Datasheet
22
c h a p t e r 1: DIGITAL IMAGING BASICS, WORKFLOW, AND CALIBRATION ■
Building a Camera Calibration
RAW processing software is designed to interpolate color information from varying
brightness of pixel sites that have a colored filter of red, green, or blue. In essence,
there is no color in the RAW data—color is inferred by measuring the brightness
of pixel clusters. The colored filters covering the tiny pixel sites are not perfect and,
manufacturing processes being what they are, there is some variation from camera
to camera even from the same manufacturer. Some RAW processing software is set
up to allow for a certain amount of adjustment to calibrate the camera’s color to
a known target. Adobe’s processing software offers this capability though sliders
in the Calibrate tab in ACR; Lightroom offers the same capability on the Camera
Calibration panel. There is also a stand-alone application, DNG Profile Editor from
Adobe (in public beta at the time of this writing), that simplifies the process.
DNG Prole Editor
DNG Profile Editor is an application designed to modify the lookup tables that Adobe
software uses to convert the raw data from the digital camera into a standard color
workspace. Every camera supported by Adobe Camera Raw or Lightroom has a
lookup table associated with it to enable the raw processor to de-mosaic, or render
color from the brightness data recorded under the red, green, or blue pixel sites on the
chip. The DNG Profile Editor uses various controls to manipulate the color result gen-
erated by the lookup table and regenerate the table, and it has a straightforward cali-
bration process that utilizes the X-Rite ColorChecker 24 Patch Classic. The software
will do an automatic analysis of an image capture of the target and generate a table
that can be saved and used by Lightroom or ACR. For our purposes, I will use the tar-
get analysis function to build an idealized color calibration.
First, select the ideal exposures from your test shots under the different color
temperatures and convert the files into DNG format. In Lightroom, go to the Library
module and select Library
> Convert Photo to DNG from the menu bar. The DNG
Profile Editor only works with DNG files, so you will need to convert all the good
exposure shots before opening them into the software. ACR requires that you save the
files as new DNGs.
Launch the DNG Profile Editor application, and you will be presented with a
simple color editor (the first tab at the top of the interface) and the helpful instruction,
“Tip: Start by opening a DNG Image from the File menu” (Figure 1.22).
Open the shot from the open shade test and then click the Chart tab
(Figure 1.23). The image should open with the white balance you set in Lightroom or
ACR. If for some reason it doesn’t, simply click on the light-gray patch to reestablish
a white balance. Four small colored dots will appear in the image; they are referred to
as “colored circles” in the on-screen instructions.
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