposed shot.

You should set your aperture to its biggest setting. Remember that aperture is measured as the bottom numbers of a fraction. f/4 means that 1/4th of the incoming light will hit the sensor. The smaller the number the larger the aperture. The larger the aperture the narrower the depth of field. So if your lens is a f/2.8 set the aperture at 2.8.

When you're dealing with a shallow depth of field you need to be sure your subject is in focus. So set your camera's focus point to just one point in its matrix instead of the normal 9 point mode. Then put that focus point on your subject. Take the photo. Look and see what happened.

Give this technique a try and post your results using our "Readers' Photos" feature. Have fun and happy shooting!

This post is the ninth part in a weekly series entitled Downtown Lens in which I will discuss a photograph and the technique that relates to it.