[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
about
an APO lens is that the lateral chromatic
aberrations
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
of
light. Secondary spectrum includes all colours not
brought to a common focus,in other words Salomon
says
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
definition
is according a D.I.N. definition.
And this an optical technician opinion:
http://geogdata.csun.edu/~voltaire/tmb/definition.html
All the best
Carlos
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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