How do cameras focus?
SLRs typically use phase detection autofocus. This Scientific American article, Focusing in a Flash, makes an attempt to describe phase detection AF, but isn’t all that clear. The basic idea is as follows: The AF system grabs strips of image from opposite sides of the lens that nevertheless project onto the same area in the focal plane. (This is typically done by using a half silvered reflex mirror and some optics behind the mirror.) When the overall image is out of focus, these two strips will be shifted in opposite directions, much like an old split prism viewfinder. When the overall image is in focus, the image captured by the two strips will be identical. The AF mechanism has two (or more) separate sensors corresponding to different parts of the lens. The lens is adjusted until these two images are the same, and the overall image is then presumed to be in focus. Non SLR digital cameras typically use contrast detection AF. While typically slower and less accurate than phase detectio
SLRs typically use phase detection autofocus. The Scientific American article, Focusing in a Flash, makes an attempt to describe phase detection AF, but isn’t all that clear. The basic idea is as follows: The AF system grabs strips of image from opposite sides of the lens that nevertheless project onto the same area in the focal plane. (This is typically done by using a half silvered reflex mirror and some optics behind the mirror.) When the overall image is out of focus, these two strips will be shifted in opposite directions, much like an old split prism viewfinder. When the overall image is in focus, the image captured by the two strips will be identical. The AF mechanism has two (or more) separate sensors corresponding to different parts of the lens. The lens is adjusted until these two images are the same, and the overall image is then presumed to be in focus. Non SLR digital cameras typically use contrast detection AF. While typically slower and less accurate than phase detection
