Digital BW, The Print

Paul,

You can see how some of these things works just by opening an image
and looking at the histogram.

Bring up the brightness/contrast slider and watch when you make a
brightness adjustment. Here all that is happening is a fixed value
(either positive or negative) is being added to every pixel value in
the image hence the histogram slides as a whole up and down the range.

The contrast slider and the curves slider can be thought of as
behaving in a similar way. As you increase the contrast the data will
fall off the sides of the histogram as more pixels are assigned values
0 and 255 and the number of tones represented in your image decreases.
You can see this as evidenced by the walls that start going up on the
left and right sides of the histogram while the number of pixels
making up the remaining tones in your image decreases the middle of
your histogram begins getting lower and lower. Make a large enough
adjustment and you will be left with just 2 tones, black and white
with no data in the middle of the histogram.

In reality the contrast adjustment is likely more elegant than the
simple scenario I described in my first missive on this subject. You
can get a picture of how it works by doing something similar with the
curves control. Place 3 anchor points equidistant on the curves
control with one in the center, resulting in the line being divided up
into four equal length segments. Now take the two non-centered anchors
and move them equal but opposite distances (resulting in an s shaped
line) and you'll see the histogram behave in exactly the same way as
with the contrast slider. The curves control can dump pixels just as
quickly as the contrast control. The big difference is that with
curves you have much more say in the re-distribution of those pixels
as opposed to the programmer that coded up the contrast implementation.

I didn't mean to say that going to 16-bit doesn't allow you more room
with editing at all. My comment at the end of my first email on the
subject was merely to state that I don't do it very often, simply
because 1. I rarely shoot 8-bit and 2. when I do I rarely am off by so
much that I need to make large adjustments to the 8-bit file.

Derek

P.S. I've been a pretty quiet lurker on this list for a couple of
years now. Its good finally contribute something in a substantial way.

> Second, the "Brightness/Contrast" adjustment is different than the
curves.
> With the former I believe you slide the entire image scale up or
down and
> are much more likely to push things out the end, with a resulting
loss of
> information.  With the curves, you'll compress one side and stretch
out the
> other, with a much lower chance of losing information.  Again, this
is in
> the context of an 8 bit file that has been converted to 16 bit such
that it
> has headroom to absorb the compression without loss of information.
> 
> Paul
> www.PaulRoark.com
>