[rollei_list] Some Experiences with Re-cementing Lenses
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: "New Rollei List" <rollei_list@xxxxxxxxxxxxx>
- Date: Wed, 20 Jul 2005 20:54:53 -0700
I posted this to the other Yahoo Rollei List and am posting
here in case its of interest. This version includes some
minor corrections. If anyone spots other errors please
notify me via private e-mail.
Some Experiences with Re-cementing Lenses
I recently re-cemented two lenses. There was a learning
curve and I want to share what I found out.
The two lenses were a Kodak Ektar f/4.5, 152mm and a
Wollensak Raptar Series 1a, 13", f/6.8
The Ektar is a Tessar type lens, the Raptar is a convertible
Protar type lens.
There were several false starts on both lenses. Initially, I
tried a two part heat curing cement. At the suggestion of a
correspondent, John Hendry (pict@xxxxxxxxxx)
I tried using an Ultra-Violet curing cement, I wound up
using the latter. Both cements were obtained from Summers
Optical.
John also suggested using Methylene Chloride to separate the
lenses. I was a little spectacle of this but tried it and
found it superior to the high temperature solvent also sold
by Summers.
Here are some of the things I found:
1, Clamping of the lens edges must be done using a machinist's
V-block. The makeshift methods suggested on various web
sites, such as using prisms, simply does not work reliably.
Proper edge clamping is imperative for proper centering.
2, UV curing cement is much easier to use than the binary
heat curing type, at least for small numbers of lenses.
The cement I used is Summers UV-74. This cement is one of
the few that can be shipped as a non-hazardous material. The
hazardous materials shipping on the other cements just about
doubles their prices. UV-74 costs about $15 US for an ounce,
enough to cement many lenses. Its shelf life is very long if
stored in the refrigerator.
3, The official method for separating lenses cemented with
synthetic cements is to boil them apart in a special solvent
at 340 degrees F. It takes about five minutes for the lenses
to come apart. However, its necessary to have a
thermostatically controlled hot plate for this. I have a
small laboratory hotplate but an electric frying pan can
also be used.
There is a definite hazard of damaging the elements when
using this method. At this high temperature the glass is
very sensitive to shock. I got edge damage on nearly all the
elements I cooked apart and will never use this method
again. In some correspondence with John Hendry he suggested
the use of Methylene chloride for de-cementing. I was at
first a little spectacle but I obtained a good supply from
Tri-S Sciences before they closed forever and tried it. The
lens is simply immersed in the solvent for several days. I
found that the UV cement would separate in about three days.
This is all done at room temperature and there is no damage
to the elements. I re-cemented both lenses several times
while learning how to do it. The edge damage from the
initial boiling apart does not seem to interfere with the
lens performance.
I obtained Methylene Chloride from Tri-S Sciences before
they closed but the same compound is available as paint
stripper from the hardware store.
For older lenses cemented with Canada Balsam decementing is
easily done with gentle heating. The traditional method is
to heat the cemented elements on a hot plate but I've found
that hot water works fine. Start with warm water and gently
heat it until the elements slide apart. I don't know the
temperature, and it probably varies, but its well below
boiling.
Canada Balsam is cleaned with Acetone and alcohol as is
described for synthetic cement below.
In the past I have tried recementing with Balsam but its
actually more difficult than with a synthetic and is not a
satisfactory cement.
Machinist's V-blocks are rather expensive new. I was lucky
to find a used one, they seem to be very difficult to find
used, but there is no substitute for one for reliably
clamping the lenses.
Method of applying cement.
I followed pretty much the instructions on the Summers
Optical web site. I also came on two books at the public
library on lens manufacture published by Adam Hilger Co., an
English maker of precision optical instruments. The methods
described were about the same. Actually, the method does not
differ substantially from that used for Canada Balsam.
Cleaning the elements.
I soaked the lenses in Acetone first. Then carefully wiped
them and rinsed them in more Acetone. Then I cleaned them
with Anhydrous Isopropyl Alcohol. This is obtainable at many
drugstores and is not expensive. I then washed the elements
in water with dishwashing detergent and rinsed them well. I
used a final rinse of alcohol just before cementing. The
room I used is reasonable dust free. Keeping the surfaces
free of dust is very important. Probably the best solution
if one has a dust problem is to build some sort of closed
chamber with access holes for your arms and a window. I did
not need anything this elaborate.
Small particles can be removed from the lens surfaces by
using sticky tape, like Scotch tape, to pick them up. This
tape does not seem to leave any residue.
I applied the cement with a wire made from a paper clip. Do
not be stingy with the cement. Make a small pool in the cup
of the concave surface. Then put the two elements together
and press down gently to spread the cement out until it
begins to leak out the edges. Air bubbles can be worked out
by gently sliding the upper lens over the lower one. Slide
them back and forth and vary the direction but do not use a
rotary motion.
After as much excess cement comes out as can be expelled
with gentle pressure, scrape off most of the excess with a
razor blade or some other convenient tool Some excess should
be left in place.
Now, place the lens, concave side down, in the V block and
clamp it. I do not have a machinist's flat so I use a small
sheet of plate glass. I clamp the V-block to the glass using
a C-clamp. The clamp for the lens is part of the V-block.
Inspect the lens to make sure its flat on the block and that
the edges are aligned. Then apply the UV light for about a
minute.
Summers UV-71 cures with an ordinary "black light" bulb. I
have a type BLB but a BL should work as well. This is a
tubular type lamp but there are now "compact" screw base UV
lamps which will fit into a desk lamp or other convenient
fixture.
After about one minute the cement will have 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 trade name
"Velostigmat". Wollensak was one of the first companies to
coat lenses and to use synthetic cement. Mechanically, their
lenses are very well made.
Both the camera and lens were probably military issue. The
specification for the lens is probably the same one that
applied to the Goerz Dagor and Gundlach-Manhattan
Turner-Reich lens, maybe others. This lens is a Convertible
Protar type with cell focal lengths of 20" and 25". It is an
excellent lens, quite sharp and with relatively little zonal
spherical aberration, a property of all double meniscus
lenses. It covers 8x10 sharp to the corners at about f/16
although its optimum stop is probably f/22. The individual
cells have very good performance with relatively little
color fringing. They are sharp to the edges at around f/32,
which is pretty good for this type of lens. Dagor halves
should be stopped down to f/45.
It is very much superior to the T-R lens, not really
surfriding.
This lens is in a Wollensak Alphax shutter. These are
excellent large size shutters, IMO better than the
equivalent Ilex types.
Wollensak lenses are of variable quality, due mostly, I
think, to design problems, but the
company was capable of making excellent lenses on occasion,
this seems to be one of them. Wollensak shutters have always
been excellent.
A note on centering.
When a lens is made it is centered after grinding and
polishing. Because the surfaces of most lenses are segments
of a sphere they will automatically center around the axis
when clamped between two ring shaped surfaces.
Lenses to be cemented rely on the edge of the lens as the
reference surface for location of the elements during
cementing. They require more precise centering than single
elements. The centering method is a fairly simple one. The
element is held on the end of a rotating tube with s
flexible cement, like pitch. A small light source is shined
on its surface and the reflection examined through a
telescope. If the lens is not centered on its axis the two
reflections will appear to orbit each other. The lens is
repositioned on the tube until the two reflections are
absolutely stationary. Then the lens is clamped by a second
tube above it and the edge ground exactly parallel and
concentric to the optical axis. For cemented elements the
absolute diameter of the lens is important because the edges
are what are used to hold the assembled elements in place
during cementing.
In some lenses the elements are not of the same diameter.
The Schneider Angulon is an example of such a lens. When
lenses of this sort are cemented the edges must be held by a
special fixture. Its possible to center the two elements in
a way similar to that used for the original centering and
edging but a process like that would be uneconomical in
production. I don't know of a practical method of
recementing unequal diameter lenses on a small scale.
In summary, I am glad I undertook this project. It was quite
frustrating at times but I now have the knowledge to
recement most lenses, which may be valuable in the future.
It was certainly worth the effort.
I've probably left something out. I will be glad to answer
any question, provided I know the answers.
Again, I thank John Handly for his advise and support.
Summers Optical is at:
(http://www.emsdiasum.com/Summers/optical/cements/default.htm)
The Summers site has a very good primer on lens cementing
including preparation and full technical details on their
various cements.
The letter referring to Methylene Chloride is at:
(http://astro.umsystem.edu/atm/ARCHIVES/AUG99/msg00231.html)
July 20, 2005
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