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On all the DSLRs I've used, there are never any autofocus points near the edges and corners. Why is that?

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Probably because camera makers consider it less useful (and it would be more costly) - although for instance Nikon's D300 has a rather large coverage. But I agree that sometimes I would have enjoyed using that possibility instead of having to "deframe", focus and reframe. –  FredP Mar 6 at 9:21
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@FredP no - it's an physical limitation, not an arbitrary decision by manufacturers –  Matt Grum Mar 6 at 10:24

3 Answers 3

Phase detect autofocus in DSLRs works by comparing patterns of light coming from each side of the lens using pairs of detectors which are separated a certain distance on the AF sensor. This distance is called the baseline, and the greater the baseline the more accurately the distance can be measured.

The need for a wide baseline and for light to travel from either side of the lens makes it impossible to have autofocus points at the very edge of the frame. The further out you go from the centre the shorter the baseline which is why the outer focus points are often less reliable.

APS-C DSLRs appear to have AF points which cover more of the frame, in reality the positions are similar to a full frame DSLR but the frame itself is smaller.

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That said, you can have sensors near the edges that lose their effectiveness with diminishing light and smaller apertures, or at the edges if the imaging sensor is small enough in comparison to the AF sensor and lens image circle (such as, say, using a Nikon D4 in DX/crop mode). It's just a matter of being willing to pay for what you ask for. –  user2719 Mar 6 at 11:27

Because autofocus needs a considerable amount of light, and the lens construction gives more light getting closer to the center. Also, lenses tend to be sharper the closer you get to the center. That is why using the middle focus point and then recomposing usually gives the sharpest result.

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-1 It's neither of those reasons –  Matt Grum Mar 6 at 10:25
    
@MattGrum: these are contributing factors, believe it or not. –  TFuto Mar 14 at 12:26
    
It's not true that autofocus needs a considerable amount of light, the 6D AF is rated to work down to -3 EV, which is the equivalent of a 60 second exposure at f/2.8 and ISO100! You do get vignetting toward the corners but not enough to stop AF working. The being able to focus on very fine detail would be compromised by a lack of sharpness in the corners, but in principle the ability to measures phase differences with coarse features is largely unaffected by lens softness. –  Matt Grum Mar 14 at 15:56
    
Interesting! I will study this further. –  TFuto Mar 14 at 19:09

Are you sure you'd even really want ones on the perimeter? For me, I turn off all of the auto focus squares except the middle one. Why? If I'm shooting through a tree, for example, I want to focus on my subject which may be 30 feet away. I don't want the camera to focus on a branch (off center) that is 8 feet away. I would never want focusing sensor squares around the perimeter of my lens. But that perhaps is just me.

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Welcome to Photography on Stack Exchange. As currently stated, this answer only indirectly answers the question. You have expressed why you don't think it is necessary, but it doesn't answer why they wouldn't bother for the rare case that someone might want to focus on something on the edge of the frame. –  AJ Henderson Mar 11 at 20:20

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