What do “aspheric” or “aspherical” mean?

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The glass elements which make up a traditional camera lens can be thought of as cross-sections of a large sphere. That is to say that the curvature of each surface is even. The problem with spherical lens designs is that light passing through the outer edges of curved camera lenses focus at different points from light passing through the centre. This causes focus errors, or spherical aberration and other forms of optical problems. Spherical lenses work fine if the recording surface is also spherical (eg: the human eyeball), but in the case of cameras it’s not - the film surface or image chip surface is always flat.

 

 

One way to fix the problem is to add additional lens elements to correct for the aberration. A generally more efficient way to fix it is to make a lens element which is not a sphere in cross section. In other words the curvature of the lens varies from the middle to the edges. Such an aspherical lens element can help simplify lens design by minimizing the number of elements required and can result in a sharper image. Aspherical elements are particularly useful for correcting distortion in wide-angle lenses.

There are three basic ways to make an aspherical element. The expensive way is to grind a piece of glass down to the right shape. This is quite difficult owing to the extreme precision required to achieve the complex geometry, and so only some L series lenses use ground aspherical elements in Canon’s lens lineup. Another way is to mould a glass lens element. Such glass moulded aspherical lens elements are used in a number of Canon’s less expensive lenses. The cheapest way is to cement a plastic resin aspheric surface to the top of a glass spherical element. Such lenses are known as replicated aspherical elements and are particularly common among point and shoot cameras.

Note that some lensmakers, particularly Sigma, use the terms “aspherical” or “ASPH” as marketing labels. However many modern lenses made by other makers such as Canon also employ aspherical elements - they simply don’t advertise this fact on the outside of the lens. It’s important to remember that lenses with aspherical elements are not automatically and intrinsically better than lenses without. Sometimes they are, though this is usually for a variety of other factors, not merely the fact that they use aspherical elements.

 

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