Re: Enlarger lens options.


"Jean-David Beyer" <jeandavid8@xxxxxxxxxxx> wrote in message
news:rbfVj.1212$OT1.79@xxxxxxxxxxx
Richard Knoppow wrote:

That's quite interesting. I suspect the Rodagon and
Componon designs are
quite similar although I don't have the actual
prescriptions. Since its
likely all the relatively modern lenses were designed
with the aid of
computers I suspect the performance should be much alike.
I think the
Focotar is an older design. One of the characteristics of
the generic
Plasmat type, which is what the Rodagon and Componon are,
is that they
can be unusually well corrected for astigmatism. Forgoing
an explanation
of what exactly that is (different in a camera lens than
in opthalmic
lenses) it leads to being able to get a very flat field.
While both
manufacturers claim superiority I suspect its pretty much
a draw. Some
think the Rodagon is mechanically superior to the
Componon. Do your
lenses have metal or plastic iris blades?

When I first got into 4x5" work, in the mid 1970s, I got a
Schneider
Componon-S f/5.6 to f/45 180mm enlarging lens, # 11 973
xxx. It works fine.
Its diaphragm has lots of blades (about 19 of them),
enough to make the
aperture look round, and they appear to be metal. At least
10 years later, I
got a Componon-S f/5.6 to f/45 150mm lens, # 14 588 yyy.
It has only 5
blades, so the aperture looks approximately like a
pentagon (except the
edges are not quite straght. I cannot tell if they are
metal or plastic; the
180 blades are shinier than the 150 and darker, reminding
me of blackened
brass (but I do not know what they are for sure), and the
150 blades are
duller, but slightly lighter in color and rougher,
reminding me of anodized
aluminum (but I very much doubt they would actually be
aluminum). Perhaps
that is what plastic blades look like.

For normal photograph use, is there any benefit to having
a round aperture?
I know in half-tone work with a sealed half-tone screen,
there is a benefit
to having a square aperture, but round holes work OK --
you just get a
little bit of a different transfer function from the
original to the half-tone.

Unless you use the lens in the hot sun or something, there
might even be a
slight benefit to having plastic iris blades: less likely
to rust or corrode.

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It appears that the shape of the iris affects out of
focus areas of the image. This is perhaps part of the effect
called bokeh by the Japanese. In any case bright points
which are not sharply focused are rendered in the shape of
the iris. This may not be as noticable for enlarging where a
flat surface is imaged onto another flat surface.
In making half-tone plates the iris is imaged by the
half tone screen as an array of spots or dots. By using a
square aperture the intestices of the dots are at the
corners so the variation is smoother. For color work each of
the images is photographed using an iris with a
lozenge-shaped aperture at a different angle. I can't
remember now if this is to prevent moir but I think it is.
In any case there is an optimum set of angles for the
apertures. These apertures are usually in the form of
Waterhouse stops and is the reason process lenses usually
have a slot in the side. My barrel mounted Apo-Artars have
the slot but the shutter mounted one does not. The barrel
mounted Artars also have 20 blade irises and a very nearly
perfectly round hole.


--
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx



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