This paper examines a novel source of depth information: focal gradients resulting
from the limited depth of field inherent in most optical systems. Previously, autofocus
schemes have used depth of field to measured depth by searching for the lens setting
that gives the best focus, repeating this search separately for each image point.
This search is unnecessary, for there is a smooth gradient of focus as a function
of depth. By measuring the amount of defocus, therefore, we can estimate depth simultaneously
at all points, using only one or two images. It is proved that this source of information
can be used to make reliable depth maps of useful accuracy with relatively minimal
computation. Experiments with realistic imagery show that measurement of these optical
gradients can provide depth information roughly comparable to stereo disparity or
motion parallax, while avoiding image-to-image matching problems.