Canon Camera Hackers Manual page 46

The other shutter type is the leaf shutter located in the camera lens.
Here, several leaves quickly open the lens and then return to their original
position. Because of this movement reversal, shutter speeds cannot be very
short. For cameras with interchangeable lenses, each lens must be
equipped with a shutter, and an auxiliary focal plane shutter is needed to
cover the light-sensitive area during a lens change.
Compact digital cameras use a simple leaf shutter that is not very fast.
Its main purpose is to shield the CCD sensor from light while the image is
read. The exact shutter speed is determined by the point of time when the
image data is read. So the CCD determines the shutter speed, not the me-
chanical shutter—and the CCD can be very fast, indeed. It also allows for
using the flash with all shutter speeds. Most Canon compact cameras use
CCD technology, though some newer models such as the SX1 use CMOS
sensors.
CMOS sensors work differently. They do not need a mechanical shutter
but work similarly to a focal plane shutter. Only a range of rows on the
sensor—comparable to the slit between the curtains of a focal plane shut-
ter—is light-sensitive at a given time. This range travels across the sensor
area just like the curtain slit travels across the focal plane. Therefore, CMOS
sensors tend to distort fast objects similarly to the traditional focal plane
shutter. They also need slower synchronization speeds when using flash.
The CHDK can take control of the electronic shutter and offer shutter
speeds down to 1/100,000 sec. That is more than the cameras can physi-
cally do, but in some cases real shutter speeds down to 1/40,000 sec. have
been reported. A list of findings for different camera models is available at
http://chdk.wikia.com/wiki/CameraFeatures.
pact cameras such as the G-series cannot benefit from the CHDK in this
respect because they use a mechanical shutter to control exposure time.)
When setting a fast Tv value in
ride Shutter Speed, the aperture (if the camera has a diaphragm) and/or the
ISO value must be adapted to achieve a correct exposure. Cameras with a
(Some of the top-range com-
ALT > MENU > Extra Photo Operations
Figure 4-23
The blade of a power hand blender
rotating at 15,000 rpm. The camera
was a Canon Digital Elph SD1100 IS.
In the CHDK Overrides, the shutter
speed was set to 1/40,000 sec. and the
ISO value to 1000. Flash was used for
illumination. By measuring the extent
of the small motion streaks on the
blade, we could determine a shutter
speed of 1/25,000 sec. The EXIF data,
however, show a shutter speed of
1/1500 sec—the official maximum
> Over-
shutter speed.
37
4.3 Exposure