Depth of Field
When taking a picture, you focus on a specific distance. In theory only points that are this distance from your camera are perfectly in focus. In practice however we find that points some distance closer and some distance farther away are also sharp. They aren't exactly in focus, but they are "close enough" for us not to notice. The range in front and behind the focus distance where focus is "close enough" is called the depth of field.
In the image above, the bar indicates how good the focus is at this distance, with white being perfect focus and black being completely out of focus (not that there are such clear limits). For the purpose of these examples I've decided that 20% gray and brighter is "good enough". This gives a depth of field as indicated by the dashed lines. As you can see the focus is on the face of the middle man, about half of the front man is in acceptable focus and the back man is out of focus. the following two images show respectively a shallower and a deeper depth of field:
In the first image only the middle man is now fully in focus, the others are affected by a mild-to-medium blur. In the second image, all three men are in focus. Note that the depth of field isn't symetrical: it extends further backwards than forwards.
The importance of depth of field
Depth of field is a very important concept in photography for two primary reasons. First of all off course there's the fact that if your subjects aren't all or fully within the depth of field, they will (partly) be rendered out of focus. Often an unfocused picture is simply due to too narrow a depth of field. This is especially important in macro or lowlight photography, as we'll see later on. Secondly, you can use Depth of Field (which from ere on I'll be abbreviating DoF) for artistic reasons. The best known example of this is in portrait photography where you want to make the subject stand out against its background by rendering this background out of focus (and thus blurred). The opposite could happen too: you might want to take a picture and have both the foreground as the background in focus.
How to affect depth of field
So how does one increase or decrease the depth of field? Well, DoF depends on three parameters
- The focussing distance (= subject distance)
- The lens focal length
- The aperture used
Focus distance
The greater the distance between the camera and the subject, the greater the depth of field. At f/4.0 and 50mm and a focus distance of 1 meter the DoF will extend from 0.98m to 1.02m for a total range of 4 cm. With the same settings but at a distance of 10 meters, the DoF will go from 8.1 to 13.1 meters for a total of 5 meters. At 100m, it goes from 29.4 meters to infinity. As you can see, the increase is not linear with distance: we multiplied the distance by 10 but the DoF range went up with a factor 125, we then multiplied the distance with 10 again and the DoF range became infinite.
This is where the macro photography gets involved: at the small distances used in that kind of photography, the DoF is extremely narrow. In the example above it was merely 4cm at a distance of 1 meter. Reduce that distance to half a meter and you have only 2 cm to play with. Often with macro photography, DoF is counted in millimeters. It becomes very important then to use the other techniques (in practice this is mostly the aperture) to increase the DoF.
Lens focal length
The smaller the focal length, the deeper the depth of field. Returning to our example above, at a distance of 10 meters, aperture f/4.0 and a focal length of 50mm, the depth of field was 5 meters. Using a 12mm lens at the same settings, the DoF becomes 231 meters, a huge increase. At 200mm however the DoF is merely 24 centimeters. Again, the relationship is not linear.
This becomes very important in situations where very long tele lenses are used, like wildlife or sports photography. 24cm of DoF at 10 meters is not enough to get an entire human in focus, let alone an elephant and 200mm isn't that incredibly huge. Toss on a teleconvertor to push that lens to 400mm and the DoF is just about 8 centimeters. Of course, generally with these long lenses you'll be at a greater distance to your subject and thus because of the previous point the depth of field is increased again. At a subject distance of 100m that same 400mm lens at f/4.0 gives a healthy DoF of 8 meters.
Conversely however with wide angle lenses you generally don't have to worry about DoF at all, since nearly everything will be in focus anyway (which is not always what you want, of course).
Aperture
The previous two ways of changing DoF were not all that practical. In practice you generally will have other constraints dictating the focus distance and/or focal length to a greater or lesser degree. We already mentioned macro photography and wildlife shooting, but there's many other cases. Portraiture, for example, is often preferred at a focal length of about 42mm. Since you want to frame just the head and a bit of the shoulders in the portrait, this limits both the focal length as the distance.
This leaves us with just one parameter to change: the aperture. The smaller the aperture, the greater the depth of field. Remember that aperture is a fraction, so f/6.3 is smaller than f/4.0. Retrieving once again our example, we had 5 meters of DoF with a 50mm lens at a distance of 10 meters and an aperture of f/4.0. Enlarge the aperture to f/2.0 and the DoF drops to 2.5 meters. Reduce the aperture to f/8.0 and the DoF grows to a bit over 12 meters.
Thus:
- If you want shallow DoF: use a large aperture (f/2.0, f/2.8, ...)
- If you want deep DoF: use a small aperture (f/11, f/16, ...)
All in all, changing the aperture is your most direct way of affecting DoF as both of the other methods change the image in other ways.
Of course aperture also controls how much light hits the sensor. For every stop you shrink the aperture, the amount of light available to the sensor halves and the exposure time or ISO will need to double. This means that aperture itself is limited by the amount of available light. For macro photography, where apertures of f/11 to f/22 are often needed to ensure sufficient DoF, lighting the subjects becomes a tricky endeavor. Conversely, in a lowlight situation when you need a large aperture to take pictures with sufficient speed, you will have to cope with a shallow DoF.
Hyperfocal distance
In the focus distance section above we encountered infinity: if you increase DoF enough then you will eventually extend it all the way to infinity. The closest focussing distance where this happens is called the hyperfocal distance which depends on the aperture and focal length only. If you have your camera focussed on the hyperfocal distance then everything starting from half this distance to infinity will be sharp. For example for a 50mm lens at f/4.0 the hyperfocal distance is 41.7 meters so when that combination is focused at 41.7m, the depth of field will extend from 20.85m till infinity. Knowledge of hyperfocal distance is important to street photographers who often do not have a chance to focus for each scene individually so leave their camera focussed on the hyperfocal to ensure the largest DoF possible.
How to calculate it all
I've tossed out some examples so far, but you may be interested in finding out the depth of field for your own lenses. To this end I've made a javascript based depth of field calculator. Before I give the link however I need to stress that to some extend DoF depends on "acceptable sharpness", which is a subjective matter which can vary from person to person and which also depends on the size and distance the final image will be viewed in. For this reason my calculator has several scenarios: the default is for a 40x30cm print viewed from about 20 centimeters, the others are a less strict one for web viewing and a more strict one for large prints.
