I'm not really that well informed on this, but practically speaking, the cross type sensors are a bit easier to use. I think they are able detect contrast in both horizontal and vertical directions; since they can exploit the contrast they can AF based on this. The line sensors only exploit contrast on lines that are either horizontal or vertical. If you have a horizontal or vertical contrast line, the line sensors are not very sensitive if the orientation is aligned. Of course the cross type sensors can deal with either orientation. Perhaps not obviously, diagonal lines are, practically speaking, a combination of horizontal and vertical lines.

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Brian... a bicoastal Nikonian and Team Member

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