A camera is manufactured not to allow light into its body until a picture is ready to be taken or exposed; light is allowed into the camera when the shutter is opened to take a picture. The iris is the adjustable opening or aperture that controls the amount of light allowed into the camera to make an exposure. The iris is adjusted to ensure the subject of the picture is correctly exposed in whatever light is available to the photographer.
F-Stops & Shutter Speed
The iris affects two parts of the process of taking a photograph; first, how much light is allowed into the camera for the single exposure, which is set using F-Stops. Second, the iris is opened for a certain amount of time as specified by the photographer altering the shutter speed. The circular camera lens and iris measure the amount of light by determining the area of light to be allowed into the camera, using F-Stops .F2.8 is the smallest iris setting available allowing the iris to open the smallest amount and let the least amount of light into the camera.
When the F-Stops and shutter speeds are combined, the aperture of the picture is affected. A large aperture opens the iris shutter for a longer time using a fast shutter speed as specified in roll film by the speed of the film. By allowing a greater amount of light into the camera, low light conditions can be used for photography where the use of flash is not available or would affect integrity of the exposure. Faster shutter speeds help to capture action shots for use in sports photography where action shots need to be captured without blurring. A large aperture does reduce the depth of field available to the photographer. In contrast to the large aperture a small aperture allows the shutter speed to be slowed without the picture becoming overexposed and creating a larger depth of field with more detail in the background of the photograph.