Canon Camera, Hackers Manual

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4.3 Exposure
41
when you need lots of light: for example, when you want to shoot with an
ultrashort exposure time (section 4.3.5).
By the way, if you enforce the manual flash but it doesn’t work, don’t
blame the CHDK; you might just need to charge the battery.
4.3.8 Using curves
Custom Curves are another option in the CHDK to control the outcome of a
shot. Curves are applied after an image has been taken; they don’t influ-
ence exposure settings such as aperture or sensor speed. They simply
modify the digital data delivered by the sensor before it’s packed into a
JPEG file.
This can make sense. The sensor delivers the image data with a finer
dynamic granularity than a JPEG file is able to capture. Each pixel consists
of 10 or more bits, allowing the capture of at least 1024 levels of bright-
ness. A JPEG file only transports 8 bits per pixel. Due to the logarithmic
scaling of pixel values in JPEG format, the JPEG file does a good job on
shadows but not so good a job on highlights. Therefore, burned-out high-
lights are a common problem when shooting in JPEG.
One solution is to shoot RAW (section 4.5). A RAW file delivers RAW
sensor data—the complete information without any loss. Using a RAW
converter, you can properly compress the tonal range of the image to avoid
drowned shadows and burned-out highlights.
Is there no solution for shooting JPEGs? In fact, the CHDK offers one.
Curves can be used to do all kinds of tone mapping, including contrast
compression. The result can be a JPEG with detail in both highlights and
shadows. Because curves are applied when mapping the RAW data onto
JPEG pixel values, they do not have any effect on RAW images. And because
the CHDK can shoot RAW and JPEG simultaneously, you can have both a
curve-modified JPEG and an untreated RAW file as a backup.
The Enable curve entry in the Custom curves submenu offers the follow-
ing options:
f
None . No curves are used for tone mapping.
f Custom . The pixel values are mapped under the control of a custom
curve. Such custom curves can be created with the help of a curve editor
(see below). The curve definition file must be placed into the folder
CHDK/CURVES/ . The curve can then be loaded under menu entry Load
curve profile… .
f
+1EV . A system-defined curve that increases shadow detail by one
f-stop. Unlike the EV correction discussed in section 4.3.1, this option
will not increase noise, open the aperture wider, or increase exposure
time. It will, however, reduce contrast.
Figure 4-26
The Custom curves submenu