The vast majority of lenses are spherical. The surface of a spherical lens shares the same shape as a portion of a sphere. The reason lenses are manufactured with this shape is because these are easy to grind. Although this shape is fine for many applications, it does not produce perfect images. The light passing through the centre of the lens will undergo a different magnification from light that passes through the edge of the lens. This means that each ray has a different focal point, dependent on its distance from the centre of the lens.
The use of aspherical lenses that have been shaped to maintain magnification across the lens will result in significantly improved images.
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