swheeler Page 2 of 34
IMT – Cholla Training Center
Focusing Your Camera
Center AF (Auto Focus)
Most digital cameras use contrast detection to auto focus (AF). Usually, the focus
point is a small rectangle in the middle of the viewfinder frame (Center AF), though
many digital cameras now also offer additional AF points (Multi-Point AF).
Center AF
If you look in the LCD monitor of your digital camera, there will usually be a
rectangle at the center of the screen, commonly called the AF Frame. When your
digital camera is set to Center AF mode, this rectangle in the middle of the screen is
your AF point.
Multi-Point AF
Multi-Point AF automatically selects between a number of AF points (the most
common seems to be 5 or 9 AF points -- i.e. 4 or 8 AF points clustered around a
center focus point) and finds the most contrasty subject among those AF points.
An Example
If your default AF mode is Multi-
Point AF and some of your shots
are sometimes out of focus, it
may be that Multi-Point AF is
the culprit.
Here is an example of how
Multi-Point AF focused on the
more contrasty background well
behind my main subject (the
lone green leaf left on the
bush).
Even though the green leaf is in
the center of the frame, Multi-
Point AF has chosen to focus on
the more contrasty background
around it instead.
The result is that the ground of
fallen leaves come out in focus,
while my main subject is out of
focus.
I took a couple more shots with
the same result before I realized
what the problem was. I
immediately went into the
camera's menu to switch AF
mode from Multi-Point AF to
Center AF. This time, I got the
result that I was after: the lone
green leaf in focus against a
nicely blurred backdrop!
Summary
Multi-Point AF works pretty well
usually but sometimes the
camera will focus on something
else besides your main subject if
http://www.photoxels.com/tutorial_center-af.html
Sony DSC-P150 Cyber-shot: 5 Area Multi-Point AF
Program AE, Multi-Pattern Metering, Macro ON
7.9mm, 1/250 sec., F2.8, ISO 400
Sony DSC-P150 Cyber-shot: Center AF
Program AE, Multi-Pattern Metering, Macro ON
7.9mm, 1/200 sec., F2.8, ISO 400