[pure-silver] Re: Swapping Out Lens Elements On Ektars
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Mon, 17 Mar 2014 20:40:30 -0700
----- Original Message -----
From: "Tim Daneliuk" <tundra@xxxxxxxxxxxxxx>
To: "Pure-Silver Mailing List" <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, March 17, 2014 5:53 PM
Subject: [pure-silver] Swapping Out Lens Elements On Ektars
I have a 127mm Ektar in a Supermatic shutter that has an
evil back element
that is showing fungus, separation, and - it looks like -
slight cracks.
It takes pictures OK, but I am not enthusiastic about it.
If I find another identical lens/shutter, can I just
remove the back element,
or both elements and put them on my shutter? The reason
I ask is that
someone is selling an identical lens with a crummy shutter
and my Supermatic
works pretty well.
I guess what I am asking is whether or not the lenses were
custom fit to each
shutter or if they're reasonably interchangeable across a
common shutter.
Similarly, say I found the same lens that someone had
reshuttered later.
Can the element be taken from such a combination on put on
my shutter.
(This is more of an academic question out of curiosity. I
assume that
if it can be done in the case above, it would also apply
here.)
Thanks,
--
I have done some element swapping on Tessars that worked
OK. In theory the front and back were chosen to work
together. In a Tessar type lens, and the 127mm Ektar is a
Tessar, the back component has the power and the front
component has most of the correction. I doubt if there is
much difference in spacing caused by the shutter. You can
try replacing the back component and see what happens at the
edges with the lens wide open, but, if you can get a whole
lens in a bad shutter why not just swap both elements? That
should give you a good lens.
Note that Tessars of all makes suffer from oblique
spherical aberration. This looks like coma although it comes
from a different place in the light path. Coma makes points
of light around the periphery look like tear-drop smears.
Its fixed by stopping down. A good Tessar should be pretty
much free of it at about 25degrees half-angle, when stopped
down about a stop and a half. For an f/4.5 lens it should be
comletely gone at about f/11 at the corners of a 4x5. This
is a larger image than the lens was intended to cover but
127mm lenses were popular on 4x5 press cameras for press
work because of the somewhat wide-angle coverage. The Ektar
will just about do it at f/11 and is sharp at f/16. Old
135mm Zeiss Tessars, once the standard lens for a Speed
Graphic, will also be sharp in the corners of a 4x5 at
around f/16. Some lenses, notably the Wollensak Raptar/Optar
must be stopped down to f/22 or beyond to be sharp in the
corners.
In any case, if you must swap cells this is a good test.
The other test is to see how much focus shift you get from
wide open to around f/8 or f/11. Focus shift is caused by
residual spherical aberration and is very common. Well
designed Tessars are relatively free of it but there is
always some. This, BTW, can be a problem when setting up a
rangefinder since the RF is fixed to the bed position and
the exact bed position for sharpest focus can vary with
f/stop.
Usually, the RF is set up for maximum opening with the
idea that depth of field/focus will accomodate the variation
at smaller stops.
A good Ektar is a very sharp lens. Even though the
Tessars were very good one can see slight differences in
texture rendition with the Ektar.
BTW, I've seen other lenses with what appear to be
cracks along some surfaces. I don't know what causes this
but all showed other signs of being subjected to high
humidity for long periods.
One more note: The coating on Ektars varies all over
the place. Kodak was not doing hard coating routinely on its
"consumer" lenses until after WW-2. These are marked on the
front retaining ring with the red L for "Luminized" Kodak's
trade name for hard vacuum coating. Some Kodak lenses were
soft coated as early as about 1939. Examples are the inner
surfaces of the lenses for the Ektra camera and the Eastman
Ektar, forunner of the Commercial Ektar. Also the lens for
the Medalist I had inside element coating. These coatings
were not baked in vacuo as were the later ones so can come
off with routine cleaning. Early small Ektars as used on
Speed Graphics, were not coated.
Since I am already way off topic I will add that some
old lenses look like they are coated because they have a
blue-ish reflection, usually from the front surface. This is
corrosion of the glass. In fact, it can act as a coating and
does reduce the reflection from that surface somewhat.
However, usually only the surfaces that have been exposed to
the atmosphere show the color.
--
Richard Knoppow
Los Angeles
WB6KBL
dickburk@xxxxxxxxxxxxx
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