Olympus-OM
[Top] [All Lists]

Re: [OM] Sunburst Lake, Bob Marshall Wilderness

Subject: Re: [OM] Sunburst Lake, Bob Marshall Wilderness
From: Ken Norton <ken@xxxxxxxxxxx>
Date: Tue, 12 Jun 2012 09:54:44 -0500
> Care to describe your "5 pass sharpening process"?

The act of scanning is actually that of taking a picture of a piece of
polyester that happens to have some form of image on it. The scanner
is a digital camera with a lens.

In a single pass scan, this is a single picture. The problem we run
into with single pass scans is aliasing artifacts where grain (or dye
clouds) are sampled in a such a way that we are likely to get wild
swings in the values of each pixel. So, for the highest level of
quality we overscan. Each time the scanner photographs the image,
there is a very tiny offset as the stepping motors don't quite get
things exactly aligned. With this multi-pass scan, we effectively
increase the diameter of the sampled area for each pixel. This reduces
the grain aliasing.

Unfortunately, overscanning like this tends to reduce sharpness. I
have found that there is no specific setting you can use to bring the
image back to maximum sharpness. For one thing, this is so film
dependant (and scanner dependant), that the settings which work for
the original Provia 100 won't work for Provia 100F. Velvia 50, 100 and
100F all scan differently too. Let's not even talk about print films.

So, overscanning reduces sharpness. OK. But does that mean that the
information is gone? Of course not. This is where the imaging software
as used by CSI comes in. I only need six pixels to be able to recover
an entire page from a newspaper. ;)

Not really.

Basically, what we have to do is apply a deconvulution style recovery
of the image. This may involve multiple passes of USM with varying
diameters.  If you apply the diameters in the wrong order you will end
up with halos and other crunchies. One thing I very much appreciate
about PWP is the sharpenening algorithms which are a bit better than
what Adobe uses. OK, quite a bit better...

I find that with a massive overscan (5-10 passes), I need about five
passes of sharpening. WIth a single or dual scan (HDR style scan), I
can only do two passes before things get nasty.

The old film vs. digital comparisons assumed a simplistic approach to
scanning and detail recovery. I am, by no means, an expert on these
things, but I'm definitely able to recover a whole lot more detail
than what the talking heads have claimed possible.

One little trick of the trade about scanned film vs. pure digital
imaging. Using a very good resize algorithm (again, like the one in
PWP), I can downsize my 20 megapixel equivalent scan to a 10 megapixel
equivalent image, apply another round of post-scaling sharpening
(again, using the awesome sharpening algorithm in PWP) and end up with
an image that may be superior to a digital camera image of similar
size. As we all know, a 10 megapixel camera is capable of some pretty
incredible stuff. Or has Bob been yanking our chains with his Maine
pictures all along? Once reduced in size and sufficiently cleaned and
sharpened, the image can then be upsized exactly in the same manner as
if it originated in a digital camera in the first place.

Now, let's talk about equivalents? Is a 20MP digital camera going to
outperform a 20MP scan? Of course! I figure that an idealized scan of
a decent slide film is going to come in at about 10-12MP. However,
those 10-12MP are per-pixel sharp with no aliasing. A sharp edge is a
sharp edge, not a two or three pixel transition. (Or in the case where
two opposite colors come together like red and green, you don't get a
gray line inbetween).

Unfortunately, we now run into other limits: Provia has the lateral
halation problem which tops out maximum usuable resolution. Lens
handling techniques, which result in defraction blurring. And then the
dreaded OM system aperture actuator vibration. It's a rare non-flash
image that will maximize a 4000 dpi scan. Flash? Sure. Even the Gary
Reese Lens Tests reveal how difficult it was to maximize sharpness.

When do I use multi-pass overscanning? Not for every shot. In fact, I
usually only use it for something that I know will end up printed big
or when there is so much dynamic range which needs to be protected or
recovered. The churn and burn stuff is usually single-pass. The latest
versions of Kodak Portra do just fine with single-pass. Slides with
extreme dynamic range, such as "Sunburst Lake", need a lot more work.
A LOT more work.

The point of this epistle, is that with a creative and disciplined
approach to scanning technique and image recovery, there is a whole
lot more detail buried in these old film images than what we generally
give the technology credit for.

When I'm in the darkroom, I use a grain focuser when making a print. I
actually have three of these grain focusers. One of them is a very
nice one which I managed to score from Bill. With my two 'affordable"
ones, I thought I could see and focus the grain. On a bad day, I can
see more detail in this highly magnified image than what my scanner
has ever been able to recover. But then with Bill's higher-end
focuser, I saw stuff I never saw before--particularily the point when
diffraction was kicking in with the enlarger lens! The point is that I
see far more detail in some of the negatives than what the
digitization process is able to reveal.

So, as an engineer, I start comparing the differences and then try
different techniques to get the two technologies to match. This is
what led to the discovery of overscanning combined with multi-pass
sharpening. As to being able to recover everything possible from a
Ilford Pan-F negative? Based on what I see, it would take at least a
10000 dpi scanner to come close. By no means have I "arrived" with a
final set of conclusions, or techniques. It remains a work in
progress.

To give credit where credit is due, I wouldn't have figured out this
process without a little hint from my Dad. He is a retired toolmaker
in the aerospace industry who spent the last dozen or so years of his
career building prototypes and one-off devices using what was at the
time the most accurate grinder in the world. (Probably still is, the
latest stuff uses lasers to get even closer). His claim to fame was
being able to actually get an additional 400% improvement in accuracy
than what the technology could ever allow.. The techniques he
developed became trade secrets, but I learned enough from him to be
able to apply a similar logic to the scanning process.

AG
-- 
Ken Norton
ken@xxxxxxxxxxx
http://www.zone-10.com
-- 
_________________________________________________________________
Options: http://lists.thomasclausen.net/mailman/listinfo/olympus
Archives: http://lists.thomasclausen.net/mailman/private/olympus/
Themed Olympus Photo Exhibition: http://www.tope.nl/

<Prev in Thread] Current Thread [Next in Thread>
Sponsored by Tako
Impressum | Datenschutz