How Aspherical Lenses Fix Aberrations and Improve Sharpness

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

Canon recently celebrated the 50th anniversary of its aspherical lens technology through a new exhibit in its virtual Canon Camera Museum. The aspherical lens exhibition has multiple articles and videos on aspherical and fluorite lenses as well as interviews with project members who discuss the development of the EOS and EF lenses.

Canon says that aspherical lenses have long been known to effectively correct for the various types of lens aberrations that can occur in optical design as well as improve overall image quality. These days, aspherical elements are pretty ubiquitous and can be found in lenses ranging from entry through professional-level optics from a range of manufacturers.

Below are two photo exapmples, one that is shot with a spherical lens and the other taken with an aspherical element:

Image shot with spherical lens element Image shot with aspherical lens element

Canon explains that conventional lenses have a curved surface that is like a slice of a sphere, hence the name "spherical" lens. There are issues with this design, however.

"Compared to light rays passing through the center of a spherical element, rays entering from its peripheries must travel a longer distance in order to reach the image plane, resulting in the light rays converging at different positions," Canon explains. "This causes an effect known as spherical aberration, where point light sources “blur” instead of being rendered as points."

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

Spherical lenses also have issues with distortion.

Aspherical lenses were known to correct these issues, but even though the supposed benefits of aspherical lenses was known, manufacturing such lenses was considered extremely difficult for a very long time. Canon says that it was challenging to achieve the precision needed in order to control the curvature at the submicron level (1/10,000th of a millimeter). Aspherical lenses were so hard to make that they were referred to as "dream lenses."

Over half a century ago in 1971, Canon finally released an interchangeable lens for SLR cameras that included aspherical lens elements. Since then, the company has continued to refine processing methods and precision technology and says that it has been part of spearheading the constant improvement of image quality via aspherical lenses.

A great deal more information about aspherical lenses and Canon's implementation of them in its lineup over the years can be found in the special exhibition on aspherical lens technology in the digital Canon Camera Museum. Within, Canon also has a great explanation of the capabilities of fluorite glass. All the information and videos are free to peruse.

_Image credits: All images provided courtesy of Canon. _

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Hulking Attachment Gave Canon FD Lens Autofocus 40 Years Ago

The Canon Zoom Lens FD 35-70mm f/4 AF was a lens originally manufactured in 1981 and features an unusual motor system that allowed it to autofocus. In Kai Wong's 13-minute video, he shows how it still works remarkably well even today.

Spotted by Fstoppers, the FD 35-70mm f/4 AF took Canon's successful lens and added a large box above the lens which housed a mechanism that would allow for autofocus. It used Canon's Solid State Triangulation (SST) system that originally appeared in the AF35ML compact camera that also launched in 1981.

According to a story on MIR, the SST mechanism used triangulation to measure shooting distance. The measurement is performed by observing a subject from two points, hence the two small lenses that are visible on the front of the large autofocus box of the FD 35-70mm f/4 AF. The two points were slightly angled so that incoming light rays that would bounce off a subject would come in and be triangulated by a small CCD image sensor inside the system. Canon describes it as such:

The SST method is a system in which information on the photographed object that enters the sensor through two fixed mirrors is converted into an electric signal and distance is measured by a microcomputer, with focusing performed by moving a distance ring with a motor. The latest fixed imaging device CCD (charge-coupled device) technology is adopted to provide high resolution and a broad dynamic range able to detect low to high luminance, making it less susceptible to the contrast and pattern size of the photographed object and enabling highly precise autofocusing. Also, as the SST method does not have a movable section in the distance measuring mechanism, no vibration or electric noise is caused, which provides high reliability fitting of a high-end SLR camera.

The system, as evidenced by Kai's video above, works remarkably well. While he is using a mount adapter, at the time the lens was compatible with any Canon FD-mount camera, making autofocus something anyone with such a camera capable of upgrading to an autofocus lens.

Canon

The company would later use a similar system for the FD mount of the Canon T80 in 1985 which was able to autofocus with one of three lenses that had integrated autofocus motors. Canon would eventually abandon autofocus on the FD mount as it moved towards the EOS system in 1987.

While the system clearly works well, its implementation on the 35-70mm f/4 lens has a clear downside: it's huge and bulky. While it isn't particularly heavy, it is a cumbersome addition to a camera system that would have normally been much smaller and easier to transport. While the technology behind the autofocus system still works, it has since been replaced with methods that are far more compact.

For more from Kai Wong, make sure to subscribe to his YouTube channel.

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