[Madaboutpix]

Hm. Kobie seems to have encountered unexpected behaviour of his K-3 III in AF.C Auto via back-button AF, where it would erratically jump to various suboptimal focus points instead of focusing on the most contrasty subject. This was different from what it did when the AF was activated with the shutter button, in which case it would invariably pick the contrasty sign in the centre of the frame as its AF target. Whatever caused this, it would seem that back-button and shutter operation yield different results in AF point selection, which clearly shouldn't happen.

You Tube

You Tube

Can any K-3 III user confirm that this is a replicable issue? If that should be the case, let's hope that people at Ricoh put this on their to-do list for the next firmware update.

[jersey]

It is true. I never use back button, but I just checked. Two pics: first it is shutter button, aimed dead centre to the most contrasty (b/w) element, second is back button and aimed just somewhere in the bottom, at white filed. Weird.




Shared album - Jerzy ?Jersey? Jab?o?ski - Google Photos




Shared album - Jerzy ?Jersey? Jab?o?ski - Google Photos

[mctaveck]

My test (quick test) : the red dot when viewing the image after the shoot does not always correspond to the black square i saw just when shooting. In viewing the red dot sometimes does not appear at all. But hand helding the camera is not a good test.
I'll check again.

[house]

This seems like a pretty difficult issue to track down successfully. I'd say you need a lot of shots, 100's, to be able to say anything one way or another. It's very easy to bias and read results from coincidence.

[licht96]

Beside the behavior of different focus using both buttons: who uses the Auto System?

Yes all cameras have included this and it´s really fully based on the “intelligence” of the algorithm….

I see this behavior in all cameras. When you press the shutter button half sometimes a different AF point or a lot´s of AF points are used….


…and since the cameras (we already discussed this in a parallel topic) is not a human: how should it know, what is the point of interest of the scene for the man behind the camera.


Yes it´s the contrast: but both of examples shows the problem for the algorithm:

Kobie: yes. The sign is in your interest…and yes, it has prominent contrast ….but there a a lot of contrast in the picture as well. For example the sky against the trees. And subject recognition may be don´t “know” a sign

Jersey: the scene is full of contrast – more hard to choose for the camera compared to example from Kobie: it´s more or less in the same distance were all the things “happen”. Why should the camera “know” your T-shirt?


Coming back to the the different button: you can do this also with one button. When you try different time, the Auto system can find a different points of interest


Before the first Firmware update came to the mirrorless Nikon Z6II, you can see the same behavior: always a different place for the AF to take place on the scene….the new firmware helps a little bit: but also for this system: how should the camera know in a scene full of contrast, what do you like to focus in AUTO mode?

I mean, all cameras shows this more or less. Canon, Nikon which are a liitle "different" in AF with a little bit constantly in choosing same area.
Why I mention the other brands? To be honest and beside all AF discussions about "our" brand: The Auto mode was really never a strength of Pentax... and now with all technical help from AE as well... it's only a little bit better.


(this topic is different to the tracking of a subject)

[Paul the Sunman]

I'm not sure I'm following. Am I just being thick? Is this single-point AF.C focus? I can't reproduce any sort of focus point jump.

[Ishpuini]

I always use back button focus. I spent a few hours trying to capture fast birds in the sky above our house with my DFA150-450, using AF-C with auto AF point selection, and I got a very nice hit rate. Never had any issues with strange AF point selection during those tests...

[licht96]

@Paul the sunman: That's the point : the topic starts talking about AUTO AF.... so ALL AF points chosen to select for their own which one can focus.

That worked - in my understanding and knowledge - never really good before, too

[Paul the Sunman]

OK, with the full AF.C AUTO enabled, and just shooting complex scenes in my study, I get fairly random AF point selection with either shutter button or back button AF. Both seem to look for edges, but any edge will do. Whether the shutter button random distribution is different from the back button random distribution would need a large test to determine, though my feeling is that they are different. But I wouldn't say one is better than the other at this stage.

[othar]

it would seem that back-button and shutter operation yield different results in AF point selection, which clearly shouldn't happen.

For AF.C I agree, that this would be a weird behavior, but one could argue, that in general there is a different mentality between back button AF and shutter button AF and therefore a different algorithm in camera would not be out of the question.
Shutter button AF is activated every time you take a picture and therefore often starts with an already in focus subject, whereas back button AF could be used to check focus on the subject it is more likely you want to focus somewhere else (at least more common than with shutter button AF)

[biz-engineer]

I doubt back button AF and shutter button would drive different auto-focus algorithm. What could make user believe that back button AF behave differently is due to initial state of focus and variability inherent to image pattern recognition. Repeated shutter half press may not give the same auto AF point selection. Testing is there is a difference between back button AF and shutter AF would require statistical sampling: focus 30 times with back button AF (make sure target doesn't move and lens initial position is always de same) and 30 times AFC with shutter button, not AF point each time, compare statistics.

---------- Post added 28-07-21 at 13:21 ----------

where it would erratically jump to various suboptimal focus points instead of focusing on the most contrasty subject.

If there was already AF lock on a point, a new attempt to focus will select the next nearest point. Typical behavior of auto-AF point selection on my K1. It's quite smart. We need to understand how our gear works before claiming it's faulty.

---------- Post added 28-07-21 at 13:24 ----------

Whatever caused this, it would seem that back-button and shutter operation yield different results in AF point selection, which clearly shouldn't happen.

The first "inconsistency problem" would be that pressing two times the same shutter button doesn't lock AF on the same point. If there are two areas in an image target which have the exact same contrast, computer have no feeling, it's will randomly switch from one area to the other depending on random error of phase detect measurement.

[OvidiuRoSb]

I think the system is working as expected because AF-C Auto Area is used for moving subjects and not on what I have seen above. I made a short test using AF-C Auto Area using half shutter-button and the system switches between focus points searching for changes in contrast (movement in the frame). Same behavior with back focus button, no difference.
To proper test AF-C Auto Area using both focus methods(buttons), you need a moving subject as target and compare the output for out of focus. If the result is OK then you have a system that's working as expected.

[FozzFoster]

AF-C is made for moving subjects.
I am 100% certain that, if that sign started moving, both the shutter button and back button AF would continuously track the moving subject.

When you select the full auto focus area, you're letting the camera decide what to focus on - and being in AF-C (camera thinks the subject will be moving), it probably looked for motion and may have picked up the leafs rustling in the background...

[jersey]

I will check next time I am will be shooting airplanes so probably not soon as my vacations are starting. But I think without movement in frame expected behaviour will be to focus in center or on same contrasty point. While it is true with shutter, not true with AF button.

[Kobie]

AF-C is made for moving subjects.
I am 100% certain that, if that sign started moving, both the shutter button and back button AF would continuously track the moving subject.

When you select the full auto focus area, you're letting the camera decide what to focus on - and being in AF-C (camera thinks the subject will be moving), it probably looked for motion and may have picked up the leafs rustling in the background...

If that's the case, then why did that NOT happen when using the shutter button? I was out there for a while and tested this on many different subjects with consistent results. I didn't shoot each one as I was only testing the AF system to see where it would focus. Flowers, plants, bees, birds. Back button and shutter button behaved differently for some reason.