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.
The term ISO refers to how sensitive to light the media is. The lower the ISO numbers means that the media is less sensitive while higher ISO ratings indicate more sensitivity to light.
The diaphragm in the lens of the camera is adjustable and the hole that allows light to pass through it is called the aperture. It operates very much like a pupil. When the aperture is wide open, more light can pass through this larger opening and land on the light collecting media. When the opening is narrowed to create a smaller aperture, less light is able travel to the sensor. The ratio of focal length of the lens to the diameter of the lens opening is expressed as f/(aperture value) and is commonly known as the f stop. An aperture of f/2.8 tells us that the focal length of the lens is 2.8 times the opening inside the lens. Because of this expression of ratio, smaller numbers in the f stop such as f/1.8 means that the aperture is larger as opposed to larger numbers like f/22 which indicate a smaller opening inside the lens.
The simplest of the three factors to understand is the shutter speed. This is how long the shutter is open to allow light to pass through. More light passes through the lens the longer the shutter is opened and the media captures more light.
All three of these factors come with other benefits or drawbacks depending on how you look at it. These side effects can be put to creative use When fully understood and properly utilized.
While all three variables are set with purpose, most photographers are concerned, first, with the ISO setting. Higher ISO settings result in more noise (or grain) which, for most film and digital cameras, increases signigicantly above ISO 400. For this reason, most film photographers use film with ISO ratings as low as conditions will allow, and similiarly, digital photographers set their camera to the lowest ISO settings that conditions will allow. Although noise or film grain is normally undesireable, there are times when they can be used creatively in a photograph to achieve desired results. Usually the ISO is determined first and is the last of the three variables to be changed.
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.
The function that controls how long light passes through the lens is called shutter speed. More light is allowed to pass through the lens when the shutter is opened longer. This also increases the liklihood of motion blur in the photo and this has to be considered when selecting these settings. Usually photographers want their photos, or at least the subject of their photos, to be sharp. There are times, however when motions blur is called for and can be used effectively.
Correctly adjusting these settings will produce a properly exposed picture. Changing any of these factors requires adjustment to at least one of the other factors to assure proper exposure remains. Changing only one of the settings will change the amount of light that is collected by the camera and will affect the final exposure. If you increase the ISO which increases the sensitivity of the media without any other adjustment, you will get an overexposed image. To avoide this you will need to increase the shutter speed or reduce the size of the aperture. Using faster shutter speeds to freeze your child speeding by on a bicycle will give you an underexposed picture unless you increase the size of the aperture to collect more light or bump up the ISO so as to increase the sensitivity to the reduced amount of light gathered.
These three variables determine the exposure of the final photograph. And because of the side effects associated with these variables, a knowledgeable photographer knows what his desired result is and will determine each of these settings prior to pressing the shutter release button.
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