Depth Of Field

The most important use of aperture to me is depth of field. Depth of field is the range from your camera that will be in focus. If your subject is 5 feet away from the camera and something 7 feet away is out of focus then you have a shallow depth of field. This means that your background (and foreground if there is anything in the foreground) will be blurry, or out of focus.

When you are doing portraits it is great to have the background out of focus. The person is your subject of interest, not the background. The color of the background is helpful, but not when it's in sharp focus.

If the lens I'm using for outdoor portraits has a maximum aperture of 2.8 then I'm going to have my camera set at 2.8. This will give me the smallest depth of field possible for my equipment, and that's great for portraits.

If I'm shooting a landscape then that's a different story. I will want everything in the picture to be in focus. For this I would want a closed down aperture like f16 or f22. This will make it so that things will be in pretty good focus from a few feet in front of you to miles away.

I took this photograph in Sedona, Arizona. My aperture was f16, and my shutter speed was 1/20 of a second. Notice the incredible depth of field I have in this photo. The stones at the edge of the creek are in focus as is the rock formation a few miles away. Notice also the blurring water I got from the slow shutter speed. This wouldn't look as good if I had stopped the motion of the water. (You may notice that the trees are somewhat blurred, but that is due to the wind. I only had one day in Sedona, and I had to take what I was given.)

Your lens focal length also plays a part in depth of field. The focal length is determined by the distance from the image sensor to the focal point of the lens measured in the millimeters. For example, you may have a 50mm prime lens or a 24-70 zoom lens. Lenses below 50mm are considered wide angle while any lens larger than 50mm is considered telephoto. Larger focal lengths make things look closer than they are. Wide angle lenses can make things look farther away than reality.

If you are using a wide angle lens at 24mm you should get a lot of depth of field. This combined with a small aperture will give you great landscape shots.

Consider the following two portraits. Which one do you like better?

I zoomed in tight and used a wide open aperture on the right. Notice how you can still make out details of the background on the left? The background on the right gives us color and texture, but it isn't as distracting. The right portrait is much better, and it draws all of your attention to her face rather than the background.

If you are using a lens at 200mm you will get smaller depth of field. This combined with a wide open aperture will give you shallow depth of field and make for a great portrait!