"John Hendry" wrote in message news:jgyRc.55606$gE.30814@pd7tw3no...
"John Hendry" wrote in message
news:intRc.52767$gE.21416@pd7tw3no...
----- Original Message -----
From: "Richard Knoppow"
Newsgroups: rec.photo.equipment.large-format
Sent: Thursday, August 05, 2004 4:25 AM
Subject: fog inside lens cell


snip
Cement is available from Summers Optical.
The easiest to use is probably their Ultra Violet curing
type UV-69 This will cure with a BL BLB UV lamp. The
technique is described on their web site.
http://www.emsdiasum.com/Summers/opt...ts/default.htm
snip

No experience with UV-69 but I know firsthand that J-91 also works well
and
cures with a black light fluorescent - 1min to set and 1 hour to cure. The
relative merits of these two isn't entirely clear from the website but
Summers are a helpful bunch on the phone. For separating cells I find
methylene chloride and patience (a day or so) is a low stress method that
works well.


One thing that has occurred to me in the past relates to alignment of the
cemented elements (I use two steel v-blocks). One generally assumes that the
elements have been ground individually with the optical centres bang in the
middle. On the last lens I did, I marked the orientation of the lens cells
with a diamond scribe (very light scratch on the ground edges) prior to
decementing so I could reorient them identically on recementing. I don't
really know whether the elements are rotated against one another in the
factory on an optical bench to minimise any slight relative eccentricity in
the grinding (slight off centre optical axes) to find the best orientation
for cementing. Can anyone confirm whether such a step is taken, or are
lenses factory cemented purely on physical alignment of the ground edges
with no regard for axial orientation? i.e. grinding stage is absolutely
perfect with centred optical axis. Having thought about it since I assume
this must be the case or you'd end up with very inconsistant quality as the
number of elements increased.


The elements are precision ground so that the edges are exactly
concentic and coaxial with the axis. If the lens were to be rotated as
you suggest it would mean that the entire cemented lens could be
mis-centered. Since even slight decentering can cause a rather large
degradion of the image its important that the cemented lenses be very
carefully centered when made.
The type of mounting commonly used for spherical surface elements
automatically centers the surfaces in the mount. The mount clamps the
lens between two rings, the minimum distance is when the surfaces is
centered. Cementing procedures must rely on the precision of the edge
grinding for alignment.
The old method of centering is still used with some
modification. The traditional method was to fasten the lens to the end
of a thin walled tube wtih flexible cement. A point sourc of light is
then reflected from the surfaces and observed through a telescope. The
lens on the tube is slowly rotated and the reflections observed. If
the lens is not centered the reflection will trace a circle. The lens
is pushed around on the soft cement until the reflections from both
surfaces are absolutely steady. A second tube is then clamped down to
hold the lens in this position and the edge ground. Modern lens making
machines probably use lasers to provide a more sensitive measure of
eccentricity but the method is still essentially the same. When the
elements are cemented they are held in something like a V block. If
the edges are correctly ground the lens will be properly centered.
Some lenses, for instance the Schneider Angulon (not Super Angulon)
have elements of different diameters cemented. The edge is still the
reference point. It is likely that some pre-war multiple cemented
element lenses were never correctly centered. The Turner-Reich lens,
which has five cemented elements is an example. Getting these centered
correctly practically means re-grinding the edges. Not worth the
effort for the T-R.
A correction to my original post partly quoted above. The cement I
am currently using is Summers UV-74. This is a UV curing cement which
does not have to be shipped as a hazardous material. The shipping cost
for some of the cements exceeds the price of the cement. UV-74 cures
fine with a type BLB black light bulb and should also cure fine with a
BL type. The difference between these is that the BLB has an envelope
of Wood's glass to eliminate visible light, the UV produced by both
lamps appears to be the same.

Richard Knoppow
Los Angeles, CA, USA