Lenses with longer focal lengths are slower (have smaller maximum apertures) than wide-angle lenses. That's why photographers mostly use wider lenses in low-light situations -- they gather more light and help create sharper images.

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Light is an essential part of the human experience. It's no wonder, then, that we've developed ingenious methods for creating, manipulating and capturing light. Cameras are one way we can record light. Whether we use an exorbitantly-priced professional camera or a cell phone, many of the principles for sensing light levels and capturing images are the same. One of the most important principles is aperture.

In consumer optical products -- most commonly cameras -- aperture describes the size of the hole that lets light into a device. Cameras use mechanical diaphragms to control how much light passes into the camera body and strikes the image sensor or film. That diaphragm works a lot like the irises in your eyes; it contracts or expands depending on how much light is needed. Aperture refers to the diameter of the opening. It's like the dark, black pupils of your eyes, the actual orifice that lets light pour through.

Cameras are by far the most common device that most of us use when dealing with the rules of aperture. However, there are plenty of other examples, such as microscopes and lasers, that are equally dependent on aperture diameter. Telescopes and aperture principles are tightly intertwined, too. Later, we'll show you how a bigger aperture can help your telescope make Jupiter look like Jupiter instead of a blurry version of Saturn.

Keep reading to get an even better understanding of just how important the subject of a simple opening -- aperture -- is to photography, and to our wide world of optical devices.