Sat 25-Feb-12 03:27 PM | edited Sat 25-Feb-12 03:29 PM by MotoMannequin
If you've ever used an old split-prism focusing screen, you can think of a phase-detect AF sensor in a similar way. These will split the lines of an out-of-focus image along the split line in the prism. As you bring the image into focus, the lines converge, like this:
Now, imagine trying to focus on a horizontal line using a horizontal split-prism; It can't be done. A horizontal split is great for focusing on vertical lines, ok for focusing on diagonal lines, and useless for focusing on horizontal lines. Some focusing screens are split on a diagonal line which gives ok focusing on horizontal & vertical lines, but still can't focus on any line parallel to the split.
An AF sensor can be thought of in a similar way. Non-cross sensors can't detect contrast lines parallel to the sensor. A cross-type sensor is like having 2 sensors, one horizontal and one vertical, which can detect lines in any orientation.