A lens is made of a curved surface, so it is thicker in some places than others. This can make it rather bulky. However, light only bends at the lens's surface. One trick for making lenses thinner and lighter is to shift sections of the lens toward the center. This is called a Fresnel lens, after its inventor, and was originally used for lighthouses. It will not provide quite as good of focusing quality as a normal lens, but sometimes this is secondary to reducing the bulk of the lens.
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