The Three Variables Affecting Exposure in Photography
From fine art photography to point and shoot snapshots, exposure of a photograph is dependant on the amount of light that lands on the exposure media. Whether you are using film or digital cameras, the result is the same. Light is captured by a frame of film or the sensor in a digital camera and the proper exposure of your images depends on the settings of these three variables: Shutter speed, aperture, and ISO. The proper setting of these three variables will ensure proper exposure.
ISO is a measure of how sensitive the film or digital sensor is that is collecting the light. The lower the ISO number, the lower the sensitivity. The higher the ISO, the higher the sensitivity.
The size of the hole that light passes through in the lens is called the aperture. Just like the pupil of the eye, the larger the hole, or aperture, the more light can enter through the lens and onto the media capturing it. Aperture is epressed as a ratio of focal length the size of the hole as adjusted by the diaphragm and is stated as f/(aperture value). If the aperture is f/2.8 it means that the focal length of the lens is 2.8 times the opening of the diaphragm. The smaller the aperture number, the larger the hole in the diaphragm. The larger the aperture number is, like f/8, the smaller the hole in the diaphragm.
Shutter speed is the easiest of the three variables to understand. It is as simple as it sounds. This is the amount of time that the shutter is open to collect light. The longer the shutter is opened, the more light is allowed to pass through the lens to be collected.
Each of these variables have other results or consequenses which, when fully understood and utilized, can be put to good use to create the desired image.
The ISO setting is usually the first variable that is set and is normally the last variable to be changed because of the increase in film grain or digital noise the higher the ISO is set. For this reason, photographers usually use the lowest ISO possible to keep the noise and grain to a minimum. Sometimes, however, digital noise and film grain are used purposefully to create a desired outcome in the photograph.
Changing the opening in the diameter of the diaphragm changes how much light passes through the lens and onto the light catching media whether it is film or a digital sensor. When changing the aperture the focus in the image captured is also changed. Smaller apertures such as f/8 and f/16 will result in more area in focus between foreground and background. Called depth of field, this is very important in the capturing of an image. Bigger apertures like f/1.8 and f2.8 narrow the depth of field so that most everything in the final picture will out of focus. This helps to highlight the subject in a dramatic way by leaving the subject in focus while distracting foreground and backgrounds remain out of focus.
Varying the length of time that the shutter is open will control how much light passes through the lens opening. This is called shutter speed. The longer the shutter remains open the more light will be collected by the sensor or film. Shutter speed also affects motion blur or lack of. When the shutter is open for longer periods, the moving parts of the image will begin to blur and the amount of blur increases the longer the shutter is open. Otherwise, opening the shutter for a shorter period of time results in freezing everything in the image in place. Faster moving objects require faster shutter speeds to capture them without motion blur, but less light is allowed through the lens when the shutter is opened for less time.
When the values for these three variables are correctly determined and set accordingly, a well exposed image will be produced. After selecting these settngs, a change in any one of these variables calls for a compensating change in at least one or both of the others. Making a change in only one of the variables changes the amount of light that was will be captured by the sensor or film. If you choose to use a higher ISO film or setting then you will be increasing the sensitivity of that media which will result in an overexposer unless you reduce the amount of light that enters the lens by reducing the aperture or shutter speed. Using a faster shutter speed to freeze a passing race car will give you an underexposed image unless you increase sensitivity of the light collecting media by boosting the ISO or unless you open up the aperture to allow more light through the lens to compensate for the shorter duration that the shutter is open.
The three variables discussed here are everything that needs to be considered when determining exposure for an image. And if you understand the implications of each variable as it applies to the amount of light that is collected by the film or digital sensor, then you can learn to properly expose your images everytime. And, if you also understand and learn how and when to apply the different effects that each of these variables also brings to the table, then you will be able to explore countless creative possibilities with your photography.
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Filed under Photo by Robert Sullivan
