Achromat, Apochromat, Superachromat – What is the Difference?
Read about the properties of light that cause chromatic aberrations and how lens design can help reduce or eliminate them. Dr. Hannfried Zügge, Group Manager of the Carl Zeiss Optical Design Department explains the difference between achromat, apochromat and superachromat lenses and what this means for image quality.
Compared with a standard lens of identical speed and focal length, an Apochromatic (APO) lens displays considerably better image quality. Dr. Hannfried Zügge, Group Manager of the Carl Zeiss Optical Design Department explains more about this fascinating topic.
Light and Colour
The visible spectrum of light contains different wavelengths which correspond to different colours. The shortest wavelength is 400nm and is violet while the longest visible wavelength is 700nm and is red. White light contains all the wavelengths and is known as polychromatic. Objects appear as different colours because of the wavelengths they reflect. An object reflecting 700nm waves (and absorbing the others) appears red.
When light passes through any lens it is bent. The different wavelengths are bent to different degrees which means they will not all focus at the same point. This causes chromatic aberrations and an image that has colour fringes and less sharpness around points of highest light contrast.
Correcting lenses for chromatic aberrations
A lens which is corrected for only one wavelength is described as a monochromat, this is useful for specialist applications such as lasers where only one colour of light is required. Aberrations of the other wavelengths remain in a monochromat lens.
Most lenses for general photography, however, are designed to focus all colours in the visible spectrum. The chromatic aberrations of these lenses must be corrected over the full spectrum to provide acceptable sharpness.
Today, an outstanding degree of correction of the chromatic aberrations of a lens is even more important than in the analogue era of photography. This is due to the regular structure of red, green and blue pixels on the sensor (e.g. the Bayer pattern) and the image enhancement in the camera (sharpening, edge contrast enhancement). Colour fringes caused by the lens are also enhanced and significantly more obvious than on pictures taken on film with the same lens.
Correcting longitudinal chromatic aberration in the primary and secondary spectrum
The most conspicuous colour aberration is longitudinal chromatic aberration (LCA). With a monochromat single lens, only the image (or the focus) of one wavelength (colour) lies in the image plane, while the focus for shorter wavelengths lies in front of the image plane, and behind the image plane for longer wavelengths. Scientists say that a primary spectrum is present.
With an achromat lens design, it is possible to correct the primary spectrum by combining converging and diverging lens elements made of different types of optical glass displaying different dispersion. To be more precise: it is possible to move the LCA for two wavelengths into the image plane (film plane or digital sensor). Residual LCA, also described as the secondary spectrum, i.e. the deviation of the focus from the image plane for all the other colours, remains so negligible that the image quality is no longer noticeably impaired.
The secondary spectrum features a profile and magnitude depending on the focal length and the lens type. The longer the focal length and the larger the aperture (speed) of the lens, the more the image quality will be impaired by the secondary spectrum. This can be so severe that the secondary spectrum is the main factor limiting the image quality. Hence, the next step required by the optical designer is to also correct the secondary spectrum in these cases. The result is the apochromat lens.
Achieving this level of correction is not simple. It requires the use of exceptional quality optical materials: long-crown glass (fluor crown glass and calcium fluoride) and special short flint glass. These specialist materials display unusual, desirable characteristics with regard to relative partial dispersion. However, they are extremely expensive and difficult to process.
The correct use of these highly specialist types of glass enables the optical designer to reduce the unwanted effects of the secondary spectrum to improve the overall image.
Aberration correction performance
Achromat: a lens which corrects light so that two colours lie in the image plane together (two LCA zeroes),
Apochromat: a lens with correction such that three colours lie together in the image plane (three LCA zeroes)
Superachromat: four zeroes of wavelength correction plus compensation for geometric tolerances
In most cases, an achromat lens will produce very good image quality. However, for some lenses, it is important to reduce the secondary spectrum and use an apochromat design. Where this is achieved you might think such a lens could be termed an “APO” lens. However, the mere use of these exceptional types of glass does not fully justify the term APO lens.
Despite the almost identical focal length and speed of the two lenses used above, superior chromatic aberration control can be seen with the APO Otus lens (right).
Are there other kinds of chromatic aberration?
The above explanation is somewhat simplified because only longitudinal chromatic aberrations (LCA) have been considered. Transverse chromatic aberration (TCA), or chromatic difference of magnification, can also play a major role. This is the case with tele lenses and retrofocus lenses such as the Distagon. Like LCA, a primary and a secondary spectrum also exist in TCA. The secondary spectrum of TCA can also be reduced by the specific use of similar specialist types of glass. Only when TCA has also been corrected for can the terms APO correction and APO lenses be used.
How do these corrections affect image quality?
When you are out taking photographs, it is not the number of zeroes in the lens specification that matters, but the overall image quality.
If APO correction is performed consistently and effectively so that the residual chromatic aberration can no longer be detected, unfortunately new problems will emerge. Long-crown and short-flint glass have very high refractive powers, so lenses using this glass will be very sensitive to tolerances. Any theoretically calculated imaging performance – however high it may be – is useless if the calculated quality cannot be reliably achieved due to excessively high sensitivity to tolerances.
What makes a superachromat lens?
To solve the problem of the high sensitivity to tolerances, individual adjustments must be made to each lens element. For example, the actual refractive indices and the precise dispersion of the glass melts can be accommodated in an adapted optical recalculation. Compensation can be made for such geometric tolerances as thickness deviations and decentration by using special compensation lenses or groups. Lenses which employ sophisticated additional compensation measures to achieve the required top-class quality, are called superachromats. An example of this is the ZEISS Sonnar T* F/5.6-250mm.