[LargeFormat] Recementing Lenses - Part 2
Richard Knoppow
dickburk at ix.netcom.com
Thu Jul 21 16:45:07 EDT 2005
cured enough to allow removal of the elements from the
clamp. At this time clean the excess cement from the edges
using Acetone and a scraper. When the edges are clean
inspect them for parallelism, they must be exactly in line
all around.
If you are cementing only two elements (as in the rear
component of a Tessar) the tacked elements are now replaced
under the curing lamp and cured for an hour. Actually, I
cure for longer and turn the lens periodically to insure
even exposure to the lamp. You can not over-cure the cement.
The Wollensak lens is a Protar type with four cemented
elements in each half. I cemented this one element at a time
tacking each before going to the next. UV cement allows
this. If you use a heat cure cement on must partially cure
each layer before going on to the next. This takes about
twenty minutes per surface where the UV cement takes about a
minute.
My original attempt at cementing this lens was a disaster. I
tried to cement all the surfaces at once. The lens slipped
in the makeshift clamp I was trying and I had to cook it
apart and redo it. Actually, I had to redo each cell a
couple of times anyway because I discovered some flaw each
time. I think its still not perfect but probably as good as
I can get it.
Once cured, the edges of the lens are painted with an
anti-reflection paint to prevent internal reflections. I
have yet to find an entirely satisfactory paint. In the past
I have recommended Krylon Ultra-Flat-Black, but find that
the small particles which are responsible for the excellent
absorptive qualities of this paint also result in the
coating being too thick. This will prevent the lens from
going back into the cell. The best material I've found so
far is a water based flat black paint from a hobby shop.
This is a flat black for model steam locomotives. I have
tried painting with marking pens of the Sharpie type but
find that the coating is not dense enough for proper light
absorption.
Lens mountings.
The Wollensak lens came in cells with threaded retainers.
The front cell has a threaded front ring. At some time in
the past this lens was worked on and someone drilled two
holes in the ring. This makes getting it off easier but is
not good practice. Generally, a friction wrench should be
used. The rear cell had a retaining ring on the back which
came off pretty easily.
The original anti-reflection paint was some sort of very
thin but very dense and very absorptive material. This may
have been 3M "Velvet", a paint intended for the purpose but
discontinued many years ago and unobtainable now.
The Ektar, as is common with Tessar type lenses, had the
rear component fixed in a "burnished" or "spun-in" mount.
The cell of this type of mount is made with a thin lip
around the opening for the lens. The lip as a ridge around
its edge on the lens side. When the lens is placed in the
mount the lip is spun down over it on a lathe. This is an
excellent type of mount from the optical and mechanical
standpoint but is difficult to open for re-cementing. I was
lucky. I tried prying up the lip. I used a small jeweler's
screwdriver to start the lip and then a larger one to get it
up all over. The lens slipped out with a little coaxing once
the lip was up. The lip on this cell was resilient enough to
withstand being pried up without breaking. When the lens was
finished I was able to fold the lip back down over the edge
of the lens using the edge of a small screwdriver and a
small block of brass.
Both of these lenses seem to work well. I checked for
decentering in two ways: the first was by simply rotating
the lens mount in the shutter and watching the reflections
of a small light in the glass. Any movement of the
reflections relative to each other indicates decentering. An
improperly centered lens must be decemented and recemented.
The other method of checking for centering was to examine
the marginal image on a ground glass screen while rotating
the entire lens. Any change in the character of the image
indicates decentering. The V-block resulted in perfect
centering.
Both visually and photographically both of these lenses seem
to perform very well. Next time I will not have the problems
with edge damage since I will not attempt to boil the lenses
apart.
Note that some types of optical glass are very fragile. Even
when cold these will chip or crack if shocked or dropped. It's
a good idea to work over a pad of some sort, even just
layers of paper towels.
A note on these two lenses.
The Ektar is one I acquired at some camera sale in dim,
distant past. For a long time I had in mounted on an
Anniversary Speed Graphic. I decided to check it for
internal haze one day and found that while the front cell
was clean the rear cell showed a haze. On closer examination
this turned out to be some turbidity in the cement layer. I've
seen this on other Kodak lenses, mainly old aerial lenses.
The haze was enough to cause flare. The only cure was to
re-cement the lens. Kodak began to use synthetic cement
early on and by the 1950's, when this lens was built, all
Kodak lenses were cemented with a synthetic. The failure
mode of synthetic cements is not the same as for the older
Canada Balsam. Balsam tends to oxidize and yellow at the
edges, and, eventually crystalize. It is also limited in the
temperature range it will withstand. If subjected to
excessive heat or cold it will become milky.
Synthetics can become turbid and also separate in large
bubbles. Bubbles also sometimes appear in Balsam layers but,
usually, they are small bubbles, often thought to be in the
glass. The bubbles in synthetic cement layers are from the
cement no longer adhering to the glass. This might be due to
defective cement, a problem in curing, or poor preparation
of the glass surfaces prior to cementing. This is the type
of failure the
Wollensak lens had. I suspect it may have been mistreated
because it also showed some signs of mechanical damage, and
had obviously been worked on before.
I obtained this lens on an 8x10 Agfa/Ansco view camera I
bought many years ago. The camera itself required
rebuilding. The cement had dried out so the camera virtually
fell apart. It was not too difficult to restore it and its
looking over my shoulder right now.
The lens was just on it and useless.
This was a post war lens, it is coated and has the Raptar
name rather than the older Wollensak
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