Pentax KP AF issue

08amczb · 03-27-2017, 01:53 PM

My first impression was very positive about the KP, but I ran into an issue. When my subject is near to the background and the background has many detail (many edges) and the subject approximetly fills the spot meter circle, but not bigger than that, then the AF locks on the background consistently. Not a backfocus issue, because the background is always sharp and I can get sharp images when the subject and the background is more separated.

Here is two examples:

The K-3/K-3ii or the other KP's do the same? I think it's a firmware bug, the center of the center focus point should be calculated in the correlation with more weight then the outside. This really anoyes me, because I can't get sharp images when I want some contextual portraits with wide open aperture because of low light or forground blur.

How to report this to Ricoh? I didn't find the appropriate e-mail adress/form on their global site.
Thanks for the opinions/suggestions.

**RobA_Oz** · 03-27-2017, 02:02 PM

Which AF mode are you using? Your text doesn't really indicate if you're using single or multi-point AF. I'd be inclined to use single-point for critical focus with static subjects like these. Focus, hold focus then recompose, to be clear.

08amczb · 03-27-2017, 02:13 PM

I was using AF-S single point select in the center and I was doing focus and recompose. The AF is set to back button only, I was focused on their face and waited until the green hexagon appeared and the center led also confirmed the focus.

rawr · 03-27-2017, 02:29 PM

Worth noting this, from the Nikon D4 manual:

I also suggest that you set your metering to multi-segment for scenes like this, so that colour information can be used to assist the AF - in the first image, the face may have enough distinct colour from the background to help the AF. Or if needed, switch to LiveView focussing, & enable Face Detection mode.

C_Jones · 03-27-2017, 02:36 PM

I use Single Point AF and AF.C. In case you happen to budge a little the AF.C with still look for focus, whereas the AF.S will be still remembering the focus point when the shutter had been pushed down halfway. Focusing on a consistently black or consistently white object with little shape is sometime difficult for the camera to AF due to the non-contrast appearance. You may want to do some tests where the subject has definite contrast with moderate light and see what your results are. I also make sure I am using an appropriate aperture for the depth I need. I noticed you were using F2.8. You may want to test with a higher number F Stop just to see what results you get. You most likely already know these things, but I am just mentioning them.

08amczb · 03-27-2017, 02:41 PM

I was using matrix metering. The metering does not influences the autofocus, only the sensor is common. The image from the sensor weighted by the metering mode and that gives the exposeur and the same source image can help the AF system. My case is definitly the second one from the D4 manual, but as an engeneer who did image processing in the past I think in this senarios the AF should find my subject.
I messured the size of the center AF point and it's approximetly the same size as the spot meter circle. My subject was filling the 90% of it at the center.

**caliscouser** · 03-27-2017, 04:45 PM

I don't have experience with the K3 or KP but what I learned anecdotally on my other camera bodies is the center focus point is bigger than you think.
In the case of the 2 photos above it looks like it focused on the most high contrast areas in and around the head. Even if the head is 90%. Those are the background wood paneling in picture 1 and the branches in picture 2.

So focusing on the subjects body instead or something else in the plane of focus is probably going to yield better results.

**clackers** · 03-27-2017, 06:48 PM

Originally posted by 08amczb

I was focused on their face.

Hi 08amczb,

Whether you're shooting Canon or Nikon or Pentax, you don't focus on a face, you focus on a line, so pick one.

If you're very close, it can be the pupil of an eye.

In your first shot, it could be the edge of the tie against the white shirt.

**Clarkey** · 03-27-2017, 07:47 PM

From the manual, page 62, it looks like you can reduce the size of the AF area, or use face detection. I haven't had a Pentax for a bit (last one K-5IIs), but it looks like the AF options have become quite a bit more sophisticated.

rawr · 03-27-2017, 09:01 PM

Originally posted by 08amczb

The metering does not influences the autofocus

It should ... talk to the AF when multi-segment metering is used along with something other than centre-spot AF, according to what we know of the K-3/K-3II, 645Z, K-1 and K-P.

Features4 | PENTAX KP | RICOH IMAGING

08amczb · 03-28-2017, 06:36 AM

Thanks, Jones. Yes I know how the AF works. The F2.8 was intented, deeper DoF certenly helps, but I would like to blur the background by a little and get sharp faces. If my subject is a face I can't choose more contrasty target sadly. And zooming out after focusing changes the focus plane too, so I can't do that neither.

Originally posted by caliscouser

I learned anecdotally on my other camera bodies is the center focus point is bigger than you think.

Thanks, caliscouser, this helped me. The KP has 3 F2.8 luminance flux AF point, so I will try the upper one. Now this is not the edge, like on my K-30. I messured the size of the center point and yes, you are right the background is in the area. I hoped some more intelligance from the AF system. It would calculate the center with more weight in the phase matching. (Of corse it could make other issues.)

Originally posted by Clarkey

From the manual, page 62, it looks like you can reduce the size of the AF area

This is true for Live View only. And yes live view may solve my problem, but I am a viewfinder shoter, and live view is slow and consumes more power. I keep this option as a my last fallback.

Rawr, trust me I am an engeneer.

I was designed algorithmys for medical imageing (X-ray tomosynthesis), I've detected cancer by AI.
To make the AE and AF system a little more clear here is how a basic system works:
The auto exposeur sees the same image as you through the viewfinder by a little 86k image sensor similar to the main sensor in the camera. In spot metering and center weighted average the metering is only a weighted average of the pixel values. (It may do it per chanel.) In matrix metering this weight matrix is modified by many parameters - like the selected AF point. It can do face detections and advanced scene analysis by deep neural networks for example. But this calculation is independent of the AF system - wich uses the same AE sensor and the same source image. The AE sensor can help the AF system by selecting the AF point in tracking mode. Because it sees a video through the viewfinder it can track the lateral movement. The AF sensor only sees the depth.
So the 86k sensor is common, but the metering mode does not affect the AF algorithm and only a few AF parameters affects the metering like the selected AF point.

frogfoot · 03-28-2017, 07:07 AM

Originally posted by 08amczb

The K-3/K-3ii or the other KP's do the same? I think it's a firmware bug, the center of the center focus point should be calculated in the correlation with more weight then the outside. This really anoyes me, because I can't get sharp images when I want some contextual portraits with wide open aperture because of low light or forground blur.

Yes it happens with K-3 and earlier cameras too. Sometimes it focuse on a background if it's more contrasty than the object. And looks like it is lens dependant.. Although I can't find an explanation to this

DA 40 is known as the lens most affected to this.

BTW A friend of mine who has Nikon D7100 said me he experienced this too..

08amczb · 03-28-2017, 07:33 AM

Thanks frogfoot, I didn't noticed it's lens dependence. If K-3 and Nikon D7100 also does this then I definitly have to improve my technique and maybe my vision with some laser surgery...

starbase218 · 03-31-2017, 02:08 AM

I've also done some testing on this and sometimes find it difficult to get precise focus as well. I think what's causing this is that the AF points of the K-3 are so large they actually overlap.

labidas · 05-13-2018, 10:50 AM

K50 here. This happens constantly and especially in low light.
I'm about to lose my last nerve here. I shot a low light event/concert. Most pictures are focused wrong.
Lenses are calibrated. Always used single center point.

It feels as if the af point is at least 10x larger than the red dot. I've come to experience, the more vivid and messy the background is the more it tends to focus on the background - like trees, branches. Also, these things really come to light during photoshoots and event photography where every shot counts. I've previously simply dismissed it as a freak accident, lens calibration error or weak technique, but now it's really getting on my nerve.

Also the af feels very unpredictable in low light. I'd say maybe 30% of my pictures are focused correctly.
(No, the AF sensor, mirrors and contacts are not dirty. I've cleaned them many times with 99% alc.)

My next camera is not going to be Pentax, unless they somehow miraculously fix their AF with the K3-III, but as the K1 and KP show, they might not even care about good AF.

03-27-2017, 01:53 PM	#1
08amczb Veteran Member Join Date: Dec 2013 Posts: 796	Pentax KP AF issue My first impression was very positive about the KP, but I ran into an issue. When my subject is near to the background and the background has many detail (many edges) and the subject approximetly fills the spot meter circle, but not bigger than that, then the AF locks on the background consistently. Not a backfocus issue, because the background is always sharp and I can get sharp images when the subject and the background is more separated. Here is two examples: The K-3/K-3ii or the other KP's do the same? I think it's a firmware bug, the center of the center focus point should be calculated in the correlation with more weight then the outside. This really anoyes me, because I can't get sharp images when I want some contextual portraits with wide open aperture because of low light or forground blur. How to report this to Ricoh? I didn't find the appropriate e-mail adress/form on their global site. Thanks for the opinions/suggestions.

03-27-2017, 02:36 PM	#5
C_Jones Pentaxian Join Date: Sep 2012 Location: NY Photos: Gallery \| Albums Posts: 6,077	I use Single Point AF and AF.C. In case you happen to budge a little the AF.C with still look for focus, whereas the AF.S will be still remembering the focus point when the shutter had been pushed down halfway. Focusing on a consistently black or consistently white object with little shape is sometime difficult for the camera to AF due to the non-contrast appearance. You may want to do some tests where the subject has definite contrast with moderate light and see what your results are. I also make sure I am using an appropriate aperture for the depth I need. I noticed you were using F2.8. You may want to test with a higher number F Stop just to see what results you get. You most likely already know these things, but I am just mentioning them. Last edited by C_Jones; 03-27-2017 at 02:50 PM.

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03-27-2017, 02:02 PM	#2
RobA_Oz Site Supporter Join Date: Nov 2008 Location: Tasmania, Australia Photos: Gallery Posts: 8,209	Which AF mode are you using? Your text doesn't really indicate if you're using single or multi-point AF. I'd be inclined to use single-point for critical focus with static subjects like these. Focus, hold focus then recompose, to be clear.

03-27-2017, 02:13 PM	#3
08amczb Veteran Member Join Date: Dec 2013 Posts: 796 Original Poster	I was using AF-S single point select in the center and I was doing focus and recompose. The AF is set to back button only, I was focused on their face and waited until the green hexagon appeared and the center led also confirmed the focus.

03-27-2017, 02:29 PM	#4
rawr Veteran Member Join Date: Jun 2009 Posts: 11,913	Worth noting this, from the Nikon D4 manual: I also suggest that you set your metering to multi-segment for scenes like this, so that colour information can be used to assist the AF - in the first image, the face may have enough distinct colour from the background to help the AF. Or if needed, switch to LiveView focussing, & enable Face Detection mode.

03-27-2017, 02:41 PM	#6
08amczb Veteran Member Join Date: Dec 2013 Posts: 796 Original Poster	I was using matrix metering. The metering does not influences the autofocus, only the sensor is common. The image from the sensor weighted by the metering mode and that gives the exposeur and the same source image can help the AF system. My case is definitly the second one from the D4 manual, but as an engeneer who did image processing in the past I think in this senarios the AF should find my subject. I messured the size of the center AF point and it's approximetly the same size as the spot meter circle. My subject was filling the 90% of it at the center.

03-27-2017, 04:45 PM	#7
caliscouser Site Supporter Join Date: Sep 2014 Location: SF Bay Area, CA Photos: Albums Posts: 1,037	I don't have experience with the K3 or KP but what I learned anecdotally on my other camera bodies is the center focus point is bigger than you think. In the case of the 2 photos above it looks like it focused on the most high contrast areas in and around the head. Even if the head is 90%. Those are the background wood paneling in picture 1 and the branches in picture 2. So focusing on the subjects body instead or something else in the plane of focus is probably going to yield better results.

03-27-2017, 06:48 PM	#8
clackers Otis Memorial Pentaxian Join Date: Jul 2013 Location: Melbourne Photos: Albums Posts: 16,397	Originally posted by 08amczb I was focused on their face. Hi 08amczb, Whether you're shooting Canon or Nikon or Pentax, you don't focus on a face, you focus on a line, so pick one. If you're very close, it can be the pupil of an eye. In your first shot, it could be the edge of the tie against the white shirt.

03-27-2017, 07:47 PM	#9
Clarkey Site Supporter Join Date: Jan 2008 Location: Brampton, ON, Canada Photos: Gallery Posts: 2,456	From the manual, page 62, it looks like you can reduce the size of the AF area, or use face detection. I haven't had a Pentax for a bit (last one K-5IIs), but it looks like the AF options have become quite a bit more sophisticated.

03-27-2017, 09:01 PM	#10
rawr Veteran Member Join Date: Jun 2009 Posts: 11,913	Originally posted by 08amczb The metering does not influences the autofocus It should ... talk to the AF when multi-segment metering is used along with something other than centre-spot AF, according to what we know of the K-3/K-3II, 645Z, K-1 and K-P. Features4 \| PENTAX KP \| RICOH IMAGING

03-28-2017, 06:36 AM	#11
08amczb Veteran Member Join Date: Dec 2013 Posts: 796 Original Poster	Thanks, Jones. Yes I know how the AF works. The F2.8 was intented, deeper DoF certenly helps, but I would like to blur the background by a little and get sharp faces. If my subject is a face I can't choose more contrasty target sadly. And zooming out after focusing changes the focus plane too, so I can't do that neither. Originally posted by caliscouser I learned anecdotally on my other camera bodies is the center focus point is bigger than you think. Thanks, caliscouser, this helped me. The KP has 3 F2.8 luminance flux AF point, so I will try the upper one. Now this is not the edge, like on my K-30. I messured the size of the center point and yes, you are right the background is in the area. I hoped some more intelligance from the AF system. It would calculate the center with more weight in the phase matching. (Of corse it could make other issues.) Originally posted by Clarkey From the manual, page 62, it looks like you can reduce the size of the AF area This is true for Live View only. And yes live view may solve my problem, but I am a viewfinder shoter, and live view is slow and consumes more power. I keep this option as a my last fallback. Rawr, trust me I am an engeneer. I was designed algorithmys for medical imageing (X-ray tomosynthesis), I've detected cancer by AI. To make the AE and AF system a little more clear here is how a basic system works: The auto exposeur sees the same image as you through the viewfinder by a little 86k image sensor similar to the main sensor in the camera. In spot metering and center weighted average the metering is only a weighted average of the pixel values. (It may do it per chanel.) In matrix metering this weight matrix is modified by many parameters - like the selected AF point. It can do face detections and advanced scene analysis by deep neural networks for example. But this calculation is independent of the AF system - wich uses the same AE sensor and the same source image. The AE sensor can help the AF system by selecting the AF point in tracking mode. Because it sees a video through the viewfinder it can track the lateral movement. The AF sensor only sees the depth. So the 86k sensor is common, but the metering mode does not affect the AF algorithm and only a few AF parameters affects the metering like the selected AF point.

03-28-2017, 07:07 AM	#12
frogfoot Forum Member Join Date: Apr 2011 Location: Moscow Posts: 70	Originally posted by 08amczb The K-3/K-3ii or the other KP's do the same? I think it's a firmware bug, the center of the center focus point should be calculated in the correlation with more weight then the outside. This really anoyes me, because I can't get sharp images when I want some contextual portraits with wide open aperture because of low light or forground blur. Yes it happens with K-3 and earlier cameras too. Sometimes it focuse on a background if it's more contrasty than the object. And looks like it is lens dependant.. Although I can't find an explanation to this DA 40 is known as the lens most affected to this. BTW A friend of mine who has Nikon D7100 said me he experienced this too..

03-28-2017, 07:33 AM	#13
08amczb Veteran Member Join Date: Dec 2013 Posts: 796 Original Poster	Thanks frogfoot, I didn't noticed it's lens dependence. If K-3 and Nikon D7100 also does this then I definitly have to improve my technique and maybe my vision with some laser surgery...

03-31-2017, 02:08 AM	#14
starbase218 Inactive Account Join Date: Sep 2009 Location: Planet Earth, Sol system, Milky Way galaxy, Universe Posts: 1,119	I've also done some testing on this and sometimes find it difficult to get precise focus as well. I think what's causing this is that the AF points of the K-3 are so large they actually overlap.

05-13-2018, 10:50 AM	#15
labidas Forum Member Join Date: Apr 2018 Location: Tartu Posts: 82	K50 here. This happens constantly and especially in low light. I'm about to lose my last nerve here. I shot a low light event/concert. Most pictures are focused wrong. Lenses are calibrated. Always used single center point. It feels as if the af point is at least 10x larger than the red dot. I've come to experience, the more vivid and messy the background is the more it tends to focus on the background - like trees, branches. Also, these things really come to light during photoshoots and event photography where every shot counts. I've previously simply dismissed it as a freak accident, lens calibration error or weak technique, but now it's really getting on my nerve. Also the af feels very unpredictable in low light. I'd say maybe 30% of my pictures are focused correctly. (No, the AF sensor, mirrors and contacts are not dirty. I've cleaned them many times with 99% alc.) My next camera is not going to be Pentax, unless they somehow miraculously fix their AF with the K3-III, but as the K1 and KP show, they might not even care about good AF.