Canon Camera Hackers Manual page 38

light caused by the shorter exposure time by increasing the sensor speed—
let's say, from 50 to 500. Manually, this is not a problem. You just dial the
new ISO speed into the Overrides. Automatic operation with shutter prior-
ity, however, is only possible on cameras with ND filters when using a suit-
able script for exposure control (section 5.7.4).
Let's return to cameras that do have diaphragms. On such a camera, if
you override the aperture only, the exposure system will select a suitable
exposure time; the camera will be in Aperture-priority (Av) exposure mode
All of the Overrides are combined in the submenu Extra Photo Opera-
tions. The first entry of that submenu is used to switch the camera between
native mode and override mode.
Disabled. The Override System is completely disabled.
f
Off. All Override Values are disabled, but the display shows the remark
f
NO OVERRIDES.
On. All Override Values are active. The display, too, shows the override
f
values if option OSD
Because stepping through the menus can be a bit awkward, the CHDK of-
fers a key combination to toggle the Overrides between On and Off. This key
combination depends on the camera type:
Cameras without DEL
f
G7: Shutter halfpress
f
All other cameras:
f
Optionally, the Overrides state (On or Off) can also be applied to the AutoISO
feature (section 4.3.2) and the Bracketing feature (section 4.6). This is con-
trolled through the entry Include AutoIso & Bracketing. If the option is set,
AutoISO and Bracketing will be enabled if Disable Overrides is off, and dis-
abled if Disable Overrides is on.
Individual entries can be switched on or off through their Value Factor
subentry. If not set to Off, this value factor is multiplied by the value of the
main entry. For the Shutter Speed entry, there is another, more convenient
possibility: when the entry Shutterspeed enum type is set to Ev Step, the
only possible value factor is 1 (and Off). The Shutter Speed Value will step
through all available shutter speeds by 1/3 f/stops (2048 sec, 1625 sec, ...,
1.3 sec, 1.0 sec, 0.8 sec, ..., 1/80000 sec, 1/100k sec) as we are used to
in mainstream photography.
Otherwise, if the entry Shutterspeed enum type is set to Factor, the fac-
tor is simply multiplied by the specified shutter speed value. This method is
1 Again, this is not possible on cameras without diaphragms—you simply
won't be able to override an aperture value.
> Show state displays is enabled (section 4.2.4).
key and SX100: Shutter halfpress
+ UP
Shutter halfpress + DOWN
1
.
+ LEFT
Figure 4-17
Overrides for exposure and other
settings are controlled through the
Extra Photo Operations submenu. An
Aperture value override is not shown
here because this camera has only an
ND filter and no diaphragm.
29
4.3 Exposure