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I understand that the difference between "regular" autofocus points and cross-type ones is that the latter consist of two diagonal "regular" ones. I also understand that this is better.

But what are examples of actual situations in which cross-type points are better? Does it really make that much difference in practice? Some reviews would make you believe that when lamenting a lack of cross-type points, but does it actually matter?

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All of them.... –  Robin Jun 6 '13 at 19:02

2 Answers 2

up vote 7 down vote accepted

"Cross" type sensors are pairs of "line" type sensors at 90 degrees, not just sensors with diagonal orientation. They are commonly arranged straight up and down like this + but can also be diagonal like this X

It matters. A line type sensor will completely fail to detect detail which is parallel to the direction of the sensor, and even worse will give highly inaccurate results with detail which is close to parallel.

Edges are usually good features for autofocus systems. More so than surface textures or shading patterns. But in our man-made world edges (of walls, floors, doors, windows) tend to be either horizontal or vertical, meaning a 50% chance of being invisible to a "regular" autofocus sensor. In my opinion camera manufacturers should orient the line sensors at 45 degrees like this / or this \ but they don't.

Another problem is most cameras don't indicate which direction a sensor is sensitive to, in any way! Some of the criticism for the outer points of the original 5D being inaccurate can be traced to the fact that some of the outer points are vertically sensitive only and some are horizontally sensitive only. Turning your camera 90 degrees and suddenly a very poor AF lock becomes a very good one.

It's not the end of the world though, there are work-arounds. Notably turning the camera 45 degrees to focus. But it's a pain to do this even if you do remember and can ruin the spontaneity of a photograph opportunity. The key is to know what's going on, then you can look for the appropriate type of detail (that has variation in all directions).

Also consider:

  • not everyone uses the outer points, some focus and recompose (centre points are almost always cross type)
  • not all scenes / types of photography require accurate autofocus

but if you want reliable and fast AF anywhere in the frame without thinking too hard (e.g. if you shoot events or weddings) then I would pay attention to how many cross type AF points are provided.

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Wouldn't a lot of the practical issues addressed by orienting the line sensors diagonally, i.e. / or \ ? –  Lars Kotthoff Jun 6 '13 at 12:23
    
@LarsKotthoff yes. Why this is not done is beyond me. –  Matt Grum Jun 6 '13 at 12:25
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I think the downside of diagonal is exactly the same as the upside: you're less likely to be aligned in the wrong way, but also less likely to be aligned in the ideal way. This is speculation but based on my experience with split-prism manual focus screens — it's faster and easier when you have a perpendicular line than an angled one. –  mattdm Jun 6 '13 at 12:40
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Because a 90º line (i.e. a vertical focus line over a horizontal feature) will be even more accurate than a 45º line will be. Higher end cameras do have some focus points that are X shaped or both + and X superimposed over each other. –  Michael Clark Jun 6 '13 at 12:45
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@mattdm I'd rather be close all of the time, than be dead on 50% of the time and miles off the other 50% –  Matt Grum Jun 6 '13 at 12:50

when you don't have cross types you have to find a detail for it to lock the focus on that is perpendicular to the AF sensor pair. this makes it twice as likely you will have to focus recompose - with a risk of changing the distance to the subject in the process, see my figure here:

How do I "focus and recompose" using spot metering?

Or switch from portrait to landscape or vice versa to focus. chances are that you dont keep this in mind, especially remembering which point works in which direction, and end up swearing why the f**king camera wont focus. This might make it safer to just manual focus.

The cross type works in two direction making it likely to catch something, if there are any details to begin with. Another ting to keep an eye out for is cross types that also are more sensitive up to F2.8 for low light focusing without annoying focus assist beams.

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