Richard - Is true APO performance focus distance specific? Tim ----- Original Message ----- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx> To: <rollei_list@xxxxxxxxxxxxx> Sent: Friday, January 05, 2007 9:11 PM Subject: [rollei_list] Re: 500/5.6 Tele Tessar for SL66 Comments > > > 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 > dickburk@xxxxxxxxxxxxx > > --- > Rollei List > > - Post to rollei_list@xxxxxxxxxxxxx > > - Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' > in the subject field OR by logging into www.freelists.org > > - Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with > 'unsubscribe' in the subject field OR by logging into www.freelists.org > > - Online, searchable archives are available at > //www.freelists.org/archives/rollei_list > --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx - Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org - Unsubscribe at rollei_list-request@xxxxxxxxxxxxx with 'unsubscribe' in the subject field OR by logging into www.freelists.org - Online, searchable archives are available at //www.freelists.org/archives/rollei_list