By Christopher Mims

When digital cameras focus, they use a complicated and slow system that often doesn't come close to being correct. Humans, however, focus correctly in an instant. Now scientists have developed an algorithm to make cameras work as well as our eyes.

For half a century, we've known that the human visual system has an exquisite ability to lock focus on an object instantaneously, a feat that no digital camera or camcorder has yet matched. Now, finally, scientists have developed a good guess about how we might do that, and have translated that method into software that could make its way into cameras that never lose focus.

Johannes Burge , a postdoctoral fellow at the University of Texas, and his advisor Wilson Geisler , wondered how it was that the eyes of humans and many other animals were able to focus so much more efficiently than most digital cameras. In a traditional autofocus system, the camera uses only one piece of information about a scene to determine whether or not an object is in focus--its level of contrast. Contrast, says Burge, isn't always a perfect proxy for focus. But it's worse than that: To determine in which direction to re-focus, a camera must first change its point of focus and compare the new image it captures with the old one, to determine whether or not the object in question has a higher or lower level of contrast. Often, the camera isn't even re-focusing in the correct direction when it captures this second image. This method of "guessing and checking" is "slow and not particularly accurate," says Burge.