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. 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. In order to create a correctly exposed image, one cannot change one of these functions without the need to alter one or more of the other settings to directly compensate. For example, if we were to set a camera’s ISO at 100, an aperture at F8 and with a shutter speed of 1/60th second, and this resulted in a correct exposure for that situation and time, to then alter the aperture to F16 in order to achieve a better depth of field, to keep a correct exposure one would then need to compensate for the now greatly reduced amount of light reaching the film or sensor by increasing the amount of time the shutter is left open. In this example that would mean the shutter must be left open 4 times as long, which is 1/15th second. Every time you alter the aperture by a full increment or stop, it either halves or doubles the aperture size, depending upon which direction the aperture is altered. In this instance, changing the aperture from F8 to F16, means the aperture is getting smaller, and by a factor of 4X. The reason is that F11 is the next aperture along the scale, which is half the size of the F8 setting, and then F16 is half the aperture size of F11. So ½ x ½= ¼.
The other way that this shift in exposure can be compensated for is to change the ISO from 100 to 400. In the case of film, this really means a change of film, yet digitally this is simply achieved by a change of setting on the camera. ISO settings are easy to understand. The lower the number, the less sensitive the film or sensor is to light, and requires longer exposures to compensate. The advantage is generally a finer grained image, or in the case of digital, a cleaner looking image. The reverse is true with higher ISO settings. The higher the number, the more sensitive the film or sensor becomes, and less light is required.
