Achromat, Apochromat, Superachromat -
What is the Difference?
by Dr. Hannfried Zügge,
group manager, Carl Zeiss optical design department
A lens which is corrected for only one wavelength (color) - e.g. for laser applications - is described as a monochromat, i.e. the monochromatic aberration has been corrected, while chromatic aberrations remain uncorrected.
Most lenses for general photography, however, are used in a broader spectrum (wavelength range). Thus the chromatic (color) aberrations of these lenses must also be corrected over a wide spectrum to provide what is generally termed "sharpness". Here, the term "achromat" is used. (Editor's note: This suggests the simple but fundamentally accurate explanation why the monster lenses used for the production of microchips are specified for an extremely narrow range of wavelengths. The level of precision required in this application many orders of magnitude higher than general photography can be assured only by specifiying monochromatic light.).
The most conspicuous color aberration is longitudinal chromatic aberration (LCA). With a monochromat, only the image (or the focus) of one wavelength (color) lies in the image plane (e.g. film plane) due to LCA, while the focus for shorter wavelengths (blue light) 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, it is possible to correct the primary spectrum by combining converging and diverging lens elements made of different types of optical glass displaying different (color) dispersion. To be more precise: it is possible to move the LCA for two wavelengths into the image plane (film plane). Residual LCA, also described as the secondary spectrum, i.e. the deviation of the focus from the image plane for all the other colors, remains so negligible that the image quality is no longer noticeably impaired.
However, the secondary spectrum features - firstly - a typical profile and - secondly - a typical magnitude, depending to a large extent 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. The only aid available for this purpose is the use of very special optical materials: long-crown glass (fluor crown glass and calcium fluoride) and special short flint glass. These can therefore be justifiably called extreme types of glass, since they not only display an unusual behavior with regard to the relative partial dispersion, but are also extremely expensive and difficult to process.
The correct use of these extreme types of glass (or crystals) enables the optical designer to reduce the disturbing secondary spectrum to such an extent that the overall image quality is no longer limited by the secondary spectrum. The term coined by users is "apochromat". The scientist's definition is usually as follows: in an achromat, two colors lie in the image plane together (two LCA zeroes), while three colors lie together in the image plane (three zeroes) in the apochromat.
In most cases, the third zero is not required in practice, but it is sufficient to reduce the secondary spectrum to meet the respective requirements. If this is successful, such a lens could be termed "Apo lens". However: the mere use of these extreme types of glass does not yet justify the term "Apo lens".
To sum up, the following can be said about the marketing-oriented, and scientifically still justifiable term Apo lens: it is possible to use the term Apo lens if, without correction of the secondary spectrum, such a lens displays an image quality which is largely determined (and impaired) by the secondary spectrum, but which, after correction of the secondary spectrum, exhibits considerably better image quality in which the secondary spectrum no longer plays a dominant role. In other words: compared with a standard lens of identical speed and focal length, an Apo lens displays considerably better image quality.
The above considerations are still somewhat simplified because only longitudinal chromatic aberration (LCA) has been examined so far. 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 (Distagon) in particular. 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 extreme types of glass. Therefore, it is correct for the user to use the terms Apo correction and Apo lenses.
The scientist goes on counting: the Achromat has two zeroes in the longitudinal spectrum, the Apochromat three, and the lens with four zeroes is called a superachromat. However, it is not the fourth zero which is the decisive factor for practical applications, but the overall image quality.
If Apo correction is performed with particular consistency and effectiveness so that the residual chromatic aberration can no longer be detected in applications, new problems will emerge: due to the use of long-crown and short-flint glass for lens elements featuring very high refractive powers, the lenses will be very sensitive to tolerances. However, any theoretically calculated imaging performance - however high it may be - is useless if the calculated quality cannot be reliably achieved on account of excessively high sensitivity to tolerances. The experienced optical designer finds a solution to this problem, too: if the tightest attainable tolerances are not sufficient, individual adjustments must be made to each lens element which take the properties of the finished optics into consideration. For example, the actual refractive indices and the actual dispersion of the glass melts can be taken into account 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 of this type, which require such additional compensation to achieve the required top-class quality, are called superachromats.
Camera Lens News No. 12, Winter 2000