In the beginning, we all used split image focusing screens to help us focus manually.  The same image is passed through opposing prisms. We lined up the two halves to achieve sharp focus.  It was always easiest to use straight lines in the subject to focus on.

With today's autofocus mechanisms, we do not even give focus a second thought.  It either works or it does not.  Autofocusing sensors are sprinkled throughout the frame, their placement indicated by focusing brackets.  Yet, they work in much the same fashion as our manual focusing techniques.

Autofocusing Sensor
Autofocus sensors work on contrast edge detection.  When a high contrast line is in focus, the transition from black to white is sharp and well defined.  When this line is out-of-focus it appears as a gradient moving from black to gray to white.  The autofocus system moves the lense elements back and forth (hunts) until this line appears well defined.

Due to the nature of this mechanism, objects that lack high contrast areas pose difficulties for the autofocus system - just try to autofocus on a smooth gray wall.  An array of closely spaced vertical lines is also problematic, as this can often appear as solid gray to the sensor (which only sees in black and white).

Autofocus sensors are typically oriented either vertically or horizontally.  The center sensor usually employs both orientations, therefore, it is the most accurate.

Use of proper focusing techniques will result in sharp, clear photos.


In this example, we are trying to photograph these two corals, but they appear out of focus.  Notice the rockwork behind the two corals is sharp and in focus. The autofocus sensor does not know what our subjects are.  It simply assumes the object closest to the active sensor is the subject.  In this case, it happens to be the rockwork.

1. To accurately focus, choose one of the corals to use for focus locking.  Here, since both corals are in the same focal plane, we will choose the brighter coral, as the AF system will have an easier time locking in.  The darker coral has less contrast, and will take a little longer to lock onto.

2. Half-press the shutter release to activate the autofocus mechanism. Once focus is locked in, the camera will indicate with a solid green circle (universal code for focus lock), and in some digicams, the camera will also emit a beep.

3. While still half-pressing the shutter release, recompose the shot so the subject is in the desired position.

4. Once we are satisfied with the composition, fully depress the shutter release to record the image.

Note:  This composition is not a spectacular one.  It is simply one that illustrates a point.  Similarly, other samples in these lessons are not always compositionally exceptional as their purpose is to supplement our discussion.  We will discuss how to take better photographs in a later section.