How Aspherical Lenses Correct Aberrations and Improve Sharpness

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What is an aspheric lens and what is it used for? Canon has shared a video that explains the problems engineers face in lens design and how the company solved them by using aspherical lenses.

Canon recently celebrated the 50th anniversary of its aspherical lens technology with a new exhibit in its Virtual Canon Camera Museum. The Aspherical Lens Exhibit contains many articles and videos about aspherical and fluorite lenses, as well as interviews with project members who discuss the development of EOS and EF lenses.

Canon claims that aspherical lenses have long been known to effectively correct the various types of lens aberrations that can occur in optical design and improve overall image quality. Aspherical elements are quite ubiquitous these days and can be found in lenses ranging from entry to professional level optics from a range of manufacturers.

Below are two sample photos, one taken with a spherical lens and the other taken with an aspherical element:

Image taken with a spherical lens
Image taken with an aspherical lens

Canon explains that conventional lenses have a curved surface that resembles a slice of a sphere, hence the name “spherical” lens. There are, however, problems with this design.

“Compared to the light rays passing through the center of a spherical element, the rays entering through its peripheries must travel a longer distance to reach the image plane, which causes the light rays to converge at different positions”, explains Canon. “This causes an effect known as spherical aberration, where point light sources ‘blur’ instead of being rendered as dots.”

Spherical aberration on a spherical lens
An aspherical lens ensures the convergence of the light rays at the same position.

Spherical lenses also have distortion issues.

Aspherical lenses were known to correct these problems, but even though the supposed advantages of aspherical lenses were known, manufacturing such lenses was considered extremely difficult for a very long time. Canon says it was difficult to achieve the precision needed to control curvature at the submicron (1/10,000th of a millimeter) level. Aspheric lenses were so difficult to manufacture that they were called “dream lenses”.

Over half a century ago, in 1971, Canon finally released an interchangeable lens for SLR cameras that included aspherical lenses. Since then, the company has continued to refine processing methods and precision technology, and says it has played a leading role in steadily improving image quality through aspherical lenses.

Much more information on aspherical lenses and their implementation by Canon in its range over the years can be found in the special exhibition on aspheric lens technology at the Canon Digital Camera Museum. Inside, Canon also has an excellent explanation of the capabilities of fluorite glass. All information and videos are free to read.


Picture credits: All images courtesy of Canon.

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