Do you know the reasoning behind the limitation? I could look it up somewhere I'm sure, but you probably know off the top of your head.
Basically, three reasons:
- Limitations on the size of the secondary mirror. Light for AF passes through the semi-transparent part of the main mirror (most is reflected up to the viewfinder), then is reflected off the secondary mirror down to the AF sensor. There is limited space behind the main mirror, based on the necessary geometry (i.e. the main mirror has to be at a 45Â° angle to the incoming light, and the secondary mirror has to be behind the main mirror and at an angle of 90Â° to the main mirror, so it's length is limited by the distance between the main mirror and the image sensor).
- Distortion. With many lenses, the edges of the frame are subject to distortion (barrel/pincushion), and that reduces the accuracy of phase detect AF.
- Vingetting. The AF system needs a certain amount of light to work. Almost all lenses vignette to some degree, meaning there might not be enough light at the edges of the frame. For example, the EF 17-40mm f/4L has >2 stops of vignetting wide open at the wide end - that means at the edges of the frame, you're below f/5.6 and AF sensors would not heve enough light to operate.
It's worth noting that none of these limitations apply to contrast detect AF, so using LiveView you can autofocus right out to the edge of the frame.