The iris is an adjustable diaphragm that regulates the amount of light striking the image sensor located inside the body of the camera at the back wall. When shooting images in low-light areas, the iris needs to be opened up to allow the maximum of available environmental light. For shooting in extremely bright light, the amount of light needs to be reduced so the image sensor isn’t overwhelmed. The area of the iris that opens and closes is the aperture and the physical diameter needs to be mathematically calculated (focal length of lens divided by the f-stop equals the diameter of the aperture) in order for the photographer to manage the light entering the camera and manage how the camera focuses on the subject matter. F-stops are mathematical notations that line the innermost rings of a lens housing. Starting at f/2.8 and rising to f/22, as the f-stop number increases, the size of the aperture shrinks.
Shutter speed is the amount of time the shutter remains open. The shutter is a moving blade that is located behind the iris and acts as a moving curtain to allow light to reach the image sensor. Fast shutter speeds will “freeze” action occurring in brightly-lit scenes. In low-light situations, a slow shutter speed will allow more of the available light to reach the image sensor.
Exposure is the result of the mathematical calculation of intensity of light multiplied by the amount of time light spends on the image sensor as regulated by the shutter. Yet, the mathematical result for exposure does not create the same image shot after shot. The image’s exposure can be adjusted by changes in the f-stop or shuttle speed shifting the depth of field from front to back depending on the numbers used in the calculation.
Depth of Field
Depth of Field (DOF) is how the area of the scene surrounding the main subject of the image is focused. The “success” of the image relies on the placement of the DOF. The depth of field is created by combing the f-stop, the focal length of the lens, and distance from camera to main image. The DOF has two types: great and shallow. Great depth of field describes a photograph where much of the scene is sharply defined. Both the main image and the background are in sharp focus creating a “competition” for the eye of the viewer. Shallow depth of focus blurs the background so the image in the foreground is left in focus. This creates a center of focus and the main image gains the attention of the viewer.
There is an “inverse relationship” between the depth of field and size of the aperture. At f/2.8, the aperture is wide open. In low light, the background blurs creating a shallow depth of field. Yet when the aperture is open and the light is very bright, a great depth of field is created from the flood of light, if not overwhelming the image sensor with light. To counter the brilliance, adjustments in other parts of the camera would need to be made in order to render the main image as the center of focus. These adjustments would need to occur by zooming into the subject, increase the shutter speed. If the camera has film, the film speed could be reduced as well.