Canon Camera Hackers Manual page 185

176
C H A P T E R 5 Scripting
Lightning
Lightning photography seems to be a passion among some photographers.
The problem, however, is that thunderstorms are quite unpredictable. The
National Weather Service (http://www.weather.gov/) and other Internet
services may help to locate thunderstorms.
The next problem—after you have located a thunderstorm—is to catch
a lightning flash. Simply opening the shutter for a longer period of time is
not really a good option; if there are other light sources in the scene, the
resulting picture might be overexposed.
Fortunately, practically all lightning flashes are preceded by a smaller
preflash. This preflash is sufficient to trigger a lightning sensor and a con-
nected camera. If the camera is fast enough, it will capture the main flash.
Such lightning sensors are not cheap. Fortunately, most CHDK users don't
need one; they can use the camera's image sensor for the same purpose.
The lightning scene program of our script is configured for speed. We
have chosen a fairly coarse 4x4 grid, a short detection interval (1 millisec-
ond), and no delay before shooting. By specifying a subsampling value of 6,
we analyze only every sixth pixel, again resulting in fast operation. Also, we
prefocus before motion detection starts unless a fixed distance is set in
parameter u. Autofocusing directly before the shot would definitely be too
slow to catch any lightning flash. Every 55 seconds (d=55), the motion de-
tection command times out. In this case, the program loops around, does
another cycle of prefocusing, and then waits again for motion. The script
reacts to changes in the YUV brightness channel (c=1) and uses a low
threshold for high sensitivity.
How should you expose for lightning? Using automatic exposure is out
of the question. Instead, you should use Overrides (section 4.3.1). As a start-
ing point, set the aperture to f/5.6 and the sensor speed to ISO 100. For
cameras without a diaphragm, dial in the ND filter (which gives you an
equivalent of f/8.0 at wide-angle zoom setting) and select ISO 200. Choose
an exposure time that results in the desired scene rendering. Then put the
camera on a tripod, start the script, select the Lightning program, press the
FUNC/SET button, and wait.
WARNING! Lightning photography is dangerous. Each year lightning kills
numerous people. Please refrain from photographing lightning in the open. If
you happen to be outside during a thunderstorm, do not seek protection un-
der trees. Instead, find a ditch or a surface depression and duck down with
your legs closed. A safe place is also inside a car (not a convertible). The safest
way to photograph lightning is from inside a building.