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If the subject has high inherent contrast with good delineation of edges or
patterns then the camera can achieve good focus.
One of the inherent problems of this system of autofocus is where the
sensor is picking up the image detail for determination of the focus point.
If we have a subject with little contrast standing in front of an area which
has higher contrast the camera may assume that the image is in focus by
adjusting to the background contrast area and not the subject!
Number, Position and Type of Autofocus Points
Autofocus Mode Selection
The robustness and flexibility of any autofocus system is primarily due to
the position and type of the autofocus points. The more points there are
the more the chances of the camera determining an accurate focus point
from the image falling on the sensor.
The Panasonic Lumix FZ200 has 23 such points which can be used in
“23 area” method. This is particularly effective when the subject may not
be in the centre of the screen, for example a flying bird.
Higher end DSLR's may have a lot more autofocus points to assist in
faster and accurate autofocus.
The type of focus point also determines the speed and accuracy at
which
autofocus can be achieved.
If the autofocus points are “cross shaped” they can determine contrast
in
both horizontal and vertical directions adding to the reliability of
detection.
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