[rollei_list] Re: 500/5.6 Tele Tessar for SL66 Comments
- From: "Ellestad" <ellestad@xxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sat, 6 Jan 2007 01:44:00 -0600
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
>
> ---
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