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Message: <blockquote data-quote="FastGlass" data-source="post: 232555" data-attributes="member: 13822">A lense that is listed, advertised or called a fast lense is one with a large aperture of typically 2.8 or larger. ( smaller numbers). The reason it is called fast is before the shutter closes and snaps the photo the cameras meter and autofocus systems are looking at the information coming through the lense to determine the exposure and what to focus on. The aperture blades don't close to the chosen aperture until the shutter button is pushed. So a lense with a larger aperture will have a bigger opening in the lense to allow more light to enter the camera. The larger the opening the FASTER the autofocus system will be.</blockquote>