Eric
I mentioned Duclós lenses yesterday, however they work with lenses
for movie cameras, they don't t mention still cameras lenses; Ferdi asked
about still cameras lenses re-coating, anyway it looks like they could do
the work for them.
Most lenses accept a minimal re-polishing degree keeping the optical
formula , in other words, keeping or recovering the original quality, the
problem for the technician is to know what is the limit to re- polish a
given lens.
Carlos
El lun., 26 de oct. de 2020 19:39, Eric Goldstein <egoldste@xxxxxxxxx>
escribió:
We need to spend a little more time with the sentence Richard included
here about polishing...
Polishing will change the curve and power of the lens. I don't think
most people realize this, and that's not a good story. I know DP and
cinematographer who sent lenses away to have a coating replaced and
were not happy about what they got back, even from highly reputable
outfits.
So the question to ask yourself is will the repair impede the
performance more than the loss of coating on one surface?
Here's a well-known, well-established Hollywood refurbisher that does
this work in the US
https://www.ducloslenses.com/pages/glass-polishing-recoating
Eric Goldstein
On Mon, Oct 26, 2020 at 6:28 PM `Richard Knoppow <dickburk@xxxxxxxxxxxxx>
wrote:
---
Coating is not trivial. Nor is re-cementing. I've tried
cementing a couple of lenses with poor success. The cement is
available, I used UV curing cement. One of the main problems is
centering. Old lenses were centered by grinding the edges, edging
its called. The old method was to paste the lens on one tube,
rotating in a vertical lathe, and shine a light on it. The
reflection is absolutely stationery when the lens is exactly
centered. The reflection is examined in a telescope while the
lens is pushed from the side. When exactly centered the lens is
clamped by a second tube from the top and the edge ground exactly
concentric and parallel to the optical axis. When this is done
the lenses were cemented and clamped by the edges. More modern
cementing is done by watching the reflections from the elements
after the cement is applied to find the exact alignment of the
elements. When centered they are cured and then the edge of the
entire cemented group is edged. I suppose the edges could still
be used to clamp the lens while it is recemented. There is
problem with lenses where the two elements to be cemented are not
of the same diameter. Those must be cemented first using the
second method and then edged.
Modern coating is done by vacuum deposition. The coating are
baked in vacuo. This is the method discovered in the US for hard
coating (although some claim Zeiss discovered it during the war,
I have seen no evidence where the U.S. discovery is well
documented). Multiple coating is a complex process which does
exactly as named, two or more coatings are applied and baked. The
coatings must be very precisely controlled for thikness. This
sort of vacuum deposition has been brought to a very high level
of perfection for use in making semi-conductor electronics.
Someone offering re-coating must have at least the means of
single vacuum coating. The early Kodak coatings, which were never
applied to outside surfaces, predated the discovery of vacuum
baking. the coating work but can be removed in the process of
ordinary cleaning.
Cementing requires curing. The cure can be heat, as for
Canada Balsam or some synthetic cements, or ultra-violet light as
in some later cements. If the surfaces are not absolutely clean
and precisely matched for surfaces (lost a word here) the cement
may not hold. Canada Balsam is sensitive to heat and, if not
sealed thoroughly at the edges, will slowly delaminate from
absorbed moisture or discolor from oxidation. The quality of the
edge paint seems to be critical. Synthetic cements will
delaminate due to incorrect curing. Sometimes the cement layer
developed gaps that look like large bubbles where the two glass
surfaces have pulled apart. Canada Balsam more often begins to
separate at the edges and in very old lenses turns yellow from
oxidation. If the ring of separation is small it will have little
effect on the performance while the bubbles will destroy the
lenses ability to form an image.
Polishing is another question. It is possible to remove a
damaged coating but if the actual surface of the lens must be
polished it will change the figure and affect the performance of
the lens. Generally, coatings are much easier to damage than
glass although some types of optical glass are pretty soft.
Be very careful of your lenses. The idea of cleaning with a
resuable microfiber cloth just makes me cringe. Nothing should be
used twice. You can make a one-time-use brush from a sheet of
lens tissue by rolling into a tube, tearing it in half and
folding the feathered edges together to form a brush. Use that to
remove dust. Any particle stuck on a cloth or brush can scratch
the surface. So called "cleaning marks" should never occur, they
are just plain scratches caused by careless cleaning.
Real lens cleaning fluid is Ammonium carbonate while Windex
is ammonium hydroxide. The carbonate is much milder while the
hydroxide, at least in theory, can dissolve some glass. I think
the glass is exposed to the cleaner for so short a time that this
is probably not a serious consideration.
I don't know where to get real lens tissue any more but
supposedly the makers of Scott and Kleenex are pretty good at
removing abrasives from their products.
On 10/26/2020 1:27 PM, F.W. Stutterheim (Redacted sender ferdi
for DMARC) wrote:
Carlos, Jan, John, Volker,
Thank you!
Ferdi.
Op 26 okt. 2020, om 09:30 heeft Volker Muth
<volker_muth@xxxxxxx <mailto:volker_muth@xxxxxxx>> het
volgende geschreven:
Hello Ferdi,
just by chance I came across Kanto Cameras in Japan. They
offer lens polishing and coating.
Here’s the link:
http://www.kantocamera.com/english/repair/repair.html
<http://www.kantocamera.com/english/repair/repair.html>
Best regards,
Volker
Von meinem HAL 9000 gesendet.
Am 25.10.2020 um 20:33 schrieb F.W. Stutterheim
<dmarc-noreply@xxxxxxxxxxxxx
<mailto:dmarc-noreply@xxxxxxxxxxxxx>>:
Yes, John van Stelten retired some years ago. I have not
found any other address for having lenses re-cemented and
re-coated. Not that I need one at the moment. Is such an
address known to members of the List?
Ferdi.
Op 25 okt. 2020, om 13:25 heeft CarlosMFreaza
<cmfreaza@xxxxxxxxx <mailto:cmfreaza@xxxxxxxxx>> het
volgende geschreven:
PS: I learnt Focal Point no longer exists,
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
WB6KBL
---
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