Page 84 - (DK) Advanced Photography Guide

82 EXPLAINING | AUTOFOCUS

AUTOFOCUS POINTS IN-CAMERA

The various AF modes set how the camera focuses—whether
it stops focusing once sharpness is acquired, or if focusing
shifts until the moment of exposure. What these modes don’t
do is select where the camera focuses; this is done by picking
the appropriate AF point-selection mode. There are essentially
▲ The sophistication of the camera
two options: either you let the camera automatically choose determines the number of AF points
where to focus, or you make the decision yourself. and the range of AF modes available.

AF SENSORS

The AF points in a DSLR’s viewfinder are linked to one of certain conditions than others. The orientation of the sensor is
three types of phase-detection AF sensors with different one factor that affects AF efficiency, as does its light sensitivity.
specifications, so specific AF points will be better suited to See your manual for details of your camera’s AF system.

HORIZONTAL/VERTICAL CROSS-TYPE

Horizontal and vertical sensors can detect and focus on lines A cross-type AF sensor is preferable to horizontal and vertical
or patterns in a scene that run in the opposite direction to the sensors because it can detect and focus on both horizontal
AF sensor’s orientation: horizontal AF sensors detect vertical and vertical lines or patterns. However, cross-type sensors are
lines, and vice versa. The problem with this is that an AF point typically less numerous, often found either at the center only
may not focus correctly if its associated sensor is in alignment or across the AF area. An even more effective variation is the
with strongly horizontal or vertical details in the scene. dual-cross AF point, which is sensitive to diagonal lines.

▼ Cross-type
AF sensors are
designed to work
effectively under
most conditions
and with most
subjects. If only
one AF point is a
cross-type sensor,
it tends to be the
central AF point.

▲ There are usually
more horizontal
and vertical AF
sensors than cross
types. The exact
number and
position of each
type is often shown
in the camera’s
instruction manual.

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