■ Just a few of the camera’s buttons you can control with
CHDK. Scripts allow you to automate the pressing of the
buttons you would normally use to set up your camera.
The set_zoom command changes the camera’s zoom
setting. Cameras zoom in steps (not in analog like manual
cameras) and the amount of zoom is specified by the
variable n. Depending on the camera, it can have 9, 15, or
129 zoom steps. Related to setting zoom in absolute
terms, the zoom can be adjusted from its current setting
with the following command:
This is a relative setting where x can be a positive or
negative number. The value of x is added to the current
zoom setting to move it in or out. A camera’s desired
zoom setting can be dependent on the camera’s current
zoom setting. To get the current zoom setting, use the
The current setting of the camera’s zoom is loaded
into the variable x. The value in the variable x can be
evaluated and acted upon with an if-then statement.
Related to the shoot command is the shoot_half
command. This command is equivalent to pressing the
shutter button only half way. Doing so refocuses the
camera and is necessary after zooming the lens since
cameras don’t refocus themselves after zooming. For a
near space mission, I might use a camera at a minimum
zoom setting to get a context picture of the ground and
then take a second picture at a higher zoom. Cool, isn’t it?
The print command displays text on the camera’s
LCD screen. For example, a script would use print
“picture” n to display the word “picture” followed by the
number stored in variable n. The following script snip
takes a picture, increments the number of pictures taken,
and displays the results on the camera’s LCD screen. It’s
useful for determining how successful a near space
mission was upon recovery, but before you get a chance
to extract the SD card.
print “picture” n
This command powers off the camera. It’s used at the
end of a camera’s sequence of actions to shut the camera
Scripts can get pretty complex. For example, a camera
can monitor a part of its field of view and when a change
occurs within that field, the camera can take a picture.
This allows people to use Canons to record wild life and
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