Digital BW, The Print

on 9/27/01 1:10 AM, Maris V. Lidaka, Sr. wrote:

> A 'good' histogram is not the holy grail - a good image is, even if the
> histogram looks lousy.  Ask Dan Margulis about histograms - you'll get an
> earful.


Maris,

You may remember I had brought up some of these topics which we are
discussing around here now on Dan's list a couple of months ago. Below is
Dan's response to me. I think some of you might enjoy Dan's take on things,
I know I always do...

Todd

********

Todd writes,
 
>>I'm rather confused about the interaction/relationship of these concepts:
Dynamic Range, Bit Depth, and the Histogram.>>

Most people would say that these are three totally different concepts, but
in fact they are linked by a couple of common factors: 1) the effect of
random noise; 2) each one has managed to give birth to a myth that has
caused a lot of trouble to the color community.

What's random noise? Well, suppose you have two digital thermometers on
your wall. One of them tells you that it is currently 73 degrees. The other
one has more LEDs, and tells you that it is currently 72.86473 degrees.

The second one *sounds* more impressive, but in reality no thermometers are
that accurate. The last four digits for sure, and probably the last five
digits, are meaningless, empty numbers, of no validity at all. And yet they
are there.

This example relates to your three terms as follows.

*Dynamic Range* means the ability to discern detail at extremes of
lightness, darkness, or color. Positive film, for example, holds detail so
subtle in the deepest shadows that no scanner or digital cameras can see
it. Drum scanners do slightly better at picking up this detail than even
the most expensive CCD devices, so we say that the drum scanners have more
dynamic range, though less dynamic range than there is in the film.

The myth is that when the camera or scanner encounters something that's
outside of its dynamic range it hits a brick wall and just clips. IOW,
assume a digital photograph, or a scan from film, of a black cat at night.
If the scanning device really can't resolve the darkest area of the cat
because of inadequate dynamic range, some people suppose that what will
show up there is pure black. Not so. The scanner won't fail altogether, it
will *think* it sees something there and will try its damndest to show it
to you. Unfortunately, what it will show you is the last four digits of the
thermometer--just random pixels, meaningless noise, not black, but not a
cat either.

*Bit Depth* is a measure of how many distinct shades might theoretically
appear in a single channel. Each bit that's added doubles the number of
potential shades. The norm, 8-bit depth, has 256 possible shades, 9-bit
512, and 10-bit 1,024. When Photoshop encounters a file with more than
8-bit depth, it converts it to 16-bit, which has 65,536 possible shades.

The myth is that expanding a file to 16-bit somehow makes it more accurate.
Assuming for the sake of argument that your scanner really can record 1,024
accurate values per channel, going to 16-bit just packs it with the last
four digits of the thermometer: useless data, random numbers, mere noise,
of no statistical validity, of no benefit to the image. Furthermore, the
more I've studied it, the more I doubt that any current scanning device is
capable of getting even 500 accurate shades per channel, let alone 65,000.

The *Histogram* is an invention of the evil one for the express purpose of
retarding progress in the graphic arts. More people have been deluded by it
than by any other feature of Photoshop.

The myth is that a smoother histogram equates to a better-looking image.
Since the histogram doesn't reveal anything about the *significant* areas
of an image as opposed to the background, it's of extremely limited utility
in evaluating image quality. Nevertheless, if you have to guess at which of
two versions of an image will look better based solely on a histogram,
you'll be dollars ahead in the long run if you bet on the one that's less
smooth. The reason, again, is noise. The more random the image, the more it
resembles the last four digits of the thermometer, the smoother the
histogram will be. In the case of the image of the black cat discussed
above, the histogram for the shadows will definitely look better in the CCD
scan that is nothing but noise as compared to the drum scanner that
captures a certain amount of detail.

Dan Margulis