Our eyes produce a very blurred picture as long as we do not look directly at the desired details (this is called peripheral vision). With a speed of approx. 3 eye-movements per second the blurred picture is "scanned" and updated in our visual memory (This is done by our foveal vision). Since we can see at will any portion of the image in full sharpness ( by looking at it ) we are only aware of the blurred picture, when we realize that we can't get an information detail - such as a number or a letter - without looking at it.
The slow-motion animation shows the first 3 seconds of "picture updating" when watching a human face. The last picture shows the complete image. As you can see the eye is "scanning" by searching for subjectively important information within the blurred image delivered by peripheral vision. In spite of its relatively slow speed the human eye can retrieve information very fast - and even increase the retrieval speed by learning where to look for important information.
The CD-ROM training programs "Eagle Eye"  and "Eagle Eye Junior" provide for diagnosis and training of this all-important ability.

Details (in German) in: Hunziker, H.-W., (2006). Im Auge des Lesers.