(50th anniversary) Aspherical lens elements: Transcending challenges in ultra-high-precision processing

Long known to effectively correct various types of lens aberrations and improve image quality, aspherical lens elements can now be found in many interchangeable lenses, whether in entry-level models or high-performance models designed for professional users. However, manufacturing such lenses requires control over curvature at the submicron level (1/10,000th of a millimeter). It was considered so difficult to achieve such precision that the lenses were once called “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, spearheading the constant improvement of image quality via aspherical lenses.

Aspherical Lenses (Created: December 2006 03'05'')

Blurring due to spherical aberration

Conventional lens elements have a curved surface shaped like a slice of a sphere, which is why they are called “spherical” lens elements. 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. This causes an effect known as spherical aberration, where point light sources “blur” instead of being rendered as points.

With aspherical lenses, the curvature changes towards the lens peripheries, ensuring that all light rays converge at the same position, thereby reducing spherical aberration.

Correcting distortion

Distortion is another type of aberration that can be effectively corrected using aspherical lens elements. Wide-angle lenses are prone to barrel distortion, whereas telephoto lenses are prone to pincushion distortion.

While aspherical lens elements can correct various types of aberration, one aspherical lens alone cannot correct everything. Rather, it must be used in combination with a number of other lenses, and the type of aberration that the aspherical lens element corrects changes depending on its relative position within the configuration of the lens. For example, barrel distortion is effectively corrected by placing aspherical lens elements near the front of the lens configuration.

The emergence and evolution of aspherical lenses

The theory behind aspherical lenses and their use in correcting aberrations has been known for a very long time. In fact, such lenses have been used in astrometric telescopes since the early 1900s. The difference in surface curvature between an aspherical lens and a spherical lens is so subtle that it is not visible to the human eye, and requires submicron-level precision measurements and processing to replicate. There were also many huge obstacles that stood in the way of producing aspherical lenses that could be used on cameras, such as mass production capability and the need to produce lenses capable of correcting aberrations at wider angles—different from the existing telescopes.

The journey to mass-produce aspherical lens elements involved the incorporation of advanced technologies developed for the space exploration program, culminating in the development of the ALG-Z ultra-high-precision aspherical lens grinder and the ALM ultra-high-precision measuring machine. The use of a laser interferometer ensured fine control over equipment grinding, which made possible surface grinding precision down to units of 100nm^* for shaping and 50nm for processing. Combined with the uniform grinding and polishing techniques that Canon had developed based on its own theories, these technologies made it possible to mass produce more than 1,000 pieces of ground and polished aspherical lenses per month.

• * One nanometer (nm) is one billionth of a meter.

Later, Canon also succeeded in developing plastic-molded (PMo) aspherical lenses, which can be mass produced in larger quantities at a lower cost, and large-diameter glass-molded (GMo) aspherical lenses, which are molded in a machine that incorporates a high-precision aspherical metal mold^*.

• * 1982: Release of the Canon Snappy 50, the world’s first camera lens for 35mm full-frame to incorporate a PMo aspherical lens element.
•    1985: Release of the New FD35-105mm f/3.5-4.5, the world’s first interchangeable lens for 35mm full-frame cameras to incorporate a GMo aspherical lens element.

In 1990, Canon developed a fourth aspherical lens production technology that produces replica aspherical lenses by using ultraviolet-light-hardening resin to form an aspherical lens surface layer on a spherical lens element.

Taking such factors as desired image quality and intended use into consideration, Canon chooses the most appropriate of its four aspherical lens production technologies for each lens in its continuing efforts to develop high-quality lenses.

Catering to diverse needs: Canon’s aspherical lens elements

Ground and polished aspherical lens elements

Plastic-molded (PMo) aspherical lens elements

Glass-molded (GMo) aspherical lens elements

Replica aspherical lens elements

Lenses with aspherical lens elements (as at May 2021)

RF lenses that employ aspherical lens elements (as of September 2023)