Depth of field (DOF) refers to the amount of a scene in the “sharp” range. Shallow DOF is typically characterized by heavily blurred backgrounds that you might see in outdoor portraits. Deep focus (opposite of shallow DOF) is typically characterized by tack sharp landscapes with no visible blur.
The most widely accepted method for controlling DOF is aperture, or f-number. This is certainly a feasible and convenient way to control DOF, but there are other factors at play. Just like exposure is controlled by three factors (ISO, shutter speed, and aperture), DOF is controlled by three main factors. Let's take a look at these three factors and how you can use them to your advantage.
The examples shown below were taken on a 1.5x crop factor dSLR and the stated focal lengths are actual focal lengths of the lens rather than a full-frame equivalent.
The f-number is probably the most widely known and used method of controlling DOF. Most intermediate/advanced cameras have “aperture priority” which allows you you set the f-number. If you've toyed with this mode on your camera, you probably found that lower numbers result in a narrow depth of field (blurry background), while higher numbers result in a wide depth of field (everything in focus).
F-NUMBER ⇓ == DOF ⇓
F-NUMBER ⇑ == DOF ⇑
TRY THIS: With a “normal lens” (40-80mm range), find a subject about 5-10 feet away from you and make sure there's some background object(s) in view behind it. Use your aperture priority and set the lowest f-number you can, and take a shot focused on the main subject. Now stay in the same spot and use the same focal length, but set the highest f-number you can (without bringing your shutter speed too low), and take another shot focused on the main subject. When you compare the two, the main subject should be in focus for both, but you'll see a difference in the background blur or the amount of focus on objects in the near distance.
Another way to control depth of field is to change your distance from the subject in focus. If you've ever shot macro, you know that the DOF is extremely narrow for 1:1 magnification. This is because you're so close the subject. On the other hand, if you've shot landscapes you'll know that it doesn't take much stopping down of the aperture to get everything in the distance nice and sharp. This is because you're so far from the subject.
DISTANCE ⇓ == DOF ⇓
DISTANCE ⇑ == DOF ⇑
TRY THIS: With a “normal lens” (40-80mm range), set your aperture to a value around f/4 or f/8. Again, find a subject that has some background element in view. Now get as close as your autofocus will allow you and take a shot. Keep the same focal length and the same f-number, but back up about 5-10 feet. Focus on the subject again and take a second shot. When you compare the two, you should see a difference in the depth of field by the amount of background blur.
The last factor in your control for DOF is the focal length of the lens you decide to use. Telephoto lenses have a shallow depth of field as compared to their wide angle counterparts. Anybody out there have a sub-20mm lens? It's pretty hard to get background blur, right? Any super-telephoto shooters out there? Just the opposite.
FOCAL LENGTH ⇓ == DOF ⇑
FOCAL LENGTH ⇑ == DOF ⇓
TRY THIS: Use a zoom lens that reaches from wide angle to telephoto (something like an 18-200, 28-135, etc.) or use two lenses (wide angle and telephoto). Again, find a subject that has some background element in view. Position yourself approximately 5-10 feet from the subject and set your aperture in the low-mid range (f/4-8, but make sure to find something that can be used for both lenses). Take the first shot with the wide angle lens or at the shorter focal length of the zoom lens. Now, hold your position and your f-number, and switch to the telephoto or use the longer focal length of the zoom lens and take the same shot with focus on the same subject. You should see a wider depth of field with the shorter focal length.
PUTTING IT INTO PERSPECTIVE
All this technical stuff is fine and dandy, but how does it translate to real world photography? The answer depends on what you're shooting with and what you're shooting at.
If you have a compact camera with no manual controls and you want a shallow DOF (say, for portraits)… zoom in all the way, get as close to your subject as possible (still preserving a decent composition), and take the shot. Also, less light will force the camera to use a smaller f-number and decrease the DOF. If you want a wide DOF (say, for landscapes)… zoom out all the way, get far away from your subject, and take the shot. Also, more light will force the camera to use a higher f-number and increase the DOF.
On the other hand, if you have a dSLR with manual controls and you want a shallow DOF… use aperture priority, set your f-number low (f/2.8-), get close to your subject, and/or use longer lenses. If you want a wide DOF… set your f-number high (f/16+), step back from your subject, and/or use wide lenses.
If you want to do some theoretical calculations on this topic, check out this handy Depth of Field Calculator. You just choose your camera, focal length, f-number, and subject distance. The calculator outputs your DOF, hyperfocal distance, and circle of confusion.
Links from around the web:
ANY OTHER TIPS?
How do you prefer to control your DOF? Any SLR shooters out there have a set of numbers that work well for narrow and wide DOF? How about some good examples of DOF in either extreme? We'd love to see 'em!
Also — any questions on this stuff? I might be jumping over a few concepts, so let me know if anything doesn't make sense.