Aperture is a measure of the opening of the iris, inside a lens, when a picture is taken. When the shutter release button is pressed on the camera a hole opens up that allows the cameras film or image sensor to catch a glimpse of the scene you’re wanting to capture. The aperture of a photographic lens will be adjusted to control the amount of light reaching the film or image sensor. In combination with variation of shutter speed, the aperture size will regulate the sensor/film’s degree of exposure to light. Typically, a fast shutter speed will require a larger aperture to ensure sufficient light exposure, and a slow shutter speed will require a smaller aperture to avoid excessive exposure.
The aperture is measured in f-stops; moving from one f-stop to the next one, f5.6 to f4, for example, doubles the amount of opening in the lens (and the amount of light getting through). Vice versa, if the f-stop is changed from f2.8 to f4, this halves the amount of opening in the lens. The f-stop number is calculated by dividing lens focal length by the diameter of the aperture opening, so a 100mm lens, with an aperture diameter of 50mm would give an aperture of f2.
Picture of lens…
Below are images showing the variation in aperture sizes in a Pentax SMC 50mm f1.7 lens:
Pictures taken at the various aperture sizes..
As can be seen in the above pictures of the books in the bookcase, the size of the aperture plays a significant role in the depth of field/focus. With the aperture set to it’s smallest number (f1.7) this creates the biggest hole which means the depth of field is the smallest. When the aperture set to it’s maximum number (f22) this creates the smallest hole however it gives the greatest depth of field over a large distance. Lenses are designed for maximum light-gathering capability. A lot of expense goes into making the lens fast or good at gathering light, so there are important reasons for cutting down the amount of light that enters the lens. One reason to stop down a lens is that the world might simply be too bright. If a high-speed film/ISO setting is being used and a slow shutter speed that must expose the film for at least 1/500th of a second, using a smaller aperture is the only way to prevent too much light from striking the sensor/film and overexposing it. Another reason is for aesthetic control of sharpness. Suppose the lens has a maximum aperture of f/2, the depth of field will be shallow. Only the object on which you focussed will be sharp. Things closer or farther from the camera will be out of focus. The range of distances for which objects are acceptably sharp is called the “depth of field”; what is acceptable in an 8×10 print viewed from across the room may not be acceptable in the same print viewed at arm’s length. What is acceptable in an 8×10 print viewed at arm’s length may not be acceptable in a 30×40 print viewed at arm’s length.
One way to influence DOF is by selecting the appropriate aperture; the rule of thumb is this:
- Select a large aperture (or small f/value or small aperture value), e.g. f/2.8, to obtain a shallow DOF
- Select a small aperture (or large f/value or large aperture value), e.g. f/8.0, to achieve great DOF
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