What is an Aspherical Lens?
The term Aspherical Lens is slightly misleading, although it one that is often used in the camera industry. As you probably know, camera lenses are comprised of several different lens elements. Each lens element serves a specific purpose, such as correcting chromatic aberration or field curvature. An aspherical lens element is commonly used to correct spherical aberration, though it is usually only one or two elements within a more complex lens design.
A spherical lens has a constant curvature to its surface. If you could extrapolate that curve, it would eventually come back around to create a perfect sphere. An aspherical lens on the other hand, has a deliberately varied curvature to create a lens that manipulates its refractive index across the lens surface, to correct the inherent misalignment of simple spherical lenses. This manipulation is done by extremely precise glass polishing, and often requires minuscule adjustments that are measured in mere microns.
Spherical Aberration
Simple spherical lenses do not refract the light at the same angle across the entire curvature of the lens surface. The result is a slight misalignment of light rays once the image projection reaches the camera’s sensor, particularly at wider apertures. This is known as spherical aberration and can be seen in the final image as a loss of fine detail, lack of contrast and even a hazy “ghosting” effect. If you have ever seen ghosting and lack of contrast in your images when shot at maximum aperture, this is spherical aberration at work. Aspherical lens elements work to correct this.
Lens Designs
When a camera lens is referred to as an aspherical lens, what is actually meant is that the lens contains one or more aspherical lens elements. As you can see in the example lens design below, sometimes a so-called aspherical lens contains just one or two aspherical lens elements within a much more complex design.
Aspherical lens elements are most common in lenses with extremely wide apertures, such as a 24mm f/1.4, a 35mm f/1.4 or a 50mm f/1.2. These are often flagship lenses in a manufacturer’s lens lineup and as such, lenses with aspherical lens elements are usually associated with extremely high image quality and high price tags.
Despite all lens elements being crucial to a design, the inclusion of an aspherical lens element is often underlined by manufacturers, or even included in the name of the lens. For example, Leica lenses with an aspherical element have the letters ASPH appended to the official lens name: Leica Summicron-M 35mm f/2 ASPH or Leica Summilux-M 50mm f/1.4 ASPH. Due to their complexity, having an aspherical element or two in your lens is somewhat like a badge of honour.
When used to correct aspheric aberration, the aspherical lens element is usually placed close to the lens’ aperture. Aspherical elements can also be used to correct distortion in wide-angle lenses, too. In this case, you will see a second aspherical element placed near the front of the lens. Again, this can be see in the example lens design for the Sony 35mm GM lens posted earlier in this post.
Reduced Lens Size
Aspherical lens elements are tricky to make, but if done well, they can reduce the overall number of lens elements that are required in a lens design. This leads to a lighter and more compact lens, but not necessarily a cheaper one. The manufacturing complexity of an aspherical lens means that they are most commonly found in higher-end lenses such as a Sony G Master or a Canon L Series lenses.
