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08-27-2018, 11:51 AM   #1
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Auto-27 logic

Hi guys,

I've been somewhat puzzled by the Auto-27 AF mode on my K-3. I was, probably wrongly, expecting it to focus on the point of nearest focus but it doesn't seem to be the case. So, I'm trying to understand how it works and decides which point to use. The idea being to get some level of predictability with this mode.

Unfortunately, the K-3 user's manual doesn't tell much about it with very few details. I've search the web and PF, but didn't find much useful information either.

The few tests I've done myself seem to suggest that Auto-27 will rather focus on the plane of focus having the most points of focus on. It also seems to use a higher ponderation for the center points (not all focus points are considered equal)...

So, is there anyone knowing which logics are used on Auto-27 to determine where the focus will be ?

08-27-2018, 11:57 AM   #2
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Interesting question and many of us can learn from the responses. The only time I tried it, it seemed random, but then I use Av or M most of the time.
I would expect P mode with face detect to yield some sort of result that causes faces to be the focal point but I have never tried it.
I guess the usefulness will be mode dependant.
08-27-2018, 12:24 PM   #3
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QuoteOriginally posted by Kevin B123 Quote
Interesting question and many of us can learn from the responses. The only time I tried it, it seemed random, but then I use Av or M most of the time.
I would expect P mode with face detect to yield some sort of result that causes faces to be the focal point but I have never tried it.
I guess the usefulness will be mode dependant.
I didn't use Auto-27 much either. Because, like you, I find it kind of random and unpredictable as to where it will focus. However , while discussing with a Canon guy, he told me that he uses auto points selection a lot because this allows him to take a shot quickly if needed. He knows that its camera will select the nearest point of focus, even if it's a single point. Since it's quite predictable, he simply make sure the main subject is the nearest thing in the AF area to focus on and take the shot. He don't even have to wait for checking wich point has been selected since the behavior of the AF is quite predictable. Very quick and efficient way of working in dynamic situations.

I've tried to do same with my K-3, but it will take at least 3-4 AF points on a subject nearer than the centre point for the camera to focus on it. On the other hand, the center point will still be chosen even if it's the only one on the nearest target. The two linears side points are never selected, unless being the only target to lock on (like a small dark line on an otherwise white background). These all combine to make the behaviour of Auto-27 difficult to predict... It could be much more useful with some additional knowledge on how it makes its decision...

Last edited by CarlJF; 08-27-2018 at 01:14 PM.
08-27-2018, 12:32 PM   #4
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It chooses the first point deemed focusable with preference given to the central area. It does not find faces, eyes, children, or dogs. The assumption is that some effort had already been given to framing the subject.


Steve

08-27-2018, 01:41 PM   #5
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QuoteOriginally posted by stevebrot Quote
It chooses the first point deemed focusable with preference given to the central area. It does not find faces, eyes, children, or dogs. The assumption is that some effort had already been given to framing the subject.


Steve
So basically its a pretty useless mode! I tried it a few time right after I bought my K-3 in 2014 and decided that it was not reliable for what I wanted.
08-27-2018, 01:54 PM - 1 Like   #6
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QuoteOriginally posted by Larrymc Quote
So basically its a pretty useless mode! I tried it a few time right after I bought my K-3 in 2014 and decided that it was not reliable for what I wanted.
I would not say it is useless, though it is a mode I seldom use. Where I have found it useful is when shooting moderate action where I want to focus my attention on framing and not be concerned about the specifics of where focus will land. For that task, auto-27 can be amazingly effective, particularly when combined with short bursts. I don't remember which of the young ladies took focus as they were dancing past, but I don't care.


Pentax K-3, Sigma 17-70/2.8-4 (C), f/4.5, 1/640s, 70mm

(...I suspect the one on the right...)


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08-28-2018, 06:51 AM   #7
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QuoteOriginally posted by Larrymc Quote
So basically its a pretty useless mode! I tried it a few time right after I bought my K-3 in 2014 and decided that it was not reliable for what I wanted.
I would not go as far as to say it's useless, but it's certainly not as useful as it could be. If the the system works as Stevebrot describes it, which really could be the case, it makes it hard to predit where the focus will be. This restrict the use of Auto-27 to the scene where your subject is the only possible target (like an airplane or bird over a blue sky), or you don't really care where the focus will be like the Stevebrot's example above. But even then, from my understanding, Auto-27 could just as well have focused on the background...
08-28-2018, 09:00 AM   #8
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QuoteOriginally posted by CarlJF Quote
I would not go as far as to say it's useless, but it's certainly not as useful as it could be. If the the system works as Stevebrot describes it, which really could be the case, it makes it hard to predit where the focus will be. This restrict the use of Auto-27 to the scene where your subject is the only possible target (like an airplane or bird over a blue sky), or you don't really care where the focus will be like the Stevebrot's example above. But even then, from my understanding, Auto-27 could just as well have focused on the background...
I just tried Auto-27 AFC and continuous medium with my 55-300 PLM shooting an AC-130J Gunship flying right over our home and the focus was scattered all over the place. The next pass the C-130J made I used spot select with AFC and continuous medium and every shot was spot on. So my test didn't prove that Auto-27 is satisfactory for even large aircraft close in with a clear blue sky background.

I have hold AF turned off so that may have affected the outcome but having hold AF turned on has always resulted in the focus being behind where I had the aiming point especially with oncoming aircraft or aircraft going away from me.

08-28-2018, 09:25 AM   #9
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QuoteOriginally posted by Larrymc Quote
I just tried Auto-27 AFC and continuous medium with my 55-300 PLM shooting an AC-130J Gunship flying right over our home and the focus was scattered all over the place. The next pass the C-130J made I used spot select with AFC and continuous medium and every shot was spot on. So my test didn't prove that Auto-27 is satisfactory for even large aircraft close in with a clear blue sky background.
Well, if it's not even useful for that, I guess it isn't of much use for anything... My own testings were done on static but decentered subjects only covering part of the frame. Most of the time, Auto-27 focused on the background unless the subject covered at least 3 of the 9 central points... But then, it's probably just better and reliable to use SEL-9 or Auto-9 and move the AF area over the subject...
08-28-2018, 12:13 PM   #10
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PDAF at best focuses on lines on subjects, not subjects, so if there's a good edge in the background ...

That's why we should control which point is used to do this, IMHO.

08-28-2018, 12:57 PM   #11
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QuoteOriginally posted by clackers Quote
PDAF at best focuses on lines on subjects, not subjects, so if there's a good edge in the background .
Sure! But the question still is, if multiple equivalent points are available, which one will be chosen by Auto--27 ? The nearest one ? The one allowing the most points to be on focus ? The first one it finds ? This is the kind of information that is currently lacking for Auto-27: what is the camera actually trying to do in addition to simply "selecting the optimum focusing point" (as written in the K-3 manual) and supposing it's trying to do something (which isn't even sure) ? What does RP means by "optimal focusing point" ? If it's the easiest one to lock on, it's okay, but it would nice from RP to specify this somewhere...
08-28-2018, 03:01 PM - 1 Like   #12
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QuoteOriginally posted by CarlJF Quote
if multiple equivalent points are available, which one will be chosen by Auto--27 ?
The first one with strong enough "signal" within conditions of a base algorithm. As clackers noted, what is being focused on are lines or what the camera thinks looks like lines. The detector is fully analogous to a split image rangefinder used in classic SLR focus screens. If a good strong "line" comes into scope for one of the set of higher priority points, that point will go red in the viewfinder screen. If a stronger "line" is in scope somewhere else, that point may also go red momentarily. Ultimately an attempt to attain will be made from the strongest signal with adequate priority. No, I have no idea what algorithm is used or how to predict behavior. If one needs that, it is probably best to select a point of zone rather than give the camera free reign.


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08-28-2018, 04:28 PM - 1 Like   #13
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QuoteOriginally posted by clackers Quote
PDAF at best focuses on lines on subjects, not subjects, so if there's a good edge in the background ...

That's why we should control which point is used to do this, IMHO.
I totally agree!
08-28-2018, 04:35 PM   #14
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QuoteOriginally posted by CarlJF Quote
Sure! But the question still is, if multiple equivalent points are available, which one will be chosen by Auto--27 ? The nearest one ? The one allowing the most points to be on focus ? The first one it finds ? This is the kind of information that is currently lacking for Auto-27: what is the camera actually trying to do in addition to simply "selecting the optimum focusing point" (as written in the K-3 manual) and supposing it's trying to do something (which isn't even sure) ? What does RP means by "optimal focusing point" ? If it's the easiest one to lock on, it's okay, but it would nice from RP to specify this somewhere...
It's a mode for people who don't know and understand the other options, like newbies, Carl, so I don't think they'll bother documenting it with a technical article, to be honest!

Once upon a time, I think in all manufacturers what it did was simply the closest thing under all the AF points, but I'm sure the algorithm has been tinkered with since.

If I were to use it shooting birds in trees, I'd just be getting all the twigs in focus in front of the subject. I even go Manual Focus at times to get rid of the camera's decision making altogether.

Like P mode or Auto ISO, be prepared to be disappointed asking any camera to know what you have in your mind about the scene.
08-28-2018, 05:44 PM - 1 Like   #15
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Thank you guys!
In the end, I conclude that it's probably better just to forget doing anything much useful or better than what can be done with other modes. So, I will continue mostly using single point in AF-S and SEL-9 in AF-C. So far, these have worked fine. And I will stop wondering about Auto-27 and what could be done better with it!
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