Panasonic Lumix DMC-FZ200 User Manual page 77

Again the camera uses light falling on specific areas on the sensor to
utilise the difference in contrast an image has when it is in sharp focus
and when it is out of focus.
By moving the focus lens element within the lens optical system the
camera can quickly find the point at which the image has maximum
contrast which directly relates to being in perfect focus.
The system works as follows. The camera reads the voltage from a
specific area on the sensor; it then moves the focus lens element a very
small amount in a known direction, usually by a small stepper motor, if the
voltage increases the camera again moves the lens in the same direction.
If there is another increase it will continue to move the lens and count
how many steps it has taken in doing so. It will continue to move the lens
until no further increase in voltage is detected. It will move the lens back
to a position which is half the number of steps it took to reach the point
where no further increase was detected.
However if after moving the initial step the voltage decreased it will
immediately reverse the direction of movement until it detects a positive
increase. If there was no change the camera would assume it was at
optimum focus.
This sounds a very slow process however the camera is able to perform
this operation 100 times per second and hence the name "light speed"
focusing was marketed.
This "contrast" based detection method relies on the imaging sensor
being able to detect the necessary difference in contrast in the light
reaching the sensor from the subject.
If the light level is low, or the subject doesn't have any contrast
differences the camera may try to hunt for focus or come up with the
focus range error in red on the LCD screen.
It usually indicates what the focus distance should be for the focal length
(amount of zoom) you have set on the lens.
In most instances where the camera cannot achieve focus when the
subject is within the range the camera is warning you that the subject
should be within is due to the lack of contrast in the subject.
It should be noted that lack of light itself doesn't mean the camera will not
be able to focus.
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