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A7RIV: is AF-C better than AF-S even for static subjects?

  
 
baltmin
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p.1 #1 · p.1 #1 · A7RIV: is AF-C better than AF-S even for static subjects?


A bit odd thread title, I must admit. Iím playing with my new A7RIV for the last 10 days and I have noticed slight misfocus in some of the landscape shots taken using AF-S. I know what Sony AF system doesnít like as Iím coming from the a7rIII (lots of foliage with relatively low contrast or very high contrast on the other side of the spectrum). I think the issue is more prominent with A7RIV probably because the high mp sensor does not forgive anything and every weakness of lens, technique or AF becomes apparent.

Has anybody tried using AF-C for everything and what are the results?

PS: I havenít noticed any problems with close focusing (1-10m) or portrait photography using eye AF with narrow depth of field. Iím talking about landscape photography that it is not always tack sharp because of slight misfocus.



Nov 05, 2019 at 06:15 PM
vdo1
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p.1 #2 · p.1 #2 · A7RIV: is AF-C better than AF-S even for static subjects?


Any chance you were using a lens affected by focus shift? AF-C and AF-S may be using different apertures when focusing,


Nov 05, 2019 at 07:15 PM
racetratr
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p.1 #3 · p.1 #3 · A7RIV: is AF-C better than AF-S even for static subjects?


I have the same experience. I get fine results at close distances wide open, and using eye-af. But like the A7RIII, the A7RIV tends to front focus slightly in AF-S using typical landscape settings. I've confirmed this using the digital focus indicators on my Batis lenses. It does this even for lenses with little or no focus shift.

However, the A7RIV's AF-C autofocus is definitely improved over the A7RIII. I now use it for almost everything, with magnified manual focus as a second option. AF-C works well for landscapes, giving me accurate focus or, sometimes, very slight back focus. Overall, it is usually more consistent and accurate than my own manual focus. Not a bad solution, actually.

I use the "stickiest" AF-C settings, including aperture drive set to focus priority. I use the back button for autofocus, and let it "settle" for a while. With the right settings, the viewfinder stays clear and unblinking at any light level. There's no worry about focus shift in AF-C, of course.




Nov 05, 2019 at 07:15 PM
baltmin
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p.1 #4 · p.1 #4 · A7RIV: is AF-C better than AF-S even for static subjects?


Thank you both for your reply. I noticed the problem with my 24-70 gm. Is it known to have focus shift issues?
I will change my settings so as to focus using the working aperture. However if this was the problem I would have consistently misfocused photos, correct? I would describe the results as rather inconsistent and unpredictable.
I will also test AF-S and AF-C on the same subject and see what happens.



Nov 06, 2019 at 01:21 AM
racetratr
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p.1 #5 · p.1 #5 · A7RIV: is AF-C better than AF-S even for static subjects?


I think inconsistent is a good word for it. If I shoot several re-focused exposures using AF-S, some will usually be very sharp. As you noted, missed focus seems to happen more at landscape distances.

I don't know if the GM zoom has any focus shift issues. If it does, they may not be the same at every focal length.

Eye-AF is another whole thing. It seems to be almost like a focus system of its own, and it is usually very accurate, in my experience.



Nov 06, 2019 at 01:36 AM
Holger
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p.1 #6 · p.1 #6 · A7RIV: is AF-C better than AF-S even for static subjects?


baltmin wrote:
A bit odd thread title, I must admit. Iím playing with my new A7RIV for the last 10 days and I have noticed slight misfocus in some of the landscape shots taken using AF-S. I know what Sony AF system doesnít like as Iím coming from the a7rIII (lots of foliage with relatively low contrast or very high contrast on the other side of the spectrum). I think the issue is more prominent with A7RIV probably because the high mp sensor does not forgive anything and every weakness of lens, technique or AF becomes apparent.

Has anybody tried using AF-C
...Show more

Yes. I use AFC professionally with A9, A7r3 and A73 for everything and didn't have any issue at all so far. I find AF quicker and nicer to use this way. After half a Million shots over several bodies over the last years, I am happy shooting this way.



Nov 06, 2019 at 02:12 AM
DaveFP
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p.1 #7 · p.1 #7 · A7RIV: is AF-C better than AF-S even for static subjects?


My understanding is that focus shift only occurs at the widest apertures (other then wide open).

Probably not the issue at f/5.6 - 8



Nov 07, 2019 at 06:05 AM
dclark
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p.1 #8 · p.1 #8 · A7RIV: is AF-C better than AF-S even for static subjects?


Sony does not provide much information on how the system focuses. I am never sure if the lens will pop open to acquire focus, whether it refocuses with the lens stopped down to avoid focus shift errors, and whether it uses CDAF to fine tune the PDAF. The setting "Aperture Drive in AF" is poorly documented.

In order to get some idea what the camera is doing I sometime look into the lens to see the iris, set the lens to be stopped down, and then watch the iris as I push the shutter release to cause the camera to focus. For some lenses I have seen the iris pop open then close. For some it remains open then closes when the shutter release is pushed. Most of the time it just says stopped down. That seems to indicate that for some lenses the aperture to acquire focus then stops down to take the photo. It is still not clear whether it refocuses stopped down. Most of my lenses don't seem to change the iris at all.

I did this with the A7R4 and the 24-70 GM. I did not see the iris move, regardless of settings. If I do this with my 200-600, the iris stays open until I focus then it closes down to the set f/#. That seems to imply the A7R4 should not have a focus shift issue due to iris movement with the 24-70.

If I do the same using the A9, the first thing I notice is that when I try to put "AF Drive in AF" into "Focus Priority" mode, the camera gives me an error message saying that mode is incompatible with the 24-70 lens.

I use AFC all the time and capture bursts of images. I believe that regardless of the lens, after the first shot in AFC the iris remains at the shooting aperture and the camera focuses with lens stopped down. That means there is some uncertainty about the first image but after that there is no possibility of focus shift. I have seen conflicting information on whether the camera will fine tune AF using CDAF. Some people claim that in AFS single shot it will do a CDAF fine, others say no. I don't know how it is possible to find out, so I assume everyone is just guessing. Unfortunately Sony provides no information that I can find.

It would be nice if we had more complete and reliable information. Maybe someone knows where such information can be found.

Dave



Nov 07, 2019 at 12:03 PM
snapsy
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p.1 #9 · p.1 #9 · A7RIV: is AF-C better than AF-S even for static subjects?


dclark wrote:
I have seen conflicting information on whether the camera will fine tune AF using CDAF. Some people claim that in AFS single shot it will do a CDAF fine, others say no. I don't know how it is possible to find out, so I assume everyone is just guessing. Unfortunately Sony provides no information that I can find.


The CDAF confirmation cycle for AF-S that follows a PDAF acquisition can be visually observed by watching the LV feed during focusing. It's especially noticeable on slower-focusing lenses such as the FE 50mm f/1.8. You'll see a quick front-to-back focus warble that follows the PDAF acquisition - that's the telltale sign of the CDAF confirmation.



Nov 07, 2019 at 12:29 PM
JohnDizzo15
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p.1 #10 · p.1 #10 · A7RIV: is AF-C better than AF-S even for static subjects?


AF-C all day, every day. Such has been the case since the A7R2 for me with every lens whether adapted or native.

The only times I've ever used AF-S are when there is a very specific need that AF-C presented problems for which are few and far between.



Nov 07, 2019 at 12:30 PM
 


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dclark
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p.1 #11 · p.1 #11 · A7RIV: is AF-C better than AF-S even for static subjects?


snapsy wrote:
The CDAF confirmation cycle for AF-S that follows a PDAF acquisition can be visually observed by watching the LV feed during focusing. It's especially noticeable on slower-focusing lenses such as the FE 50mm f/1.8. You'll see a quick front-to-back focus warble that follows the PDAF acquisition - that's the telltale sign of the CDAF confirmation.


Yes I have observed that. The only time I see that is if the f/# is high enough that it won't focus in PDAF. Maybe it is fast enough that it's not visible. I have seen lots of opinions and speculation that in AFS single shot mode the camera always does CDAF after PDAF, and consequently it is more accurate. It seems that if that was true the OP would not have observed the problem he sees. It's also possible that CDAF is not working that well.

It would be nice if there was a source of authoritative information on AF with different lenses in different camera modes. Looking at iris movements and focus warble can tell us something but it would be much better if Sony would publish complete information. The result is there is a plentiful supply of opinions and speculation and a dearth of facts.

Dave



Nov 07, 2019 at 12:51 PM
DaveFP
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p.1 #12 · p.1 #12 · A7RIV: is AF-C better than AF-S even for static subjects?


So when would you all typically use AF-S ?

I have tended to use it with static scenes and Flexible Spot (Small)

I guess AF-C + a burst is more predictable?

Leaves a lot of images to review.....

The price of one good one I suppose.



Nov 07, 2019 at 01:08 PM
snapsy
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p.1 #13 · p.1 #13 · A7RIV: is AF-C better than AF-S even for static subjects?


dclark wrote:
Yes I have observed that. The only time I see that is if the f/# is high enough that it won't focus in PDAF. Maybe it is fast enough that it's not visible. I have seen lots of opinions and speculation that in AFS single shot mode the camera always does CDAF after PDAF, and consequently it is more accurate. It seems that if that was true the OP would not have observed the problem he sees. It's also possible that CDAF is not working that well.


That would be the CDAF-only fallback case. I'm speaking to the CDAF confirmation cycle, which occurs for the cases where PDAF is employed in AF-S. It's easiest to see the post-PDAF CDAF warble when triggering the AF while using a zoomed-in LV display - starting with the A7III/A7rIII the bodies support AF in the zoomed views.

dclark wrote:
It would be nice if there was a source of authoritative information on AF with different lenses in different camera modes. Looking at iris movements and focus warble can tell us something but it would be much better if Sony would publish complete information. The result is there is a plentiful supply of opinions and speculation and a dearth of facts.


Agreed. Sony's stopped-down AF behavior varies not only by model and lens but actual firmware versions as well. It's a mess.



Nov 07, 2019 at 01:16 PM
kimknapp
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p.1 #14 · p.1 #14 · A7RIV: is AF-C better than AF-S even for static subjects?


I typically use AF-C, but it will miss focus if the subject you are focusing on is low contrast and the background is high contrast, even with a small flexible spot and the background is outside the spot. I have seen this on the A9, too.
So, the other day I was experimenting with using magnify while in AF-S mode and it worked really well. Plus, you can visually see that you are perfectly focused.



Nov 07, 2019 at 01:21 PM
snapsy
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p.1 #15 · p.1 #15 · A7RIV: is AF-C better than AF-S even for static subjects?


dclark wrote:
I have seen lots of opinions and speculation that in AFS single shot mode the camera always does CDAF after PDAF, and consequently it is more accurate. It seems that if that was true the OP would not have observed the problem he sees. It's also possible that CDAF is not working that well.


Just remembered I took a video demonstrating the CDAF confirmation cycle last year. This was using a FE 50mm f/1.8. I compare it to AF-C, which naturally doesn't employ the CDAF confirmation for performance reasons:




Nov 07, 2019 at 01:21 PM
snapsy
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p.1 #16 · p.1 #16 · A7RIV: is AF-C better than AF-S even for static subjects?


baltmin wrote:
A bit odd thread title, I must admit. Iím playing with my new A7RIV for the last 10 days and I have noticed slight misfocus in some of the landscape shots taken using AF-S. I know what Sony AF system doesnít like as Iím coming from the a7rIII (lots of foliage with relatively low contrast or very high contrast on the other side of the spectrum). I think the issue is more prominent with A7RIV probably because the high mp sensor does not forgive anything and every weakness of lens, technique or AF becomes apparent.

Has anybody tried using AF-C
...Show more

Which lens? Some lenses have noticeable focus shift issues - since Sony's AF-S focuses at the largest aperture on some lenses if those lenses have focus shift issues then focus will be off if the shooting aperture is smaller. This was also a problem with some of the early Sony 24-105mm f/4 lenses, which required a service recall to correct.



Nov 07, 2019 at 01:26 PM
dclark
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p.1 #17 · p.1 #17 · A7RIV: is AF-C better than AF-S even for static subjects?


DaveFP wrote:
So when would you all typically use AF-S ?

I have tended to use it with static scenes and Flexible Spot (Small)

I guess AF-C + a burst is more predictable?

Leaves a lot of images to review.....

The price of one good one I suppose.


I would say that AFS single shot would be the usual AF choice for landscapes, as the OP was using it. The reason for that is the expectation that PDAF is followed by fine tuning with CDAF. The problem is it did not seem to work for the OP, for reasons that are not clear. Thus far all the comments seem to say it should have worked, but, since we don't really know what's going on, give AFC a try.

When I shoot landscapes (not that often) I shoot manual focus.

Dave



Nov 07, 2019 at 01:29 PM
racetratr
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p.1 #18 · p.1 #18 · A7RIV: is AF-C better than AF-S even for static subjects?


snapsy wrote:
The CDAF confirmation cycle for AF-S that follows a PDAF acquisition can be visually observed by watching the LV feed during focusing. It's especially noticeable on slower-focusing lenses such as the FE 50mm f/1.8. You'll see a quick front-to-back focus warble that follows the PDAF acquisition - that's the telltale sign of the CDAF confirmation.


I used to think that the Sony fine tuned AF-S focus using CDAF, but that turns out not to be the case. In AF-S, all recent Sony bodies focus wide open. Jim Kasson has done a lot of work on this. He shows, among other things, that any focus shift that a lens may have is fully represented in AF-S-focused images. (For instance: https://blog.kasson.com/a7riii/sony-a7riii-autofocus-accuracy-summary/).

Jim also showed that, at least with the A7RIII, AF-C is more accurate than AF-S but less precise. That is, average focus is grouped around the correct focus point, even for lenses with focus shift. But it can vary significantly from one focus attempt to another.

My experience with the A7RIV is that, depending on settings, AF-C is more reliable for landscape subjects than it is with the A7RIII.



Nov 07, 2019 at 01:36 PM
snapsy
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p.1 #19 · p.1 #19 · A7RIV: is AF-C better than AF-S even for static subjects?


racetratr wrote:
I used to think that the Sony fine tuned AF-S focus using CDAF, but that turns out not to be the case. In AF-S, all recent Sony bodies focus wide open. Jim Kasson has done a lot of work on this. He shows, among other things, that any focus shift that a lens may have is fully represented in AF-S-focused images. (For instance: https://blog.kasson.com/a7riii/sony-a7riii-autofocus-accuracy-summary/).

Jim also showed that, at least with the A7RIII, AF-C is more accurate than AF-S but less precise. That is, average focus is grouped around the correct focus point, even for lenses with focus shift. But it
...Show more

The fact AF-S's precision is higher than AF-C is your first statistical clue that AF-S is using CDAF confirmation. That can be visually confirmed by actually observing AF-S's behavior while focusing - the contrast-seeking warble from the CDAF conformation is clearly visible. Focus shift will still occur even after the CDAF conformation cycle because that cycle is performed at the same wide-open aperture that the PDAF cycle employed, thus will not correct for focus shift.



Nov 07, 2019 at 01:44 PM
racetratr
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p.1 #20 · p.1 #20 · A7RIV: is AF-C better than AF-S even for static subjects?


snapsy wrote:
The fact AF-S's precision is higher than AF-C is your first statistical clue that AF-S is using CDAF confirmation. That can be visually confirmed by actually observing AF-S's behavior while focusing - the contrast-seeking warble from the CDAF conformation is clearly visible. Focus shift will still occur even after the CDAF conformation cycle because that cycle is performed at the same wide-open aperture that the PDAF cycle employed, thus will not correct for focus shift.


AFAIK, there has never been any technical documentation of CDAF confirmation, by Sony or anyone else. IMO, the "warble" in the finder has nothing to do with that; I think it may have to do with resetting the finder brightness after stopping down. Interesting that you think CDAF confirmation would be done wide open. Why wouldn't it do its confirmation when the aperture was open for PDAF instead of stopping down and then opening again?

I expect that AF-S precision is higher in some situations because it is focusing wide open, and therefore has more light available, and lower depth of field. AF-S works well for me for wide open photography and close subjects, as long as focus shift isn't an issue. But in my landscape work, I find AF-S to be far from precise. I can focus on foliage or a building and refocus several times, while watching the distance scale on my Batis lenses jump around from one attempt to the next. The variability in AF-S focus is borne out in the images. With AF-C on the A7RIV, I get more accuracy and more consistency.

Here's more by Jim Kasson on non-correction of focus shift: https://www.dpreview.com/forums/post/60663729



Nov 07, 2019 at 02:19 PM
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