Newbie question: lenses

I don't have any lenses of this kind at all, but I believe I've
read that they're relatively difficult to make well - something
about precise centering of the cemented elements. And at any
rate, for whatever reason, one lens might be quite a bit better
or worse than the next. Whole different world from today's
Schneider large format lenses like the Symmar XL.

So, you never know, your odds are good but it's something to check.

Donn

(CamArtsMag) wrote in message ...
I believe this lens has an image circle of 154mm and an 81 degree angle of
coverage (these numbers are taken from a secondary source, not from
Schneider, so it's possible they're off a
little).

Any thought of identifying your secondary source? It might be helpful to the
questioner and to others who might have the same question if they new of a
place to go look for this type of info.

steve simmons

The 81 degree figure is from the Schneider web site. This is for
f/16. At smaller stops the coverage is greater, up to about 90 degrees
at f/45. Schneider's claims for this lens have varied over the years.
Because of the oversize outside elements the coverage before
vignetting is quite large. Schneider's old claim of 102 degree
coverage is true for _illumination_ but the quality of the image is
poor beyond about 90 degrees. This is no greater than the Wide Angle
Dagor. The performance of the Angulon for distant objects should be
slightly better than the Dagor because of the slight assymmetry, I
don't know if this is true in practice. I think one reason the Angulon
has a bad reputation is Schneider's somewhat poor pre-ww-2 quality
control. However, that seems to have been turned completely around for
post-war lenses.
As I mentioned in my first post the original Angulon had a design
error. I don't know what they did wrong but the actual lenses show
large color fringing. A computer analysis of the patent prescription
shows a large amount of spherochromatism. I have a very early Angulon,
In fact it was built before the patent was issued and has a DRPa on it
(patent applied for). The serial number dates it at 1929. The color
fringing is bad enough to make the lens useless. Too bad, its in mint
condition. Later Angulons do not have this defect and are decent
lenses. I am quite puzzled as to how any production lenses could be
made with this awful performance. Surely, they must have discovered
the problem in optical shop models. I thought my lens was simply the
result of poor QC until the similar poor performance of the patent
design was pointed out to me. It would be interesting to hear from
others with very early Angulons about what their performance is like.
Angulons have the same problem with focus shift due to zonal
spherical aberration that Dagors do, this is inherent in thick
meniscus lenses of this general type.
Super Angulons, which are of a completely different type, are
superior in a lot of ways but are larger and heavier so the Angulon
remains a useful lens.

"Donn Cave" wrote in message news:1075615453.736833@yasure...
I don't have any lenses of this kind at all, but I believe I've
read that they're relatively difficult to make well - something
about precise centering of the cemented elements. And at any
rate, for whatever reason, one lens might be quite a bit better
or worse than the next. Whole different world from today's
Schneider large format lenses like the Symmar XL.

So, you never know, your odds are good but it's something to check.

Donn

Cemented lenses must be centered so that all of the elements are
exactly coaxial and the axis is perfectly concentric. This is done
by precision grinding of the edges. The Angulon is particularly
difficult because the front and rear elements are larger than the
others. So, not only must it be accurately ground but the lens must be
clamped in a precision fixture after cementing.
All lens elements are centered but those to be cemented must have
much greater precision of gringing of the edges since the edge is the
reference surface for cementing.
The old procedure is to place the element on a tube of something
less than the lens diameter. The lens is held on by a thick resin. A
point source light is shown on the lens and its reflection observed
through a telescope. If the lens is not centered the reflection
rotates. The lens is moved on the tube until the reflections from both
surfaces stand still during rotation. The lens is then clamped by
another tube from the top and the edge ground so that it is exactly
concentric. In the usual case where the diameters of all the cemented
elements are of the same diameter the edge is used to clamp the lenses
together. The more elements to be cemented the more critical the
centering operation becomes. For air spaced lenses the precision of
the edge is not so important because it is not used to center the
lens. The usual method of mounting lenses is to clamp the element
between two rings, one on each side. The pressure of the rings on a
spherical surface will force the lens to be centered in them. The edge
has just enough clearance to allow this. This trick works only for
spherical surfaces, aspherical surfaces must be individually centered
in the mount by optical inspection methods.
Cemented lenses must also be individually polished to final figure
so that the cemented surfaces fit accurately. This, plus the
additional centering operation, and the additional operations needed
for the actual cementing, make cemented surfaces expensive.
Cemented surfaces were popular with lens designers before good lens
coatings became available. A cemented surface reflects very little
light so their use is a way of reducing lens flare. Lens coatings
reduce glass-air surface reflection to nearly the same level so air
spaced designs are much more popular now than before about 1945.

question about the lens, a Schneider Angulon 90mm f/6.8. I know it's old;
the serial number is is mid 5 million, making it a two years my senior.
The
trouble is, I can't seem to find any information about it on the web. What
I
did find conflicted with everything else. I'm looking for just the angle
of
coverage and coverage at infinity focus. It was mere curiosity before; now
that I have it in my hands, I'm concerned.

http://www.schneideroptics.com/info/...format_lenses/

http://www.schneideroptics.com/info/...enses/angulon/

http://www.schneideroptics.com/info/.../6,8-90mm.html