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01-16-2015, 10:47 AM   #1
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3 Questions about autofocus

For all questions assume same equipment except for component listed...

1. Is autofocus speed determined by the processor? (apart from considerations like SDM etc)

2. Is autofocus accuracy determined by the sensor?

3. Does a faster aperture lens focus more accurately? (as in shallower depth of field would make the in focus area smaller, so, focus would be even better stopped down after that)

01-16-2015, 11:01 AM - 1 Like   #2
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1) In PDAF (viewfinder), autofocus is handled by a separate set of sensors. Light coming in is redirected to them and they make the measurements by trying to match the phases of two different beams of light that enter from opposite sites of the lens. In the much slower CDAF, the imaging sensor does the measurement. That is tied to the main processor of the camera, I believe.

2) There are many sensors in a camera. See above. PDAF doesn't touch the main imaging sensor at all but CDAF (live view) involves it.

3) Sometimes. Faster apertures mean more light, which is generally a good thing. But fast apertures can mean poor contrast when wide open, and that gives the autofocus unit trouble (yes, contrast matters even in PDAF because the phases are not measured 100% accurately.) You'll note that focus points are rated to an f stop. The middle ones on a K3 are all f/2.8 while only the center one on the K-5/II/IIs. Basically, because they can't guarantee the performance of fast lenses wide open, they can't say their AF unit will always work with them well.
01-16-2015, 12:04 PM   #3
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QuoteOriginally posted by MadMathMind Quote
1) The middle ones on a K3 are all f/2.8 while only the center one on the K-5/II/IIs. Basically, because they can't guarantee the performance of fast lenses wide open, they can't say their AF unit will always work with them well.
Makes one wonder what the obsession with f1.x lenses are then? Perhaps it's the camera's fault and not the lens that some have such a bad rep for inaccurate AF?
01-16-2015, 12:19 PM   #4
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Just because the AF doesn't guarantee that they work with 1.X lenses doesn't mean they don't work. I've had no issues with my FA77 f/1.8 or FA50 f/1.7. I regularly shoot them below the f/2.8 specification of the AF module and get great results. In fact, I regularly use non-center AF points, which aren't even listed at f/2.8, and it still works for me. I've had minor issues with my Sigma 18-35 f/1.8 (far, far fewer than other have) but I do not believe them to be related to the f/1.8 aperture.

A common thread among all those lenses is that they have excellent contrast wide open. The F50 f/1.7 I had before had more issues with AF missing at large apertures; its contrast is very good at f/1.7 but the FA is just a couple notches better. I'm sure that makes all the difference in the AF accuracy.


Really, it's a matter of guarantee, not "it won't work." It does work with apertures below f/2.8, but there may be limitations. DoF is another concern as well. No AF module is 100% accurate. If you focus a 77mm lens 60cm away at f/1.8, your AF needs to be absolutely spot on because you've got about 3mm of DoF. No AF tech is going to be that accurate. If you back up to 2m and increase to f/2.8, you get 8cm of DoF. It's not a lot, but certainly a much welcome improvement. That 8cm may be within the tolerance of the AF system.

Remember that while DoF decreases with focal length, the AF system might also change with focal length (long focal length ==> more coherent subject ==> more accuracy), thus giving enough accuracy to make the guarantee.

01-16-2015, 12:41 PM - 1 Like   #5
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QuoteOriginally posted by nomadkng Quote
Makes one wonder what the obsession with f1.x lenses are then?
Its certainly not auto focus....

People like the bokeh, the depth of field, etc...

If I go wide open often I would just manually focus. Its about the image not so much the speed to get there.

With my k5 I manually focused a lot....with the k3 not nearly as much.
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