Focus - Canon Camera Hackers Manual

4.4

Focus

The focal system of Canon compact cameras is dominantly based on an
autofocus system. Only some cameras in the higher price range allow
manual focusing. The autofocus system is augmented by special focus
modes such as Macro or Infinity, which can be set manually. Over the years,
the focus system has become more and more advanced. One of the more
recent improvements is face detection, allowing the camera to focus pre-
cisely on the faces in a scene.
Like any of the focus systems in other compact cameras, the Canon
AF system is also contrast-based. When focusing, the camera moves the
lens back and forth until it finds a position where the contrast in selected
image areas is maximized. This takes some time; the phase detection-
based focus systems in DSLR cameras are much faster.
In the area of focusing, the CHDK also adds functionality. An explicit
subject distance can be dialed in precisely to the millimeter. This overrides
the autofocus system as well as selected focus modes such as Macro or
Infinity. The value range is between 0 and 65535 mm, with 65535 repre-
senting infinity. The easiest way to dial in infinity is to start with zero and
4.4 Focus
Figure 4-31
Dream landscape obtained with the
curve shown in Figure 4-30. See the
tonal inversion in the central sky area.
Figure 4-32
The entry Override Subj. Dist. Value in
the submenu Extra Photo Operations
can be used for manual focusing. The
override can be switched off via the
subentry Value Factor. The labeling of
this subentry is somewhat misleading.
Unlike its equally named siblings,
this value factor does NOT define a
value factor by which the main
entry is multiplied. Instead, it defines a
step width for modifying the main
entry. So, the subject distance set here
is 770 mm (77 cm) and not 7700 mm
as you might expect.
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