Why do DSLR autofocus points rarely reach the edges or corners of the frame?
Asked 3/6/2014
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On the DSLRs I’ve used, the autofocus points are clustered near the center and don’t extend to the extreme edges or corners of the frame. Why is AF coverage limited like this on DSLRs?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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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.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
12y ago
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Generated from our catalog & community — verify before relying on it.
In DSLRs, autofocus points are usually limited by how phase-detect AF works. The AF system compares light coming from opposite sides of the lens using paired detectors on a separate AF sensor. Those detector pairs need a usable separation (“baseline”) to measure focus accurately. Near the edges of the frame, that geometry becomes harder to maintain, so AF points can’t practically extend all the way to the corners. As points get farther from center, they also tend to be less reliable for this reason.
Light and image quality also tend to be stronger toward the center of the lens, which helps AF performance there.
This is also why APS-C DSLRs can seem to have wider AF coverage: the AF point layout may be similar to full-frame models, but the smaller sensor makes that same cluster cover a larger percentage of the frame.
In practice, many photographers still prefer using a central AF point and recomposing, especially when off-center objects might distract the AF system.
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