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Bizarre AF behavior

kimcarsons

By kimcarsons
in Fuji X-T2 / Fuji X-T20

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kimcarsons

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In addition to the bizarre behavior where the X-T2 stops down the lens for the entire time focusing in AF-C, I just noticed a new weird behavior tonight.

 

This affects AF-S mode, and it is very odd indeed.

 

It's most obvious in zone focus mode with the largest PDAF focus area.

 

This is with the 35mm F2.

 

Set the lens to f/2 and try focusing on something close and then something distant. The AF slowly hunts (3 seconds in my tests--about the same as the X-Pro1, so seems to be using CDAF).

 

Now set the lens to f/16 and try it. Boom, focusing now takes less than 1 second.

 

Now, as I mentioned in the other thread, in AF-S mode the X-T2 focuses with the lens wide open (or nearly). This is still true in this case. The lens doesn't actually stop down to f/16 until the focusing is done. But for some bizarre reason, and even though the actual aperture during focusing is f/2ish, it's far quicker with the aperture ring on the lens set to f/16.

 

WTF, Fuji?

 

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kimcarsons

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This would be due to DOF at the relevant aperture. F/2 has a very narrow field of focus vs f/16 which is basically everything in focus to start with so locks on straight away.

 

Same thing happens with all fast lenses when using them wide open.

 

Sent from my LG-H850 using Tapatalk

 

Are you saying that Fuji's AF algorithm is enabling a faster/lower accuracy focus mode for higher aperture numbers (the speed increases dramatically at f/4 and higher)? Or, conversely, forcing a slower AF mode when the aperture number is lower (than f/4)?

 

If this is necessary, why do you think DSLR manufacturers don't do this (AF speed is consistent no matter the aperture setting on every DSLR I've ever used...)

 

Are you aware of any setting/option to allow users to opt out of this behavior any use the fast AF mode even at f/2?

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kimcarsons

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UPDATE: This behavior seems less consistent in brighter light (in bright light focusing appears to be using the 'fast mode' with the lens set to f/2 most of the time). It's not entirely clear to me what causes the Fuji to fall back on CDAF. Low light is when you need f/2 the most, so it sucks that focus is faster with the lens set to f/4... It's like a choice is between getting a grainy shot at f/4+ or missing the moment entirely at f/2, f2.8 (because the camera takes 3 seconds to focus). 

 

The real disappointment here for me is that in the situations I most often shoot in, the X-T2 is focusing just as slow as the X-Pro1 does.

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