[rollei_list] Re: 500/5.6 Tele Tessar for SL66 Comments

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Fri, 5 Jan 2007 19:11:21 -0800

----- Original Message ----- From: "Carlos Manuel Freaza" <cmfreaza@xxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, January 05, 2007 6:30 PM
Subject: [rollei_list] Re: 500/5.6 Tele Tessar for SL66 Comments

I had found Bob Salomon's definition on the basis of
that obscure DIN standard (according I understood
then, he discussed with Richard about the topic):

According Bob Salomon, a more precise definition
an APO lens is that the lateral chromatic
of the secondary spectrum are reduced to within a
minute percentage of the focal length of the lens. I
add about lateral chromatic aberrations of the
secondary spectrum that "Lateral", as in lateral
colour, means at right angles to the optical axis in
the plane of focus. It fundamentally indicates a
difference in image size for different wavelengths
light. Secondary spectrum includes all colours not
brought to a common focus,in other words Salomon
the lateral chromatic aberrations of the secondary
spectrum are corrected to within a percentage of the
focal length of the lens, he said that the
is according a D.I.N. definition.

And this an optical technician opinion:


All the best

This is a good essay on the definition of apochromatic and modern lens design. It was brought to my attention by Brian Caldwell, a very well known lens designer, that often one can not calculate the chromatic correction of a lens from the patent prescription because the glass constants are not given fully. We all know that glass bends light when light strikes it at an angle. The amount that the light is bent or "deviated" is proportional to a constant known as the Index of Refraction. Index of refraction is defined by the amount light is deviated and also is the ratio of the speed of light in the glass to the speed in a vacuum. What is less realized is that the Index is not constant but varies with wavelength. In general, it increases inversely to the wavelength. That is, glass bends blue light more than red. This effect is known as "dispersion". Dispersion is the property of glass that splits up white light into a spectrum when passed through a 30 degree prism. The index of refraction is really an average of the indices over a given range of wavelengths. For the "old" types of glass, that is glass types known before the researchs of Abbe and Schott, the dispersion was pretty much locked to the index, that is, as the index increased so did the dispersion. What the new or "Jena" glass acomplished was to produce glass types which had lower dispersion for a given index than existing glasses. That allowed a different arrangement of elements to be used to correct for chromatic aberration. Because the relative position of positive and negative elements could now be changed while retaining color correction it became possible to correct for other aberrations. The most important was astigmatism resulting in astigmatic lenses. There is still a third property, namely Anomolous Dispersion. Normal dispersion varies pretty uniformly with wavelength but Anomolous Dispersion results in an change which become increasingly rapid as the wavelength approaches the cut off of transmission of the lens. It affects mostly the blue end of the spectrum. So, a lens can be well corrected over most of the range and still have serious deviation at the blue end. The way to correct this is to choose glass having the right average index and dispersion but similar anomolous dispersion. Modern rare earth glasses or the use of materials other than glass (Flourite or fused quartz for example) allow for better matching of blue end dispersion. It is the anomolous dispersion which is often left out of the glass specifications in patents and other published literature. It is necessary to know this constant if the complete chromatic aberration of a lens is to be calculated and plotted. Hence, it is often not possible to tell of a given lens design is an achromat or apochromat or something else. The Abbe definition of Apochromatic has been generally accepted for microscope and telescope objectives for a century or more but has never been formally adopted for photographic objectives. This is one reason for the sloppy use of the term APO. BTW, since the complete glass characteristics for lenses like the Apochromatic Artar are not published its not possible to tell if even this well respected lens is truly apochromatic. The Artar was intended for use of photomechanical process cameras to make color separation printing plates so, if it is not a true apochromat it must be pretty close. It is an example of a lens relying on symmetry to correct lateral chromatic.

Richard Knoppow
Los Angeles, CA, USA
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