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Addendum
Page 253
A Word About Auto Focus Sensors
Sensors are either vertical, horizontal or cross type. Pentax K10D, K20D and K-7 all
have 9 cross type sensors and two vertical sensors. Vertical sensors detect the
sharpest horizontal contrast within its area of coverage and lock the focus on that
point. Alternatively, the horizontal sensors detect the sharpest vertical contrast
within its area of coverage and lock the focus on that point. You guessed it; the
cross type sensors detect the highest vertical or horizontal contrast and lock the
focus on that point. Since we use the center focus point of our camera, which is a
cross type sensor, our chart was deliberately designed without any vertical lines in
the center so that the focus can only lock at the intersection of the black and white
portion of our target. Note that in your viewfinder, the little red square indicates the
focus area but the cross sensor is not necessarily dead center. It merely shows you
the area of the sensor, but is not an exact focus point as the sensor will lock on the
point of maximum contrast.
Difference Between 45° And 30° Charts
The above charts, one at 45° and one at 30°, illustrates why the depth of field is
proportional to the angle of the chart.