Depth of Field is the amount of your image before and beyond your focus point that will be in focus. Depth of field tells you whether or not your subject and background can be sharply focused at the same time.
Depth of field is determined by several factors:
Aperture/F-Stop
Lens
Subject Distance
While the camera can actually only focus on one tiny point in space, the depth of field determines how much of the image is in an acceptable focus to the human eye.
In subjects such as landscapes, a large depth of field is often desired so that the entire scene appears to be in focus. With subjects such as portraits, a small depth of field is often used to blur the background and reduce distractions from the main subject.
Aperture describes an adjustable opening inside your camera lens that controls the amount of light striking the cameras sensor or film. As the size of the aperture changes, the angle of light striking the film also changes. It is this angle change, much like wearing a pair og glasses that changes the angle of the light, therefore making changes in depth of field.
As i mention before aperture is measured in F-Stops. F-Stop settings represent a ratio derived from the size of the lens opening and focal length.
A simpler way to remember the relationship between F-Stop/Aperture and Depth of Field is:
Large F-Stop = Large Depth of Field = More Light Needed
Small F-Stop = Small Depth of Field = Less Light Needed
This means that larger F-Stops, such as F11or more, will require slower shutter speeds and produce images with a larger depth of field. Smaller F-Stops, such as F4, will allow faster shutter speeds and produce images with a shallower depth of field.
The focal length of a lens also plays a big part in determining the depth of field, of your images. Think of your lens strength as a limiting factor for your aperture capabilities. The higher the magnification factor, the smaller the depth of field will be, even with large F-Stop settings.
Depth of Field Progression for a 70-300mm Lens
70mm = largest DOF
100mm = large DOF
200mm = small DOF
This effect is more pronounced in macro photography where being close to the subject and high focal lengths result in depths of field that are sometimes less than an inch.
Much like lens strength, subject distance, plays a big part in determining the possible size of depth of field. The closer you are to your focal point/subject, the less depth of field is possible. To illustrate this effect, hold your hand at arm's length in front of your face. Even when focusing on your hand you can probably see a good bit of the surrounding environment in reasonably clear focus. Slowly move your hand towards your face until you reach the half-way point. Notice how much less of the area surrounding your hand is now in focus. Continue moving your hand towards your face until it is as close as your eyes can focus on it. Very little of the area surrounding your hand can now be seen.The same effects occur with your camera lenses. This effect, combined with high magnification factors, results in the tiny depth of fields seen in macro photography. It also makes the huge depths of field in many landscapes possible when using a lower magnification factor lens.
