Digital Photography Tips and Techniques

The layouts of the auto focus (AF) sensors in the viewfinders of some Canon, Nikon and Sony DSLR cameras are compared with rule-of-the-third grid lines superimposed, thanks to the DPReview forum member Matt Cham.

If you want to shoot with the rule-of-the-third composition, you may find it a little bit tricky on these viewfinders to auto focus at exactly the 1/3 points. The focus and recompose technique can still be useful, even with so many AF sensor points on some models.

Source: http://forums.dpreview.com/forums/read.asp?forum=1032&message=29295041