[rollei_list] Re: Lens Tests
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Sun, 27 Mar 2005 15:03:05 -0800
----- Original Message -----
From: "Dennis Purdy" <dpurdyphoto@xxxxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Sunday, March 27, 2005 8:50 AM
Subject: [rollei_list] Re: Lens Tests
> Eric
>
> Thank you for going to such effort to create and share
> your results. I
> personally appreciate compulsive obsessive behavior.
>
> On my monitor, the second close up of the machine screw
> head seems far
> and away sharper in the 3.5E image. Your analysis says it
> is slightly
> less sharp and contrasty than the Mamiya 7 image. Perhaps
> you got the
> jpgs mixed up? Of the 4 cameras on that test the 3.5
> Xenotar looks
> significantly sharper.
>
> I was personally of the opinion that pristine, haze free
> lenses are
> nearly impossible to find on Rolleis older than the F.
> You undoubtedly
> made certain of that. A little bit of haze would create a
> bit of flare
> and perhaps flatten the image some and maybe lower
> apparent resolution?
>
> I am glad to see someone else finally noticed the flare of
> Planar
> lenses. You mention that both the 2.8E Planar and the
> Hassy Planar had
> flare bleeding off the image. I had owned several Rolleis
> for sometime
> but always Xenotar or Tessar. Then last year I decided I
> wanted to get
> the best and I picked up a 2.8F Planar. The first thing I
> noticed was
> flare, especially using close up Rolleinars. With a back
> lit situation
> I got so much flare the image was unusable, with bleed off
> into the
> next image. The people here in RUG assured me that
> Planars don't have
> more flare than Xenotars. I decided that I must have a
> bad one. I
> could detect with a flashlight shinning through the lens
> that there was
> a slight flaw to the front element coating. I finally got
> so
> frustrated with the flare that I sent the camera in to
> John Van Stelten
> to have the lens polished and recoated. With shipping
> costing me 400
> dollars. 3 months later I got the camera back and the
> flaw was gone
> and the lens was crystal clear. I immediately did several
> flare tests
> and the lens was a little better but the flare was still
> there. It
> still bled off into the next image if there was sky or
> back lit
> situations.
>
> 3 days after finally getting that 2.8F Planar back from
> Stelten I ran
> across a very late 2.8F Xenotar on ebay. It was in the
> 2957xxx serial
> numbers and looked to be nearly pristine, and it had been
> posted on
> ebay late at night for $865 "buy it now". I bought it.
>
> When I got that camera I checked the lenses and they were
> like brand
> new, unmarked and absolutely clear. I had gone way over
> my budget for
> Rolleis so I had to decide right away which of the two to
> sell. I ran
> some very precise tests with both the Planar and the
> Xenotar using the
> same heavy tripod and switching the cameras on the tripod
> after every
> test so that the cameras were in exactly the same position
> and
> everything. For sharpness I did a large wall of news paper
> making very
> certain to have the cameras squared up to the wall and
> paper. And for
> flare I did a series of tests with and without Rolleinars
> shooting a
> dark subject against back lighting.
>
> For the sharpness test I put the negs in my enlarger at
> maximum height
> and looked at them through a grain focuser. They were
> identical at all
> fstops. The Xenotar might have been just a tiny bit
> sharper in the
> middle at f8 but I nearly gave myself several new floaters
> in my eye
> trying to see it. I already knew the Planar was sharp so
> mostly I just
> wanted to see that the Xenotar was just as sharp and it
> was.
>
> For the flare test I just looked at the film on a light
> box. The
> Planar lost that contest big time. The Xenotar had none
> of the obvious
> bleed off the edges of the exposure that the Planar had.
> And I could
> see that the Planar negs had significantly more density in
> the shadow
> (zone 1,2,3) areas. The extra density was flat looking
> and obviously
> from light scatter. The Xenotar negs were perfect.
>
> I know that Richard has said on numerous occasions that
> there should be
> absolutely no difference between the Xenotar and Planar in
> either
> sharpness or flare, but at least in the cameras I tested
> there was a
> difference. The Planar I had wasn't absolutely pristine,
> but dang
> near. I think it had one light scratch on the rear
> element. And there
> was a very fine (nearly invisible) mark left on the inside
> by Van
> Stelten when he recoated the lens.
>
> So I am done stressing over which camera I should have. I
> am happy as
> can be with my 2.8F Xenotar and I sold the Planar for
> quite a lot more
> than I thought I would get on ebay. Now to spend more
> time taking
> pictures.
>
> thanks
> Dennis Purdy
>
This and other posts speak of "bleed" at the edges as
flare. If, by this, is meant the spreading of highlights its
more likely spherical aberration. Flare takes two forms: one
is the reduction of shadow contrast, the other is ghost
images. The first kind of flare can be caused by the lens or
by reflections in the camera. It doesn't affect highlight
contrast much but reduces shadow contrast in much the same
way as a long toe film. Ghost images are obvious. They are
usually of very bright objects in the image but sometimes
can be of extra-image objects. Sometimes they are fairly
sharp, in other lenses they are blurred to blobs. They move
as the camera is moved. Flare is caused by reflections
between surfaces between two media of quite different index
of refraction. There is some reflection from cemented
interfaces but the cement typically has an index close to
the average for glass so its very small. Between glass and
air the difference in index is very larg so glass has a very
bright reflection unless treated with a coating. The coating
provides a thin layer of material with an index midway
between the glass and air. When made the right thickness it
will completely eliminate reflection at one wavelength and
from one angle. When multiple coatings are used the indeces
are tapered in a way that extends the low reflection over a
wider range of wavelength.
Spherical aberration is shows up as a broadening out of
the image since it is really a combination of many images of
slightly different focus. Spherical is uniform everywhere in
the field unlike coma or oblique spherical which are
proportional to the image angle. One effect of spherical is
the creation of a blur or cloud around bright highlights.
This is the effect desired in soft focus lenses.
It is surprizing to me that actual Planars have more
spherical than Xenotars. Both types are very well corrected
but the data generated from the patent prescriptions for the
two show the Planar being very slightly better as far as
spherical than the Xenotar. I am speaking here of the
original five element f/2.8 Planar used in Rolleiflex TLR's.
The Planar used in Hasselblads shares the name but not the
design. It is a traditional Planar or Biotar type with six
or seven elements. The f/3.5 Planar and Xenotar used in
Rolleiflexs is also a Biotar rather than a five element
type. All of these are capable of extremely good correction
which is why the Biotar/Opic type has been the standard for
f/2 lenses for decades.
Note that the broadened out highlights can also come from
defocusing. Even very slight focus errors can result in this
effect. I would double check the lenses being tested on film
by examining the aerial image using a very well corrected
low power microscope to see what the lens is actually doing.
This eliminates focus errors due to mismatch between finder
and taking lens or film plane inaccuracy.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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