Sony a7iii af-s vs af-c focus accuracy

ventureout · Jun 28, 2018

Hi

i was was wondering how people are finding the focus accuracy on their Sony a7iii In af-s vs af-c focus, I have read reports about af-c being better which is different to how I use my other brand cameras etc, so after some feedback from the dpreview wisdom

thanks

DaFireMedic · Jun 28, 2018

ventureout said:
Hi

i was was wondering how people are finding the focus accuracy on their Sony a7iii In af-s vs af-c focus, I have read reports about af-c being better which is different to how I use my other brand cameras etc, so after some feedback from the dpreview wisdom

thanks

I almost always use AF-C anyway, but I’ve noticed no difference in AF-S in terms of speed or accuracy.

bayindirh · Jun 28, 2018

ventureout said:
Hi

i was was wondering how people are finding the focus accuracy on their Sony a7iii In af-s vs af-c focus, I have read reports about af-c being better which is different to how I use my other brand cameras etc, so after some feedback from the dpreview wisdom

thanks

Generally I'm using AF-A (AF-S, but switches to AF-C if something is moving), but I use AF-C if I need to track. Both are tack sharp without any visible difference.

Jefenator · Jun 28, 2018

I use AF-C for moving people, now. (Pretty freaking cool, the way it will refocus on an eye between frames during a hi-speed burst.)

I use AF-S for stopped-down still life product shots. (Have my AEL button set to recall a custom setting that acquires focus that way.) That's handy because it opens the lens up to acquire focus then stops back down automatically. (I default to manual focus so it's not constantly refocusing when I don't really want it to.)

Different modes & behaviors for different situations. I can't imagine life any other way now.

Roland Schulz · Jun 29, 2018

I hardly use AF-S anyway, even not with my Nikon D4 system due to a simple reason. When focusing on objects that are not plain flat it´s sometimes diffcult for the AF system to decide on what to focus, the closer or the detail more far away. There´s no difference in AF-S or AF-C about this but in AF-C you note this because the system is "searching" or pulsing a bit. When I note that the AF system is unsure or "nervous" I try to focus on a slight different detail, simply move the AF sensor a bit so that the AF system becomes steady, even in AF-C (for PDAF, low light CDAF always hunts a bit but that´s the "system").

voronspb · Jun 29, 2018

I use the AF-C for people and other dynamic objects. No problems with speed or accuracy. Also I have mapped a dedicated button to force the camera to focus on middle point with AF-S (if there's not enough light or contrast). But mostly I use AF-S for static objects.

bayindirh · Jun 29, 2018

voronspb said:
I use the AF-C for people and other dynamic objects. No problems with speed or accuracy. Also I have mapped a dedicated button to force the camera to focus on middle point with AF-S (if there's not enough light or contrast). But mostly I use AF-S for static objects.

voronspb · Jun 29, 2018

Yes, I use the same button, remapped to a more sophisticated action: switching to AF-S, choosing the smallest dot at center and then focusing.

bayindirh · Jun 29, 2018

voronspb said:
Yes, I use the same button, remapped to a more sophisticated action: switching to AF-S, choosing the smallest dot at center and then focusing.

Entropy512 · Jun 29, 2018

ventureout said:
Hi

i was was wondering how people are finding the focus accuracy on their Sony a7iii In af-s vs af-c focus, I have read reports about af-c being better which is different to how I use my other brand cameras etc, so after some feedback from the dpreview wisdom

thanks

Depends on the lens.

In theory, AF-S is more accurate due to CDAF being more accurate than PDAF.

In reality, many lenses have enough backlash/overshoot/inertia to ruin AF-S. Especially the Sigma MC-11 when an SGV lens is attached...

--
Context is key. If I have quoted someone else's post when replying, please do not reply to something I say without reading text that I have quoted, and understanding the reason the quote function exists.

JimKasson · Jun 29, 2018

ventureout said:
Hi

i was was wondering how people are finding the focus accuracy on their Sony a7iii In af-s vs af-c focus, I have read reports about af-c being better which is different to how I use my other brand cameras etc, so after some feedback from the dpreview wisdom

https://blog.kasson.com/a7riii/sony-a7riii-autofocus-accuracy-summary/

Some of the high spots:

In AF-S mode, the a7RIII focuses with the lens diaphragm wide open, or very close to that. That means that the images are prone to suffer from focus shift. Some of the above lenses have remarkably little focus shift, so the way the a7RIII works is not always problematical, but others have significant focus shift.
I don’t know of a workaround for this AF-S behavior other than using AF-C, which works because, in AF-C mode, the a7RIII focuses with the lens diaphragm at taking aperture, and focus shift is not a problem.
So what’s wrong with setting the camera to AF-C almost all the time? AF-C gives more focus variation than AF-S.
Using face detection can reduce focus plane variability.
Large spot sizes tend to create less variability in the focus plane than small or medium ones. That’s probably why face detection also helps, but that doesn’t explain the next one.
Using eye detection can reduce focus plane variability.

I expect these things to apply to the a7III.

Jim

--
http://blog.kasson.com

geoffreyhdt · Jun 29, 2018

JimKasson said:
JimKasson said:

[...]

Click to expand...

Using eye detection can reduce focus plane variability.[...]

I actually found that I get more accurate focus (i.e.: sharp where I want it to be) if I use face focus vs eye focus (in AF-S) where there is no point focusing on the eye specifically (i.e.: when getting the eye in focus gets the entire face in focus, like when shooting at 28mm f2.0 at a reasonable distance, for instance).

My moto is:

if you do not need eye-AF, use face AF

if you do not need face AF, use regular AF

if you do not need lock-on AF, use regular flexible spot

if you do not need AF-C (which does not lock the focus hence tends to focus a bit in front, right on spot, a bit behind, right on spot, etc...), use AF-S

and if you can...use DMF

that way, you narrow down the possible causes of misfocus that, as good as the technology is, may still happen in various situations.

PS: I am aware of what focus-shift is but have not been affected by this in AF-S yet. I have mainly use Sony 28mm f2, 50mm f1.8 and 85mm f1.8. I got the Samyang 35mm f1.4 and Sigma 16mm f1.4 (used in APS-C mode to get 24mm f2 equivalent) but have not used them enough to confirm whether or not there is focus shift in AF-S.

techjedi · Jun 29, 2018

ventureout said:
Hi

i was was wondering how people are finding the focus accuracy on their Sony a7iii In af-s vs af-c focus, I have read reports about af-c being better which is different to how I use my other brand cameras etc, so after some feedback from the dpreview wisdom

thanks

I usually use AF-C, but I don't notice an accuracy difference between the two as long as I keep my body fixed position from subject after getting a lock in AF-S. With AF-C I have more freedom to move while focusing and still get great AF.

Note that the user's selection of the following menu choices will affect the perception of accuracy:

Priority Set in AF-S -> AF, Release or Balanced Emphasis

Priority Set in AF-C -> AF, Release or Balanced Emphasis

If someone always had "Release" priority, they might report poorer accuracy than someone who selected "AF" or "Balanced Emphasis"

IdM photography · May 10, 2019

I just received my Sigma 40mm f/1.4 Art, a very sharp lens... Doing some tests with the diaphragm wide open, I noticed that the photos are noticeably sharper (I assume better focused) when using AF-S rather than with AF-C. The difference is systematic.

I did not notice such differences with my Sony 24-105 f/4, but evidently focusing differences are more difficult to notice with an f/4 lens than with an f/1.4...

juanmaasecas · Jul 3, 2019

IdM photography said:
I just received my Sigma 40mm f/1.4 Art, a very sharp lens... Doing some tests with the diaphragm wide open, I noticed that the photos are noticeably sharper (I assume better focused) when using AF-S rather than with AF-C. The difference is systematic.

I did not notice such differences with my Sony 24-105 f/4, but evidently focusing differences are more difficult to notice with an f/4 lens than with an f/1.4...

sabin_csl · Aug 4, 2019

yes i had the same problem,a few months ago it got a lot better since the a7iii firmware 3.01

as for samyang 85 1.4 with firmware version 4 the focus accuracy in af-c got better, noticeably better then version 3.

photofan1986 · Jul 20, 2020

I am "happy" to read those observations, as I am too struggling with focus accuracy with my new A7III and mostly the Tamron 28-75 (but other lenses are affected too). I'm trying to isolate the culprit but the issue seems quite random, which is a shame...

Here is an example, shot at f/2.8 with C-AF on the left and S-AF on the right.

I'm pretty disappointed in this as C-AF was the only way of making the Tamron acceptably responsive on the A7III, because in S-AF when shooting stopped down, the camera is ridiculously slow to react.

Hmm, I'm starting to think I'll not get along with those Sony cameras :-(

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Sony a7iii af-s vs af-c focus accuracy

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