[rollei_list] Re: changing lens formulas
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 4 Nov 2007 16:42:39 -0800
----- Original Message -----
From: <aghalide@xxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>; <rollei_list@xxxxxxxxxxxxx>
Cc: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Sent: Sunday, November 04, 2007 3:49 PM
Subject: [rollei_list] Re: changing lens formulas
After 4 years at /RIT and two weeks working for the
fashion editor of the Ladies' Home Journal I was
recommended by C.B. Neblette for a position at Modern
Photography Magazine to replace the technical person who
left to write books and brochures about Polaroid products.
That was 1957. Upon returning from 6-months active duty in
Arkansas I worked with Optical Expert Ben Sherman to
create a lens testing method. Ben decided to use the U.S.
Airforce targets and test for resolution and call it lens
sharpness.
This worked most times, but failed manytimes. Zeiss lenses
for the Zeiss Contarex tested badly for resolution and
great for sharp images. Zeiss seemed to be designed to
give high contrast, as did many Nikkors which also failed
in resolution. Minolta did well with resolution and fair
with contrast. Leica lenses did well in both cases. So we
didn't publish low contrast lenses because we were really
testing for high and low resolution not contrast. If the
lens failed a test we didn't publish the results.
Many years later they added contrast to the lens test. Ed
Meyers
Edge contrast and resolution are often trade-offs in
design. Kingslake discusses this in his books on lens
design. It can be measured optically but can also be
calculated using one of the lens optimization programs.
These will show the energy distribution in the image for a
point source and other measures. One of the compromises is
the balance of high order aberrations. The eye generally
interprets edge contrast as sharpness, in film this is
called acutance but there is no specific term for lenses.
One can design a lens with both good edge sharpness and high
resolution but it becomes more complex. Cost and difficulty
of manufacture are factors in any lens design as they are
for other products. Also, in general, the more complex a
lens is the more sensitive it is to variations in the
components. This is also a cost problem.
A problem with using test charts is that its difficult to
analyse field flatness unless many tests with slight shifts
of focus are made. This can be seen more easily by examining
the aerial image on an optical bench. One can also look for
coma and oblique spherical this way although its difficult
to distinguish one from the other.
The lens optimization programs will tell you much more
than you want to know about a design. There is a pretty good
freeware program called OSLO available on the web. The free
version is limited to I think 12 surfaces. Two must be the
object and image planes but that leaves you with enough to
analyse six element Planar and Plasmat types. I don't know
of a freeware program for more complex lenses.
Most of the lens prescriptions available come from patent
literature and are often incomplete or unreliable,
especially older ones. The incomplete data is usually the
complete glass dispersion spec.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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