Continuing with a discussion of terms, the ability to adjust shutter speed and aperture are probably the most important controls available on your camera. When using a film or digital camera you must control the amount of light that strikes the film or sensor. Too much and the image is “burned out” or overbright; too little and everything is black. There are many reasons for choosing a shutter speed or aperture yourself rather than letting the camera do it for you.
Aperture is the term for the size of the lens or lens opening. It can be used to describe the maximum opening of a lens, for example your camera may be described as having a f/2.8 lens. In most cases, at least for the same film or sensor size, the larger the diameter of the lens, the “faster” it is, that is the more light gets through it. A f/2.8 lens lets more light through it than a f/5.6, and will likely be more expensive. Actually, the physical diameter of the lens will also depend on its focal length, but let’s stick with aperture and save focal length for another posting.
Although a few inexpensive cameras may have a fixed aperture lens, most add an iris to change the working aperture of the lens the same way the iris in your eye adjusts for changing lighting conditions. Apertures are described by numbers called “f stops.” The term can be a bit confusing in that the smaller the number the larger the aperture and the more light strikes the sensor. Reducing the value by one stop lets 1/2 the amount of light through the lens as the previous number. Typical values include f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128 and so on, however most cameras only include a portion of the range. Note that doubling the number (for example from f 2.8 to f 5.6 diminishes the light by two stops or 1/4, not 1/2, as might be expected.
Click on “Read the rest of this entry” to find out how changing the aperture can improve your images.
Although changing the aperture of the lens is often used to increase or decrease the amount of light striking the sensor, it also can be used to select the portion of the image that is in focus.
The term depth of field describes how much of the image from the front of the lens to infinity will be in focus. A wide depth of field is created by stopping down the lens to a small opening (example – f/16.) Objects near the camera, the subject you focused on and the background will all be in focus.
Conversely, a narrow depth of field can be produced by shooting with the lens wide open (example f/2.8.) If you want to isolate your subject from the background, use a wide aperture and focus on the subject. The foreground & background will be blurred.
Neither a wide or narrow depth of field is “correct” – there are times you want everything in focus. An example might be an image of someone standing in front of a landmark or sign that you want to include in the image. Landscape photographers often use small apertures to keep everything in focus. On the other hand, it is often useful to isolate the subject from the background. If your subject is standing in front of a busy street or trees, they may blend and get lost in the background. By choosing a wide aperture to limit depth of field and careful focusing, your subject stands out. A sample of each is shown below. You can, if you wish, click on the image to link to a 700 pixel wide version located on my web site.
Wide Aperture, Narrow Depth of Field – f/2.8
Small Aperture, Wide Depth of Field – f/22
Another reason to use aperture priority is to force your camera to use a slow or fast shutter speed. Set the aperture to the smallest opening possible (for example, f/16) and your camera’s meter will choose a slow shutter speed – useful to blur water, streak headlights & taillights at night, etc. Choose a large opening (for example f/2.8) and your camera’s meter will choose a fast shutter speed – useful for limiting movement blur, freezing the spray from a wave, etc. Of course you could switch to shutter priority and set the shutter speed, but you may find the amount of light on the scene is outside the range of your lens. I’ll discuss using shutter priority in the next entry.
If your camera is set on full automatic or program mode, you have little control – it chooses the shutter speed & aperture for you. To gain control of depth of field you must set your camera to aperture priority. Not all cameras have this capability – check your manual or look for an “A” in a string of controls that include “P” “A” “S” “M”.