It consists of a series of overlapping thin metal blades that can swing toward the center of a hole in a flat metal ring. This is the device most cameras use to form and adjust the aperture. This kind of measurement is used because a given focal ratio produces the same image brightness, requiring the same shutter speed for a given ISO setting (film speed or equivalent sensor light amplification) without regard to focal length. This is the ratio of the focal length of the lens to the size of the aperture. Because every camera has an aperture, usually adjustable, and if not, at least has the edges of the lens as an aperture, the aperture size setting is what is normally called the "aperture".With a lens, it also blocks rays of light that would pass through far from the center, where the lens glass may less closely approximate (usually with various easy-to-make spherical surfaces) the shapes that would focus it perfectly (usually much more complex aspherical surfaces), causing aberrations. Like the pinhole in a pinhole camera, it blocks rays of light except those that would, even without a lens, tend to form an inverted image by passing through that central point to a corresponding point in the opposite direction on the film. This is the adjustable hole through which light passes on its way from the subject, through the lens, to the film (or digital sensor). You'll need to know these in order to make sense of the rest of the article. Familiarize yourself with some of the basic concepts and terminology.
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