Canon® EOS Digital Photography Photo Workshop

3.3. EXPLORING FOCUSING OPTIONS

Today's modern lenses offer photographers amazing optical quality, and whether you choose to focus using one of the automatic focusing modes or to focus your lens manually, you can obtain razor-sharp images with the lineup of Canon lenses. Focus can be defined as the point at which light rays from the lens converge to form a sharp image. This is achieved by adjusting the lens or the distance between the lens and the subject. I usually use some sort of autofocus (AF) as I know my eyes aren't as accurate as Canon's AF, but I do manually focus in some situations where AF tends to fail or become a hindrance. In 3-9, you see how AF can render even a fast-moving subject tack sharp; in 3-10, I used manual focusing as the closeness of the subject and the shallow DOF made using AF mode impractical.

3.3.1. AUTOFOCUS SYSTEMS

Autofocus (AF) systems rely on one or more sensors to determine correct focus. Some AF systems rely on a single sensor, while others use an array of sensors. Most modern digital SLR cameras use through-the-lens optical AF sensors, which also function as light meters.

The speed and accuracy of through-the-lens optical autofocusing is now more precise than what you can achieve manually with an ordinary viewfinder. Autofocus accuracy within a third of the depth of field at the widest aperture of the lens is not uncommon in professional AF dSLR cameras. There are, of course, some situations where manually focusing provides better results ...