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The Three Variables Affecting Exposure in Photography

May 18, 2008
Whether it's a snapshot or fine art photography, exposure of a picture is depends on how much light registers on the media (film or digital camera sensor) that gathers that light. The correct exposure of a picture is a product of the three factors that control how much light is gathered: aperture, Iso, and shutter speed. When these three variables are in the proper ratios, the proper exposure of pictures will result.

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 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.

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.

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.

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 these three variables are set properly, a properly exposed image will result. Once set properly, a change in one of the variables will require a change in at least one of the other variables to compensate. This is because changing only one of the variables will increase or decrease the amount of light that lands on the film or sensor or affect the sensitivity of that film or sensor. Increasing the sensitivity, or ISO, of the sensor or film will result in an overexposed photograph if the light landing on that sensor or film is not reduced by increasing the shutter speed or decreasing the aperture. Increasing the shutter speed to freeze a rodeo rider will result in an underexposed photograph unless the size of the aperture is increased to allow more light in or the ISO is increased which results in more sensitivity to the decreased amount of light.

Together, these three factors result in whether or not the picture is properly exposed. A good photographer understands these factors and the effects that they have on the final image. He will use this knowledge and take care to ensure the proper settings are utilized to produce exactly what he wants before he ever presses the shutter release button.
About the Author
Robert Sullivan is a Photographer and Artist who loves to find a great nature photo he lives in South Florida. If you are looking for a great picture of florida alligator he is the expert you have been looking for.
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