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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 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 exactly what it sounds like. The length of time that the shutter is opened to collect light is called shutter speed. More light passes through the lens and onto the media the longer the shutter is opened.

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.

Knowledgeable photographers understand the causes and effects of these factors and will set them before ever taking a picture. Usually, a photographer is concerned first with ISO. Higher ISO settings result in grainier images. For this reason most photographers normally keep ISO settings as low as possible. Even though noise and film grain are usually unwanted, sometimes they are used to achieve this effect in the image.

Varying the aperture changes the diameter of the diaphragm which controls how much light can pass through the lens and onto the film or sensor. When varying the aperture the most noticable result in the final picture is how much of the scene is in focus. When a smaller aperture is used (higher numbers such as f/8 and f/16) more of the area from foreground to background is in apparent focus. This is called depth of field and is extremely important in the production of an image. Larger apertures (lower numbers such as f/1.8 and f2.8) narrow this depth of field which results in most everything in the image to be out of focus which brings attention to the in-focus subject in dramatic fashion and often eliminates distracting backgrounds.

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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