Primary Color Correction: Tonal Correction Tools - The Art and Technique of Digital Color Correction - page 60

thinking: “Wow! I'm unclipping all of the detail in the sky.” But you need
to see on the picture monitor whether you are really just introducing
noise and banding or posterizing. Be like a doctor: “First, do no harm!”
One of the problems that begins to emerge in this image as the tonal
range improves is that a lot of noise and artifacting of the video becomes
evident. If you have no means of correcting this noise, then you may have
to either limit how far you take your correction or determine that the noise
is a small price to pay for the improved contrast. Many color correction sys-
tems have noise reduction built in to them, and you can use that feature to
fix the noisiness that you introduced. The thing with noise reduction is that
it must be used with a very gentle hand. Always check the image before
and after noise reduction to make sure you haven't applied too much.
Isolating Tonal Ranges with Curves
Another cool thing about working with curves is that you can isolate
specific tonal ranges using points that you don't move so that other cor-
rections on either side don't affect them.
Let's try an example. To best understand exactly what this isolation
is doing, open the “grayscale_neutral” file from the DVD and load it
into an application with curves, like Color, Avid, Photoshop, or After
Effects.
With this example, what we want to see is how we can isolate a spe-
cific tonal range so that it is not affected by the corrections that you make
in another area. First, let's see what happens when we don't isolate the
tonal range.
With the “grayscale_neutral” clip loaded, look at the image on the
waveform monitor and on the video monitor. The levels are almost cor-
rect. The highlights are a little below 100IRE, but the black levels are cor-
rect and the gammas are just about right ( Figure 2.30 ) .
Now, let's adjust the shadow area with a point about 25 percent up
from the bottom of the master curve. Because the range is 1-255, that
that would equal an input value about 63. Hover the cursor over the
diagonal line in the master curve while watching the input value just
below the master curve graph. When it gets to around 63, click on the
line and drag that point down to around 40 while watching how the
trace of the waveform monitor reacts. You'll notice that while the major-
ity of the correction is in the bottom of the waveform monitor (the shad-
ows), the midtones and even the highlights are affected to some extent
( Figure 2.31 ) .
In some real-world corrections, you may want to adjust the deep
shadows without changing your midtones or highlights. The key to doing
this with curves is to add points on the curve that you do not move at all.
These points will protect the rest of the curve from moving.