Apochromatic lenses use a group of three lens elements to reduce colour fringing (as above). Achromatic image left; Apochromatic image right

An APO or apochromatic lens is usually an anonymous design feature. You may see APO on your lens.

A fundamental problem with camera optics is that the light that forms the image on the film or sensor comprises a range of wavelengths. Uncorrected, using a simple lens design, these different wavelengths focus at slightly different distances relative to the film or sensor plane. This results in soft images and chromatic aberrations where details in the image break up into rainbow colours. This is called colour fringing. Lens designers have fought to keep this property of light under control for many years and an APO lens is one of the more sophisticated solutions.

The simpler solution is to combine two lens elements into a group that accurately focuses two primary wavelengths, for example red and blue. This is an achromatic design. It’s inexpensive, simple and brings tangible benefit, especially for black & white photography. But the job is not entirely done, as a third primary colour wavelength, green, for example, will remain uncorrected. This is where the APO lens comes in as an apochromatic lens, typically using a group of three lens elements, each having carefully matched low optical dispersion properties to accurately focus red, green and blue light.

Achromatic lenses have been around for over 100 years, but apochromatic lenses have only been widely used in more recent decades. ‘APO’ marked on a lens, in theory, identifies it as an apochromatic design, but manufacturers have also used the term as a marketing tool to identify a lens as a high-performer – even if reviews sometimes contradict this claim, while other lenses not claimed to be ‘APO’ can garner rave reviews.

Apochromatic designs are usually found in high-performance telephoto lenses and, unsurprisingly, in telescopes.