A little further reading – Apple Shake 4 User Manual
Page 439
Chapter 15
Image Processing Basics
439
These types of controls are paralleled in the LogLin node with the black and white
point parameters. Every 90 points represents one stop of exposure. You can therefore
control exposure with the LogLin node.
Note: Some Kodak documents state that 95 points represents one stop of exposure.
Once images are converted to linear color, they can be treated like other “normal” linear
images. When a linear image is broken down to its steps of brightness, there is equal
distance between two steps in the dark areas and two steps in the light areas. In a
logarithmic file, however, the digital negative sees light areas differently than it sees
dark areas, as described by the logarithmic curve that it is stored in. The distance
between two steps in the dark areas is different than the distance between two steps
in the bright areas. This is why color corrections and compositing should be done in
linear color. This is explained in “
The Hazards of Color Correcting in Logarithmic Color
Once you have finished your compositing, the images must be converted back to
logarithmic representation (another LogLin node set to “lin to log”) and then rendered
to disk. These images are then properly treated by the film recorder.
A Little Further Reading
Two websites are recommended for more information on this subject. The first is the
specification for the logarithmic conversion and all of the parameters. It is somewhat
dense, but contains useful information:
The second recommended site contains a nice discussion of the film negative’s
response to light:
The Hazards of Color Correcting in Logarithmic Color
If the logarithmic format is so great, why bother to convert back to the linear color?
First, logarithmic color is unnatural to the eye—you have to convert back to linear color
to see it properly. More importantly, compression also means that any color corrections
applied in log color produce unpredictable results, since shadows, midtones, and
highlights have an uneven application in many color correctors.