A spherical lens (left) brings light rays passing through its edges to a different focus point than light passing through its centre. An aspherical (or non-spherical) lens (right) can be designed so that all the light rays converge on the same point.

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Aspherical lens elements help to compensate for distortion in wide-angle lenses, and compensate or eliminate spherical aberrations in lenses with a large maximum aperture. They also allow the Canon to produce more compact lenses than was previously possible using only spherical lens elements.

The degree of asphericity is so minor that special manufacturing processes were created to stay within the 0.1 micron* tolerance. Measuring the curvature requires even greater accuracy. It wasn’t until 1971 that the first SLR camera lens with an aspherical lens element was produced. However, it wasn’t perfect. In fact, it took another two years before manufacturing techniques reached the levels required to really achieve large gains in image sharpness.

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Canon designers discovered that an aspherical (non-spherical) lens shape would eliminate these spherical aberrations, because by altering the curvature of the lens could be used to converge the light rays to a single point.

In the early days, all lenses were spherical – i.e. they had a perfectly curved surface. They are the easiest lens shape to make, but are not best suited to rendering a sharp image as they cannot make parallel rays of light converge at the same point. This causes a problem called spherical aberration.

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Aspheric lens elements manufactured today are so accurately ground and polished that if the degree of asphericity is even 0.02 micron* (2/100,000mm) away from ideal, the element is rejected.