Common photography myths (2)
Myth #1: Autofocus works by measuring the distance to the subject
September 5th, 2009 - 08:28:44 PM:
While there are indeed a few AF systems that measure the distance to the subject, most of the more modern systems don't work that way. A few older systems (mostly in early AF point-and-shoot and some SLR cameras) use ultrasonic sound to measure the distance between the camera and some object in front of the camera. I specifically say “some object” here because it's not necessarily the subject. For example, when you shoot through a window, these systems focus on the glass instead of the real subject behind the window. All in all these so-called “active AF systems” don't work too well. Although they can do some things that modern AF systems can't (e. g. they can focus in total darkness), they are not very precise and don't work well over longer distances.
All modern AF systems are passive, i. e. they don't send signals and use the echo to focus, but they only look at the light entering the camera through the lens. With the help of phase detectors sitting at distinct points of the viewfinder image, they determine whether the current focus is in front of the object under the sensor or whether it's behind, and how much it is out of focus qualitatively. They then turn the lens in the right direction until contrast of the image under the sensor is maximized. So in a way they work just like a photographer focusing manually.
The primary result of this process is, of course, that the lens is focused on the
object under the sensor(s). The distance to that subject is a secondary result,
but it's neither required for the AF process to work nor is it measured by
the AF sensors. The distance is often extracted after focus is acquired,
for example by reading the position of the focusing shaft directly or by
indirectly calculating the distance from the number of turns of the AF drive
and information on how one turn translates into change of distance. The distance
information can then be used for other things, e. g. for flash exposure,
advanced program exposure or DOF calculations.