Depth of Field (DOF)
You will hear this term tossed around by photographers who are discussing their images. Simply put, depth of field is the zone of focus, from foreground to background, that is sharp. This zone of focus increases if you close down the lens to a larger aperture
number (toward f22), and the DOF zone will become shallower the wider open you have the lens (f4, f2.8, or a smaller number). The focal length of the lens you are shooting with adds other dimensions as well. A wide-angle lens, 35mm or wider, will have a greater DOF than a longer lens.
As a rule, the wider the angle of your lens, the more DOF it offers. Conversely, a longer lens, from 50mm, to 85mm, to 120mm and so on, will give you shallower zones of focus
or DOF. And this DOF becomes shallower and shallower with longer and longer lenses. A long lens with a shallow DOF is very often used for creative effect, and you can press a button, usually located on the front of the camera or lens, to close down the lens to
the predetermined aperture so that you can preview the DOF through the viewfinder.
How Aperture and Shutter Speed Affect Exposure
Your camera lens has internal blades that open or close, allowing light in or decreasing the amount of light. These f-stops are represented by the numbers f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, and so on. Moving from one f-stop to the next doubles or halves the amount
of light striking the chip. These numbers represent the ratio of the aperture’s opening created by the blades to the focal length of the lens. The f-stops work interactively with the shutter speed to control exposure. In other words, the two factors that affect exposure
are shutter speed and aperture. These are interrelated; to maintain the same exposure, the shutter speed is moved up accordingly, and the aperture is changed in the other direction.
If the exposure is 1/125 at f4, and you want to increase the shutter speed to stop the action
more effectively, then you could go to 1/250 at f2.8, or 1/500 at f2.
If you wanted to change the aperture to get greater or less DOF, and your exposure was 1/125 at f4, you could move the aperture to f5.6, and the shutter speed would then drop to 1/60, to maintain the same exposure.
Shutter Speed
I used to shoot NFL football, and my rule of thumb was to use a shutter speed of 1/500 of a second to stop the action. If I went to 1/250, a bit of blur would start to appear in moving feet and arms. Remember that the longer the shutter stays open, the more movement will be recorded.
Shutter speed can also be used for 55
creative effect, using a long exposure—say, 1/2 second—to capture the effect of a lot of movement in the frame, such as people
moving on the street. Another example would be opening the shutter in a long exposure to capture the rotation of the Earth shown by the stars streaking across the frame in a two-hour exposure.