a7RIII AF-S does not correct for focus shift

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JimKasson
JimKasson Forum Pro • Posts: 21,140
a7RIII AF-S does not correct for focus shift
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I, and everyone I know who has expressed an opinion it here,  have always believed, without testing explicitly, that, at least with Setting Effect on, that a7RII and a7RIII AF-S corrects for focus shift caused by spherical aberration by doing a CDAF tweak at taking aperture. A couple of days ago I turned a focus error setup that I had devised for measuring Nikon D850 PDAF errors to measuring a7RIII errors.

My test uses a LensAlign fixture and measures the position of the focal plane by looking at the variation in sharpness of the ramp, using Brenner's focus criterion.

In the case of the a7RIII and the 100-400, 70-200/4, Batis 85, and Sony 90 macro, it is not the case that focus shift is corrected.

http://blog.kasson.com/a7riii/a7riii-af-accuracy-batis-85/

In AF-C mode, focus shift is corrected, but at a cost of greatly increased sample scatter: the focus is on average more reliable, but each picture is more likely to be OOF.

http://blog.kasson.com/a7riii/a7riii-af-c-accuracy-with-batis-85-1-8/

I went back and checked the Batis 85 with the a7RII and saw the same kinds of systematic errors.

http://blog.kasson.com/a7riii/a7rii-af-s-accuracy-with-batis-85-1-8/

At least one other person has observed this with the 24-105 on the a7RIII.

My questions:

  • Was focus shift ever corrected in AF-S in the a7RII and/or a7RIII?
  • Are there some lenses for which is it still corrected?
  • Are there some firmware versions for which it is corrected?
  • If the answer to the above questions are all no, how did the correction get to be conventional wisdom?

In addition:

Are there some modes in which the a7RIII in AF-S does correct for focus shift?

I tried Medium Spot, Release priority AF, Setting Effect On only.

Jim

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Nikon D5 Fujifilm GFX 50S Sony a9 Nikon D850 Sony a7R III +4 more
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