-- "How Do I Know When to Stop?"
If you're new to using a scanner, or perhaps even not so new, the process of getting
an acceptable image as well as what constitutes a final image can at first be a
total mystery -- it sure was for me. A paucity of useful books on this subject
only aggravates one's attempts to address one's ignorance. No matter how good
appearing the scans one gets, unless one develops an understanding of the
underlying concepts and principles of operation, one is left with the nagging
suspicion that the final scan could have been better.
Another reason for such a measure is that if you
have a high-end scanner and want that last 2% of performance -- which
differentiates the scans of an excellent scanner from an average one -- you need
to be able to quantify just how good each image is.
else does one justify the premium paid for such a scanner?
“I Can Tell My Scanner Is Good Because
the Results Look Great.”
You see, that was the nub of the problem. Once
one acquires a minimal competence in adjusting images one is able to adjust
images scanned under a wide range of scanner settings and get acceptable results on the
monitor. I would make several scans
of the same image, each with different settings, and after tone and color
adjustment end up with images of apparently equivalent quality.
Which one to save? Obviously you want to save the best one, but they all look the
same. What is needed is an
objective measure of scan quality that eliminates variables such as monitor
performance and the subjective judgments of tone and color adjustment.
Because I tended to use my scanner intermittently, I accumulated a pile of
notes over the years. In an effort to organize and consolidate these notes,
mostly for my own convenience, I decided to put them into Web pages. Hopefully
these also may be beneficial to others. Basically these pages outline, more or less,
the evolution of a personal approach to scanning.
To be honest, this is not simple material, but when I reflect on how many
hours I've wasted making scans, I realize that I would have saved most of those
hours if I had spent just a few of them learning this material beforehand.
The Context for This Tutorial
Often a properly scanned image will look much better than an image scanned
carelessly at twice the resolution. This tutorial emphasizes a quality of
data approach to film scanning over quantity (resolution). And as a
practical result, it advocates a
two-step, professional-type of workflow for image production:
In the scanning step, one scans for maximal data quality, i.e. capture
image data as accurately possible. The resultant image constitutes the
In the image production step, one or more final images are derived from
the final scan in an editing program and are modified through tools such as
filters, cropping, sharpening, etc. What distinguishes these final images is
that they are modified to address different needs by abstracting from the final
scan. A final scan with high data quality as a resource has more potential
for meeting more needs.
Most amateurs and beginners attempt to accomplish all this in a single step and as a
result must re-scan the image as different needs arise. If the
final scan contains all capturable data, re-scanning is superfluous. This is the
main theme of this tutorial. Histograms and curves are merely the tools that are used
as aids in extracting every bit of data and thereby implement this approach.
Goals of a Scanning Procedure -- First Principles
To me, getting a quality scan should almost be a mechanical process. I don't
mean that one works like a mindless robot; only that at each step one is able to
read and interpret the diagnostic tools available and from them know exactly
what to do next. Part of this requires a good scanner with software with the
necessary features. The other important component is, of course, a competent operator who
knows what to do.
The abiding idea the operator follows is that in scanning the name of the
game is accurate and comprehensive data capture while maintaining the
relationships among the pixels. If this sounds a bit dry and mechanical, I would
argue that's how it should be. Having good original scans as a resource gives
you more data to work with when you exercise your creative impulses later in the
image-editing program. That's where you alter and make abstractions of your
original scans. A good final scan may not look snappy and may not grab your
attention, but it will stand-up to your manipulations in the image-processing
program so that the final product does look snappy and grabs attention.
Scanner Sensitivity -- The Limiting Resource
The typical consumer film scanner reads each pixel as an analog signal from
an array of charge-coupled-device (CCD) sensors, which sweep longitudinally
across the film plane. An analog to digital converter translates it to an
integer from 0 to 255 (in 8-bit mode) or 0 to 4095 (in 12-bit mode). This is
done for each of the red, green, and blue color channels. These readings are
passed to the scanner software, e.g. NikonScan, which uses a user-specified
function to transform the tone and color to an output number, also from 0 to
255, which is stored in the resultant image file. Unfortunately the CCDs in
non-professional scanners have a very limited sensitivity range and are often
incapable of capturing all of the tonal values in an image. In addition, because
they operate at the limits of their capabilities, they are susceptible to
electrical noise artifacts. To minimize these artifacts and make optimal use of
the limited range, the user is forced to define how the image is to be scanned
by assigning weights to areas of the image.
The 256 tones are just sufficient to form a visually continuous gradient. The
problem is that once the scan is made one is working with discrete (integer) values.
each adjustment of contrast or brightness, some of those values are aggregated. After several adjustments,
an area containing a smooth tonal gradation may degenerate into visibly distinct
LS-2000 Process Schematic
© 2000 - 2008 by D. Kosaka. All rights reserved