Let's look a little closer at what actually happens during the autofocus process. For this example we will
use an image of a house, and focus on the house number.


The individual mages projected by the SIR optics onto the central part of the AF sensor is shown below.
The limited size of the image sensor used to take this images is the reason for concentrating on this part of
the sensor. The four middle areas in “+” shape correspond to the f/5.6 sensitive sensor range. The four corner
areas make up the "x" f/2.8 sensitive part. The real AF sensor arrays do not capture these images but only
the areas marked with red.

When all the individual SIR images cover the same area, as shown in the previous image, the picture is in focus.
When not in focus as shown below, the individual SIR images start to move, the more they move the larger
the degree of defocus.  The original image postion has been overlain in order to show the moving direction and
displacement of the SIR images. Note that the f/2.8 images are displaced more than the f/5.6 images


To make the movement even more evident there is a link to a small video below. The left half of the screen
in the video is what sensor sees and the right part is the view through the viewfinder. Notice also the
difference between how fast the real image on the right blurs in compararison to the SIR images on the left.
We will get back to the reason for this shortly.

Right click on the link, save the video to your harddrive, and play it from there.


The AF Module
5D SIR and AF sensor
7D SIR and AF sensor
The AF sensor view
The SIR image shift