I was reading the D4/D800 brochures awhile ago and it made me wonder how does the cross type behave compared to the line sensors? I really don't have an in-depth understanding on how these AF sensors function.


Regards,

Glenn

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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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.

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Brian / Larry - thank you for the informative information. I appreciate it. All I know before was that the cross-type AF sensors provided better AF performance.


Regards,

Glenn

It should also be noted that the less-sensitive single-axis sensors are usually placed in the outer edges of an AF array (in the more expensive models) but can constitute the majority of an AF array in entry and mid-level cameras. They are also "leaning" to one side or another to optimize their effectiveness against subjects with straight vertical or horizontal edges.

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