Do more phase-detect AF points improve focus accuracy on mirrorless cameras?
Asked 11/20/2020
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I'm comparing mirrorless cameras such as the Nikon Z6 II, Z7 II, and Sony A7 III, and they advertise very different numbers of phase-detect autofocus points. Does a higher AF point count actually make focus more accurate, especially for things like eye/iris detection? If it does, how? Or are factors like AF point size/coverage, autofocus algorithms, and the way phase detect works with contrast detect more important than the raw number of points?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
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On-sensor PDAF works much more like a split prism focusing screen as compared to the DSLR's dedicated PDAF sensor. A pair of photosites (pixels), or a pair of photosite groupings, have their lens(es) biased to intercept opposing virtual images (left/right, upper/lower, or both), and when those virtual images differ in phase (angular offset/position) the image is determined to be out of focus.
The number of focus points is a tradeoff in terms of size. Due to the offset photosite lenses, on-sensor PDAF points inherently receive less light (virtual image rather than the total image/light), this can result in exposure lines/banding in an underexposed/recovered image. It is NOT necessarily less light than a DSLR's dedicated PDAF sensor receives, it may be more (max aperture AF) or less (stopped down "live view" AF). But the more photosites that are dedicated to PDAF reduces overall fill efficiency (exposure sensitivity). When there is plenty of light this is of minimal concern.
Smaller focus points can detect finer details, but that is equally more dependent on the quality (sharpness/clarity/contrast) of the image being focused. If the lens projected image doesn't have the resolution to project that fine of a detail clearly, then having that small of a focus point is essentially useless (image will still be focused as well as possible).
What is most relevant in terms of the number of focus points is the area of coverage (IMO). I would rather have a larger number of focus points because they cover a larger portion of the image area, as opposed to having a larger number due to them being smaller and covering the same portion of the image area.
The eye AF issues of the Nikon Z series is primarily due to the algorithms in use (IMO). Nikon is relatively new to the advanced mirrorless systems, and eye AF was a firmware add-on for the Z6... Sony has been doing advanced mirrorless for a lot longer.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
5y ago
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Not directly. A higher number of phase-detect AF “points” does not automatically mean better focus accuracy.
AF points are really areas/zones of sensitivity, not infinitely small dots. More points can help with coverage and subject placement, since there may be a focus area closer to the subject detail you want to lock onto. But accuracy also depends on the size and design of those AF areas, how the camera’s AF algorithms work, and how phase-detect and contrast-detect systems are combined.
On-sensor phase detect in mirrorless cameras works by comparing light from different parts of the lens image, somewhat like a split-prism. Because of that design, there are tradeoffs in sensitivity, light gathering, and point layout. So point count is only one part of the system.
In practice, eye/iris focus performance is often influenced more by subject detection/tracking software and how precisely the AF area can identify the eye than by the raw number of points alone. More points may improve coverage, but they do not guarantee more precise focus by themselves.
So yes, point count can matter somewhat for coverage and placement, but it is not a reliable stand-alone measure of autofocus accuracy.
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