Exposure is basically the length of time that a camera’s shutter must be open to allow sufficient light to enter the camera and reach the film plane or digital sensor. This is determined by a direct relationship with the size of the aperture and the ISO set at the time of exposure.
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The smaller the aperture (the higher the number such as F16, F22 etc.), the less light that can reach the film plane or sensor, and so in order to have a correct exposure, the shutter must remain open for a longer period than if the aperture is larger (a smaller number like F2.8 or F4). Likewise, when a higher ISO is selected, this means that the type of film used, or the function of the sensor, is allowing less light to be needed to enable correct exposures. These relationships between shutter speed, aperture and ISO settings are always directly related.
