[rollei_list] Re: changing lens formulas
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 4 Nov 2007 17:06:53 -0800
----- Original Message -----
From: "Marc James Small" <marcsmall@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, November 04, 2007 4:53 PM
Subject: [rollei_list] Re: changing lens formulas
Several of you, especially Eric and Richard, have
made really solid contributions to this thread.
One point to bear in mind, as both of them
pointed out, was that lenses were hand-assembled
and hand-calibrated up to World War II. What
they failed to point out was that automated lens
assembly methods came into use during the War,
especially in the US and Germany, and also to a
lesser degree in the UK and Italy. Automated
assembly is a compromise: you get rid of the
bottom level of poorly assembled lenses, but you
miss out on the very best which only human
assembly can provide. Since the 1950's,
automated assembly has been mated to automated
calibration, so that lenses produced for the past
thirty years have been much more uniform and much
better calibrated, all in all, than was the case in earlier
years.
I would not automatically toss a Prewar Magnar
out of the window, of course: some of these were
excellent. But they were all hand-assembled by
guys in the Zeiss telescope department, and
hand-calibrated. So some Magnars are better than
others, albeit MINE is a great lens.
By 2970, everything was automated and the only
human intervention was with a lens which would
not assemble or a lens which would not
collimate. This happens, of course. The East
Germans discarded such lenses, and the West
Germans reworked the lenses to make them
work. Now that the Germanies are united again, I
am not certain just how Zeiss or Rollei or Leica
handle those alarm-buzzer cases.
Marc
msmall@xxxxxxxxxxxx
Cha robh bàs fir gun ghràs fir!
I have heard many references to automated production of
lenses but I am not sure what this could include. The
process of making a lens begins with a blank. In the old
days this was a chunk of glass the right size broken out of
a "pot" of glass but a later method was to mold the glass
into the approximate size and shape for the element. The
elements were then ground on machines which ground many
lenses at once. Several steps are used to grind the lens
from rough grinding to finished lens. About the only hand
work I know of was the making of aspherical surfaces which
is done now by automated machines. AFAIK such operations as
centering and cementing must still be largely hand work
although the determination of the center can be done by a
sensor working an automatic centering machine. Centering is
very important especially for elements wich are to be
cemented.
Lens mountings must be quite precise but are the sort of
machining that can be done by CNC machines.
To some degree the exact curvature and thickness of an
element can be varied to compensate for small variations in
glass constants. Usually, the glass is measured before its
use in production to verify its constants. While glass
manufacture has advanced a great deal there are still some
small variations. These can affect all optical aberrations.
I simply don't see that automated manufacure and
assembly results in more precise production although it
probably lowers cost. The fact is that some very fine lenses
were made before automation.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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