Exposure; Overrides - Canon Camera Hackers Manual

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C H A P T E R 4 Teach Your Camera New Tricks
4.3

Exposure

Most compact cameras come with automatic exposure only. Usually they
support a variety of automatic program modes such as portrait, landscape,
sports, kids&pets, night shot, sunset, snow, etc. However, the smaller and
lower-priced models usually do not feature aperture priority, exposure
time priority, or a fully manual mode. There are simply no provisions to dial
in an aperture value or a shutter speed. If they offer a manual mode, it only
allows dialing in an exposure correction value, and that's it.
The CHDK has set out to remedy this situation. It allows overriding the
values determined by the camera's exposure system with values dialed in
by the user. This is possible for aperture (or neutral density filter), exposure
time, sensor speed, and flash power. In addition, the CHDK offers a highly
customizable Auto ISO system and advanced features for exposure control,
such as a live histogram and zebra areas. We will discuss those features in
the following sections.

4.3.1 Overrides

As already mentioned, the CHDK allows overriding aperture, exposure
time, ISO speed, and flash power. This works in all camera modes, auto-
matic and manual. So, for example, if the camera is in automatic mode and
you override the exposure time only, the camera will automatically choose
the right aperture; the camera will be in Shutter priority ( Tv) exposure
mode.
But not all cameras can do this. Cameras equipped with an ND filter
instead of a diaphragm cannot adjust the aperture and completely revert
to manual mode. In this case, you must adjust the ND filter state and the
ISO speed yourself to obtain a properly exposed image. The density of the
ND filter depends on the maximum aperture—the ND filter simulates a
setting of f/8 at the focal length at maximum aperture. So, if the maximum
aperture is f/2.8, the density of the ND filter will be three f-stops. With a
telephoto setting where the aperture is perhaps only f/4.9, the aperture
simulated by the ND filter would be around f/14. With the CHDK in place,
you can normally keep the ND filter out and use very short shutter speeds
when conditions become too bright. An ND filter makes sense if you need
rather long shutter speeds for compositional reasons (waterfalls, architec-
tural images, etc.).
So, with diaphragm-less cameras your options for exposure control are
somewhat limited. You can't just open or close the aperture to compensate
for variations in shutter-speed. Instead, you must adapt the sensor speed.
For example, if the camera computes a shutter speed of 1/100 sec and you
override it with 1/1000 sec, you have to make up for the 3 1/3 stops less