[rollei_list] Re: mirrors
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Tue, 15 Aug 2006 09:10:40 -0700
----- Original Message -----
From: "Mike Kovacs" <mskovacs@xxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Tuesday, August 15, 2006 4:39 AM
Subject: [rollei_list] Re: mirrors
Yes, and of course the prewar mirrors are shellac coated
silver, not aluminum. Quite often the shellac turns
yellow and slightly hazy on these old mirrors, so even if
there isn't any obvious black spots or missing silver, the
mirrors have terrible transmittance.
I have not found a way to clean these silver mirrors. The
shellac is the culprit but any sort of solvent will remove
the coating and the silver.
Richard is correct about the bloom on the viewing lens.
That viewing lens was among the haziest lenses I have ever
successfully cleaned. It literally took quite a scrubbing
with kim wipe tissues using 1:1 household ammonia :
household hydrogen peroxide. However, the "coating"
remains even after this treatment. I am certain it would
not have been possible to clean if it were a coated lens
with Zeiss T coatings of that era.
The only real defect on the camera that detracts from it
is the loss of plating on the front nameplate which shows
as brass-coloured spots. Its not a collector's piece but
really razor sharp with a nice uncoated look.
http://www3.sympatico.ca/mskovacs/pv/Rolleiflex/PrewarAutomat2.jpg
Here's some photos I took with the Automat II, including a
self-portrait I took with it, holding my venerable SL66
behemoth. Enjoy!
http://www3.sympatico.ca/mskovacs/pv/Rolleiflex/0306-2_Bridgedetail.jpg
http://www3.sympatico.ca/mskovacs/pv/Rolleiflex/0306-4_Ally.jpg
http://www3.sympatico.ca/mskovacs/pv/Rolleiflex/0306-6_Emily.jpg
http://www3.sympatico.ca/mskovacs/pv/Rolleiflex/0306-5_SPSL66.jpg
The tarnish does not affect the lens performance. If its
progressed far enough it acts like a lens coating and
improves the lens. However, tarnish usually appears only on
unprotected front elements. Some optical glass is more
vulnerable to oxidation. Modern glass catalogues carry a
"stain" rating indicating the sensitivity of the glass to
oxidation.
Usually, pure Isopropyl alcohol will take off internal
haze. The old standby lens cleaner for optical assembly is
reagent grade Acetone, but Acetone can dissolve paint and
lens older type Canada Balsam lens cememt, so it must be
used with descretion. I don't know what causes the haze but
its very common. My guess is that it is something
evaporating from the internal anti-reflection paint in the
lens cells. I have found some Rollei finder lenses, c.1950's
have what appears to be dropletes of oil on their internal
surfaces. I have no idea what this is but it comes off with
alcohol.
Older mirrors appear to be front surface silvered
mirrors. The silver surface must be protected by a lacquer
or else the silver surface will tarnish very rapidly. I
don't know if Rollei ever used aluminized mirrors. Aluminum
coatings are very resistant to oxidation. They do not have
quite as high reflectance as Silver to visible light but
maintain their reflectance for very long periods. Actually a
sort of reverse lens coating is used on aluminized mirrors
to increase the reflectance, which can then exceed silver.
It is possible to remove the lacquer with either alcohol
or sometimes a soluton of Sodium carbonate, depending on the
type of lacquer. In any case amateur telescope makers have
mirrors vacuum coated with Aluminum so services are
available to do this at reasonable prices. Probably the
Rollei mirror could just go in with a telescope mirror. Look
for an amateur astronomy club locally to find some telescope
builders.
The main cause of dimness in Rollei finders, assuming the
lens and mirror are in good condition, is the ground glass.
Rollei uses quite fine grain GG which has good diffusion,
however, much of the light from the lens continues through
the GG surface in the same direction it left the lens so
there is a hot spot in the center and dim corners. The hot
spot will move if you look at the GG from an angle. The
solution is to use a field lens to focus the light to about
were the eye will be. The usual field lens is a Fresnel type
consisting of concentric prisms molded into a flat surface.
The Rolleigrid is such a Fresnel field lens and works very
well. It just drops onto the ground glass. The Rolleigrid
was intended for post war Rolleis which have little spring
catches in the finder to hold it and also to hold the mask
for the Rolleikin. Most of the fancy replacement screens are
just Fresnel field lenses combined with the ground glass,
often on opposite sides of the same glass. There is only a
certain amount of light available from the lens so one
screen is not really brighter than another, the appearance
of increased brightness is due to the uniform distribution,
that is, the center is about the same but the periphery of
the ground glass _is_ brighter because the light transmitted
by it is being directed toward the eye.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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