[LargeFormat] Recementing Lenses - Part 2

[Richard Knoppow]

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