CDLC: How Does Canon's Traditional SLR AF Work?

From the Canon Digital Learning Center:

by Rudy Winston
 
Today’s digital SLR cameras usually have two distinct Autofocus systems — what we now think of as traditional AF you get through the optical viewfinder; and (usually) a separate AF system for focus using the LCD monitor to view your scene, when shooting with Live View or recording video. They’re very different:
 
Viewfinder AF:
Usually relies on a totally separate AF sensor for focus detection, and always uses what’s called Phase Detection technology.
 
Live View and video AF:
Uses pixels on the actual camera imaging sensor to read focus information. Early systems simply read blurriness or sharpness off the image sensor; this is called Contrast Detection AF. (This is still used in some competitive brand interchangeable-lens cameras.) More sophisticated systems utilize individual pixels, or groups of pixels, on the image sensor and make comparisons of data to detect when a subject is focused or not…this approach is also called Phase Detection.
 
We’ll discuss the many advantages that traditional through-the-viewfinder AF systems offer in this series of AF articles. But before we dive in, it may be helpful to have a quick understanding of how AF has typically worked in our SLR cameras since these systems were introduced back in the mid-1980s.

Read the entire article on the Canon Digital Learning Center.

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