Variable aperture zoom lenses come in different focal length combinations.
Creating a photograph involves exposing light-sensitive material to light. Cameras regulate how much exposure a given media such as film or a digital sensor receives by adjusting three basic settings -- aperture, shutter speed and ISO. While the shutter speed and ISO settings are built into the camera body, the aperture resides in the lens. A lens has a maximum aperture value it can use that is indicated on the body of the lens.
Aperture
The diaphragm or opening in a lens is called the aperture. The aperture size, called the f/stop or f/number typically is stamped on a lens to indicate the aperture range. This maximum f/stop on a lens lets the photographer know the largest aperture available for a particular lens. The maximum f/stop is indicated by the number one, followed by a colon and the f/number and is stamped on the front barrel of the lens. For example, a lens with a maximum f/stop of 3.5 is shown as 1:3.5. The aperture regulates the size of the opening light passes through to reach the camera's film or digital sensor.
Zoom Lenses
Zoom lenses come in various focal lengths, sizes and aperture combinations. Most people think of a zoom lens as a lens that allows you to "zoom in close" on your subject. The truth is a zoom lens represents any lens that has a variable focal-length range. For example, a 35 mm camera lens with an adjustable focal length of 10 mm to 20 mm functions in the wide-angle, focal-length category, but because it has an adjustable focal length, it still represents a zoom lens. Most zoom lenses have a variable maximum aperture range, although some more expensive zooms have a fixed aperture.
Variable Aperture
The variable aperture range means that the maximum aperture of the lens varies based on the focal length set on the lens. For example, a lens with an 80-300 mm focal length might have a maximum aperture range of f/3.5 to f/5.6. This means with the lens used at the 80 mm setting the photographer can set the maximum aperture to f/3.5. As the photographer moves farther up the focal-length range of the lens, the maximum aperture size changes. When the photographer reaches the 300 mm focal-length setting, his maximum aperture is now f/5.6 not f/3.5. One reason manufacturers make variable-aperture zoom lenses is because they are less expensive to make than their fixed-aperture zoom counterparts.
Considerations
Variable-aperture zoom lenses cost less than fixed-aperture zooms, but they also have certain drawbacks. Fixed-aperture zoom lenses allow the photographer to use the same aperture, shutter speed and ISO combination settings throughout the entire zoom range of the lens. With a variable-aperture zoom, in order for the photographer to maintain the same exposure, she must adjust the shutter speed or ISO setting when the lens reaches its highest maximum aperture setting. Additionally, fixed-aperture lenses typically have better optical performance than variable-aperture lenses.
