[rollei_list] Re: xenar test roll is back
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Fri, 5 Feb 2010 01:49:08 -0800
----- Original Message -----
From: <daxelb@xxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Thursday, February 04, 2010 5:01 PM
Subject: [rollei_list] Re: xenar test roll is back
Hey Richard,
I defer to your (far) greater knowledge and I'd thought the
same thing-- my lens is old and may not be as clean as it
should be--but I've printed with two other lenses ( also
old) that pretty much correspond with contrast of my lens.
I'll certainly use your recommended flash light method and
lord knows I've had that lens for over twenty-five years and
I don't doubt that I will find the evidence you describe but
I have used my friend's lens on old negs and gotten superior
results-- not sharper just more contrasty than I was ever
able to effect originally. I think, but wouldn't argue, that
it is just an inherently more contrasty lens.
Have your heard of this before? I guess my question is: are
there variations among same (new) lenses? Or is consistency
the non-hobgoblin of little lenses?
David
Its possible that its something else but I would
certainly check for haze, its very common. There are
aberrations that mimic flare, particularly spherical
aberration. This is what most soft focus lenses depend on
for their softness. SA, unlike some other aberrations is
constant over the field. It varies with the stop. In a
well-designed lens the SA over the normal field will be gone
at about two stops down. Other longitudinal aberrations show
up more as you move away from the center but are also
reduced by stopping down. Since the Nikon enarging lenses
are amoung the best I rather doubt if any have enough
residual SA to cause this problem. I could be wrong but
would look elsewhere first.
Beside internal haze any damage to lens surfaces will
also have the effect of scattering light. Also, if there is
any degradation of a cemented surface it can also affect
contrast. The latter can also often be seen using the
transmitted light examination. Old lenses were cemented
using Canada Balsam, a natural resin. More modern lenses are
cemented with synthetics. The synthetic cements are stronger
and are not affected by extreme temperatures as is the
balsam but they can still fail. The failure is usually due
to some error in their application. If the cemented surfaces
were not perfectly clean of if the cement was not cured
properly it can develop wrinkles or even voids. I've seen
wrinkles in some old Kodak lenses. The lens appears find
until light is shown across the cemented surface and
examined with a magnifier. Then you will see a sort of
orange peel look. That is diffusing enough to seriously
affect contrast. Other lenses may appear to have large
bubbles between the elements. This is where the cement no
longer adheres to the glass. The history of the lens
probably also has some effect on this. I've seen the bubbles
in both Wollensak and Zeiss lenses.
Its also possible for lenses to lose correction through
mechanical damage of the mounts, that is, the lens spacing
or parallelism is lost. Usually that comes from a lens being
dropped hard but also usually the damage is apparent. I do
not think this accounts for your low contrast lens.
Cleaning internal surfaces requires disassembling the
lens. This varies in difficulty depending on how the lens is
made. Some lenses have slots or dots on the retaining rings
for a spanner, which makes life easier, others have plain
retaining rings which must be removed by use of a friction
tool, essentially a tube with something sticky on one end.
Other types of retainers are caps on the back. In any case,
beside getting these loose one must be very careful to note
any spacers or shimming washers in the lens and their exact
orientation along with the correct orientation of the lens
components. Even simple lenses can be confusing: for
instance, the finder lens used on Rollei cameras is a simple
Triplet type. However, the center element is nearly
symmetrical. It is very difficult to tell which way its
facing visually, but, if its installed facing the wrong way
the lens will barely even form form an image.
For the most part cleaning a lens is something which can
be undertaken at home provided one is careful and has a
clean place to work. The haze will respond to any ordinary
lens or glass cleaner. I use Kimwipes to clean lenses and
use each wipe once only. Of course, if the lens appears to
be hazy it can be sent off to one of the lens experts who
clean them professionally.
Cleaning should be mentioned here. I do not approve of
"microfiber" cloth or fine camel's hair brushes or anything
else that is used more than once. While a microfiber cloth
may be excellent at picking up dust from the lens it also
holds anything it picks up so that any grit it picks up is
there to scratch the next time its used. Its better to use
disposable tissue. Kimwipes or Tekwipe are intended for this
kind of use. For routine cleaning a one-time brush can be
made by rolling the tissue up, tearing it in half and
folding the feathered ends together. Use this ONE TIME and
toss it. For routine cleaning with lens cleaner place the
dry tissue on the lens, apply some lens cleaner, and drag
the now wet tissue off it with no downward pressuer. Toss it
and, if necessary do it again. Use lens caps, its easier to
keep things clean than to remove dirt once it gets on.
So called cleaning marks, which are clusters of tiny
scratches cause a lot of image degradation and are
completely avoidable.
FWIW, a single deep scratch has little optical effect.
In fact, a lens can be cracked in half and still perform
well if the two sides are in intimate contact, but bunches
of fine scratches act just like ground glass.
--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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