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Digital Camera Basics
If you've ever wondered why some photos have a sharp subject but everything else in the foreground and background are out of focus, but other photos are sharp from near to far, it's all about depth of field (DOF).
|DOF is the distance in front of and behind the subject that is appears in focus. Focus falls off in front of and behind the subject.
The good news is that you can control the depth of field. If you've just unwrapped your first digital SLR, that control can be right at your fingertips. Let me show you how.
Shallow depth of field is useful when you want a distinct subject to stand out from everything else. Photojournalists, on the other hand, may prefer a deep depth of field to take in the environment surrounding a subject, or to depict simultaneous action near and far in the same photo.
Three pillars of DOF
Depth of field is controlled by three things: the focal length of the lens being used, the distance between lens and subject, and the aperture being used. Let's look at each.
Lens length: The longer the lens, the narrower the depth of field, all other things being equal. Conversely, the shorter the lens, the deeper the focus.
Exercise: Take a zoom lens, zoom to the longest tele setting, and focus on something at a medium distance. You'll see the background go out of focus. Now lock focus and zoom out slowly. Take note of what happens to the blurriness as you zoom out.
Wide-angle inclusiveness: I wanted to show all the detail in this shot in the Florida Everglades, so I used an 18mm wide-angle lens, and everything in this scenic is sharp. The added benefit? It took a lot of detail in.
Focus distance: The closer you are to your subject, the narrower the depth of field, again all other things being equal. The farther away you are, the deeper the focus depth. For example, if you are photographing a flower up close, objects a few inches away from the focus plane will appear blurred, but if you're photographing a scenic, nearly everything will appear sharp.
Real world test: If you have a lens that focuses down to 1:4, 1:3, etc., you will see this effect work as you look through the viewfinder. Try it now!
Aperture: The wider the aperture (or "f-stop"), the narrower the depth of field. The smaller the aperture, the deeper the focus depth. Many photographers use this simple formula to manipulate how much of the scene they're photographing will be in focus, and will also use aperture to affect how out of focus the background will appear.
A caveat: There is a price to pay for shooting small apertures, which is that your shutter speed must slow down, and/or you need to boost your digital camera's ISO setting. Both can adversely effect image quality, so use image stabilization if you've got it, or bring a tripod. (Same can be said for shooting macro or with telephoto lenses. That's why tripods are such essential tools for serious photographers.)
Tip: If you're a DSLR newbie, be sure to set your camera on its Aperture Priority setting so you can adjust the lens's aperture. Directions, different for each camera, are in the manual.
No change in distance--but look at how focal length changes focus!
In the following three photos, I focused on the same plane of focus without moving or changing the aperture; all I did was zoom. The two clothespins are approximately equal distance to where I was standing. But look at what happens to the background!
This first shot was made at the 50mm setting. Note how distracting the background is. Not a very useable composition.
In this shot I've zoomed in to 100mm--doubling the focal length. Look at the big difference it makes in separating the subject from the background. You can even see the spider web!
One of the reasons sports photographers love long telephoto lenses is that, in addition to bringing you right into the action, these lenses have naturally narrow depth of field, which helps sports shooters isolate their subjects for compelling shots.
But as you can see here, even a more modest focal length range can still give you a lot of control--especially now that you know how the three elements of DOF control work together