Autofocus technique for off-center subjects

devorama · 11-25-2014, 11:03 AM

I would have to say my biggest frustration with my Pentax cameras is the autofocus performance. I used to have a lot of front focus issues with my K-5 under tungsten light. That's better with my K-3, but now I found I have front/back focus issues with lenses that didn't miss focus on my K-5. This last weekend I had another issue: shallow depth of field shots for an off center target.

I was shooting my K-3 with a Pentax-F 50mm F/1.4 lens at F/2. My subject was off center with his face closest to one of the edge focus points. Not the most extreme side ones, but the one next to that. I believe these are cross type points with F/5.6 sensitivity. Now I know there are 3 center AF points that are F/2.8 sensitive. But those are so far from my subject's face that I was afraid that a focus then recompose method would give too much change in distance. So I shot at F/2 while using the cross sensor nearest the subject's face. I reviewed my focus on the camera's LCD and thought it looked a bit softer than it should be. I'm aware these large aperture lenses are softer near the edge, but I felt it was not as sharp as it could be. So I decided to try to switch to Live View and use contrast detect autofocus. I moved the focus point to directly where his face was so there was no recomposing. After later review on my computer, these shots seem softer than the previous ones.

So what's a person to do in this case? Magnified Live View and manual focus? Any other ideas?

FantasticMrFox · 11-25-2014, 11:21 AM

Originally posted by devorama

So what's a person to do in this case? Magnified Live View and manual focus? Any other ideas?

a) Find out whether your lens has any focus issues. If yes, try and correct them with the internal focus correction. If that doesn't work, send the lens and camera in for adjustment.

b) If the AF is not faulty, you pretty much have to do with life view and manual focus.

**nicoprod** · 11-25-2014, 11:27 AM

Do AF points have differences in sensitivity? I didn't know that. Can someone explain or direct me towards some information? Thanks (sorry for hijacking your thread)

**luftfluss** · 11-25-2014, 11:28 AM

Is it possible you move slightly when shooting, throwing off the focus when using narrow DOF? I did this a lot with my K100D & 50/1.4.

What about the in-body lens adjustments?

Don't have any other ideas, I use a K-01 and shoot dont have any AF lenses faster than F/2.4.

**Miguel** · 11-25-2014, 11:44 AM

Originally posted by devorama

So I decided to try to switch to Live View and use contrast detect autofocus. I moved the focus point to directly where his face was so there was no recomposing. After later review on my computer, these shots seem softer than the previous ones.

So what's a person to do in this case? Magnified Live View and manual focus? Any other ideas?

It would help if all the facts were presented, like what your shutter speed and ISO were.Plus, were you using a tripod? That helps screening out motion blur as a cause of the soft images.

Why don't you attach the image?

M

VisualDarkness · 11-25-2014, 11:46 AM

Instead of recomposing by moving the camera from the same point you take a big step to side. That won't change the focus distance but is hard to perform in practice.

nomadkng · 11-25-2014, 11:49 AM - **1 Like**

if you haven't calibrated your lenses, that's the first step. The K3 is very unforgiving in terms of technical quality. Especially if you are shooting at such shallow DoF, you must have calibrated lenses for THAT camera. I found that every single one of my lenses had radically different calibration points on my K3. Some that didn't require any adjustments on my K5iis were either front OR back focused on my K3. My FA28 was so out of calibration that I sold it as defective. I could not get it close enough on the chart to even tell if it was FF or BF (and that was 28mm at F2.8)

Your F50 may simply not play well with your K3. That's one of the downsides of using legacy glass and a 24MP sensor.

The K3 is a fickle camera. Like dating a supermodel, you can no longer "get away" with things that you used to neglect in the past. It requires an attention to detail on many levels.

stevebrot · 11-25-2014, 01:27 PM

Originally posted by nicoprod

Do AF points have differences in sensitivity?

Yes, they do, but only with faster lenses. The three points vertical at center have a focus sensitivity equivalent to f/2.8* while the others are sensitive to f/5.6.

Steve

* What this means is that the AF system is able to detect out-of-focus the same at f/2.8 as at wider apertures. This has to do with the optics (prism angle) that service the focus point.

---------- Post added 11-25-14 at 12:33 PM ----------

Originally posted by devorama

Now I know there are 3 center AF points that are F/2.8 sensitive. But those are so far from my subject's face that I was afraid that a focus then recompose method would give too much change in distance.

I would be more concerned that your subject would move. Magnified manual focus (w/ focus peaking) in live view is your most sensitive option. More practical would be manual focus using a high-performance matte field focus screen (e.g. Canon type-S). Or you could simply focus and recompose and shoot stopped-down to f/4. After all, you do want something other than the leading edge (tip) of the subject's nose in focus, eh?

Steve

devorama · 11-25-2014, 02:45 PM

I did check the focus calibration on the lens before my trip, but only for the center point. I made a point to do this because I got burned a month ago because I neglected to check the focus on my K-3 when using a 24mm F/1.8 Sigma that I'd used plenty on my K-5. So I check the lens for center point and it was spot on. I was not shooting with a tripod, so some forward/back movement might have been the culprit also. It sounds like the only more reliable way to do this would be to stop down a bit or manual focus in magnified Live View.

**Miguel** · 11-25-2014, 03:16 PM

What was your shutter speed?

M

devorama · 11-25-2014, 04:15 PM

Originally posted by Miguel

What was your shutter speed?

M

Shutter speed was pretty fast, and shot with SR on, so I don't think it was motion blur.

devorama · 11-26-2014, 09:00 AM

I went back and reviewed the detailed info on the photos on the camera to see which ones were shot with what focus method. Then I looked at the corresponding photos on my 23" monitor. From least to most sharp, they are:

Center point focus with focus/recompose
Edge focus point right over subject's face (no recomposing)
Live View focused with focus point over subject's face (no recomposing)

I originally said I was disappointed with the focus. So I did some more controlled testing with PDAF vs. Contrast Detect AF vs. Live view manual focus. And I found that I really couldn't do much better with live view and manual focus than I could with the contrast detect autofocus. So I guess the image quality was lacking more in resolution and contrast than it was in focus. I guess I expected a bit too much out of the lens near the edge with an F/2 aperture.

**sinus007** · 11-26-2014, 10:46 AM

Hi,
Sorry, but I don't understand why you can't use re-compose method with a disabled (AF) shutter release button? Focus at whatever part of the object with the center cross with AF button, re-compose and then shoot.

devorama · 11-26-2014, 02:47 PM

Originally posted by sinus007

Hi,
Sorry, but I don't understand why you can't use re-compose method with a disabled (AF) shutter release button? Focus at whatever part of the object with the center cross with AF button, re-compose and then shoot.

The problem with the focus/recompose method is that recomposing the picture can change the distance between your intended focus target and the imaging sensor. This effect is amplified with close focus distances or narrow depth of field shooting. Typically if you use the center point to focus on a subjects face or eye, then you aim the camera down to compose, you will get a back-focused image.

Here is an explanation with pictures:
The Problem With The Focus-Recompose Method - Digital Photography School

**sinus007** · 12-01-2014, 07:19 AM

Originally posted by devorama

The problem with the focus/recompose method is that recomposing the picture can change the distance between your intended focus target and the imaging sensor. This effect is amplified with close focus distances or narrow depth of field shooting. Typically if you use the center point to focus on a subjects face or eye, then you aim the camera down to compose, you will get a back-focused image.

Here is an explanation with pictures:
The Problem With The Focus-Recompose Method - Digital Photography School

Devorama,
If your intended focus target is the face, not an object behind it, it won't matter if you use a side cross to focus without turning the cam or, if you use center cross to focus and then recompose to have the face and the background object inside the frame - the distance to the face will be the same. Therefore the face will be in focus and the object behind will be blurry. If, OTOH, you want both the face and the object behind to be in focus, then you need to find a "common ground": focus on something in between so both are within DOF or change the aperture.

Thanks for the article, BTW. It's funny that the author, at the end, even provided a proof that hypotenuse is larger than cathetus.

11-25-2014, 11:03 AM	#1
devorama Veteran Member Join Date: Apr 2009 Location: Chicago, IL Photos: Gallery \| Albums Posts: 638	Autofocus technique for off-center subjects I would have to say my biggest frustration with my Pentax cameras is the autofocus performance. I used to have a lot of front focus issues with my K-5 under tungsten light. That's better with my K-3, but now I found I have front/back focus issues with lenses that didn't miss focus on my K-5. This last weekend I had another issue: shallow depth of field shots for an off center target. I was shooting my K-3 with a Pentax-F 50mm F/1.4 lens at F/2. My subject was off center with his face closest to one of the edge focus points. Not the most extreme side ones, but the one next to that. I believe these are cross type points with F/5.6 sensitivity. Now I know there are 3 center AF points that are F/2.8 sensitive. But those are so far from my subject's face that I was afraid that a focus then recompose method would give too much change in distance. So I shot at F/2 while using the cross sensor nearest the subject's face. I reviewed my focus on the camera's LCD and thought it looked a bit softer than it should be. I'm aware these large aperture lenses are softer near the edge, but I felt it was not as sharp as it could be. So I decided to try to switch to Live View and use contrast detect autofocus. I moved the focus point to directly where his face was so there was no recomposing. After later review on my computer, these shots seem softer than the previous ones. So what's a person to do in this case? Magnified Live View and manual focus? Any other ideas?

11-25-2014, 11:44 AM	#5
Miguel Site Supporter Join Date: Sep 2007 Location: Near Seattle Photos: Gallery Posts: 2,743	Originally posted by devorama So I decided to try to switch to Live View and use contrast detect autofocus. I moved the focus point to directly where his face was so there was no recomposing. After later review on my computer, these shots seem softer than the previous ones. So what's a person to do in this case? Magnified Live View and manual focus? Any other ideas? It would help if all the facts were presented, like what your shutter speed and ISO were.Plus, were you using a tripod? That helps screening out motion blur as a cause of the soft images. Why don't you attach the image? M Last edited by Miguel; 11-25-2014 at 11:52 AM.

11-25-2014, 01:27 PM	#8
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by nicoprod Do AF points have differences in sensitivity? Yes, they do, but only with faster lenses. The three points vertical at center have a focus sensitivity equivalent to f/2.8* while the others are sensitive to f/5.6. Steve * What this means is that the AF system is able to detect out-of-focus the same at f/2.8 as at wider apertures. This has to do with the optics (prism angle) that service the focus point. ---------- Post added 11-25-14 at 12:33 PM ---------- Originally posted by devorama Now I know there are 3 center AF points that are F/2.8 sensitive. But those are so far from my subject's face that I was afraid that a focus then recompose method would give too much change in distance. I would be more concerned that your subject would move. Magnified manual focus (w/ focus peaking) in live view is your most sensitive option. More practical would be manual focus using a high-performance matte field focus screen (e.g. Canon type-S). Or you could simply focus and recompose and shoot stopped-down to f/4. After all, you do want something other than the leading edge (tip) of the subject's nose in focus, eh? Steve Last edited by stevebrot; 11-25-2014 at 01:33 PM.

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11-25-2014, 11:21 AM	#2
FantasticMrFox Veteran Member Join Date: Oct 2014 Location: Munich Posts: 2,339	Originally posted by devorama So what's a person to do in this case? Magnified Live View and manual focus? Any other ideas? a) Find out whether your lens has any focus issues. If yes, try and correct them with the internal focus correction. If that doesn't work, send the lens and camera in for adjustment. b) If the AF is not faulty, you pretty much have to do with life view and manual focus.

11-25-2014, 11:27 AM	#3
nicoprod Site Supporter Join Date: Nov 2010 Location: New Berlin, WI Photos: Gallery \| Albums Posts: 528	Do AF points have differences in sensitivity? I didn't know that. Can someone explain or direct me towards some information? Thanks (sorry for hijacking your thread)

11-25-2014, 11:28 AM	#4
luftfluss Site Supporter Join Date: Jun 2011 Location: NJ Photos: Gallery \| Albums Posts: 11,619	Is it possible you move slightly when shooting, throwing off the focus when using narrow DOF? I did this a lot with my K100D & 50/1.4. What about the in-body lens adjustments? Don't have any other ideas, I use a K-01 and shoot dont have any AF lenses faster than F/2.4.

11-25-2014, 11:46 AM	#6
VisualDarkness Veteran Member Join Date: Nov 2011 Location: Uppsala, Sweden Photos: Gallery Posts: 4,439	Instead of recomposing by moving the camera from the same point you take a big step to side. That won't change the focus distance but is hard to perform in practice.

11-25-2014, 11:49 AM - 1 Like	#7
nomadkng Veteran Member Join Date: Sep 2008 Location: Phoenix, AZ Posts: 1,448	if you haven't calibrated your lenses, that's the first step. The K3 is very unforgiving in terms of technical quality. Especially if you are shooting at such shallow DoF, you must have calibrated lenses for THAT camera. I found that every single one of my lenses had radically different calibration points on my K3. Some that didn't require any adjustments on my K5iis were either front OR back focused on my K3. My FA28 was so out of calibration that I sold it as defective. I could not get it close enough on the chart to even tell if it was FF or BF (and that was 28mm at F2.8) Your F50 may simply not play well with your K3. That's one of the downsides of using legacy glass and a 24MP sensor. The K3 is a fickle camera. Like dating a supermodel, you can no longer "get away" with things that you used to neglect in the past. It requires an attention to detail on many levels.

11-25-2014, 02:45 PM	#9
devorama Veteran Member Join Date: Apr 2009 Location: Chicago, IL Photos: Gallery \| Albums Posts: 638 Original Poster	I did check the focus calibration on the lens before my trip, but only for the center point. I made a point to do this because I got burned a month ago because I neglected to check the focus on my K-3 when using a 24mm F/1.8 Sigma that I'd used plenty on my K-5. So I check the lens for center point and it was spot on. I was not shooting with a tripod, so some forward/back movement might have been the culprit also. It sounds like the only more reliable way to do this would be to stop down a bit or manual focus in magnified Live View.

11-25-2014, 03:16 PM	#10
Miguel Site Supporter Join Date: Sep 2007 Location: Near Seattle Photos: Gallery Posts: 2,743	What was your shutter speed? M

11-25-2014, 04:15 PM	#11
devorama Veteran Member Join Date: Apr 2009 Location: Chicago, IL Photos: Gallery \| Albums Posts: 638 Original Poster	Originally posted by Miguel What was your shutter speed? M Shutter speed was pretty fast, and shot with SR on, so I don't think it was motion blur.

11-26-2014, 09:00 AM	#12
devorama Veteran Member Join Date: Apr 2009 Location: Chicago, IL Photos: Gallery \| Albums Posts: 638 Original Poster	I went back and reviewed the detailed info on the photos on the camera to see which ones were shot with what focus method. Then I looked at the corresponding photos on my 23" monitor. From least to most sharp, they are: Center point focus with focus/recompose Edge focus point right over subject's face (no recomposing) Live View focused with focus point over subject's face (no recomposing) I originally said I was disappointed with the focus. So I did some more controlled testing with PDAF vs. Contrast Detect AF vs. Live view manual focus. And I found that I really couldn't do much better with live view and manual focus than I could with the contrast detect autofocus. So I guess the image quality was lacking more in resolution and contrast than it was in focus. I guess I expected a bit too much out of the lens near the edge with an F/2 aperture.

11-26-2014, 10:46 AM	#13
sinus007 Site Supporter Join Date: Jul 2008 Location: Cleveland, OH, USA Photos: Gallery Posts: 292	Hi, Sorry, but I don't understand why you can't use re-compose method with a disabled (AF) shutter release button? Focus at whatever part of the object with the center cross with AF button, re-compose and then shoot.

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11-26-2014, 02:47 PM	#14
devorama Veteran Member Join Date: Apr 2009 Location: Chicago, IL Photos: Gallery \| Albums Posts: 638 Original Poster	Originally posted by sinus007 Hi, Sorry, but I don't understand why you can't use re-compose method with a disabled (AF) shutter release button? Focus at whatever part of the object with the center cross with AF button, re-compose and then shoot. The problem with the focus/recompose method is that recomposing the picture can change the distance between your intended focus target and the imaging sensor. This effect is amplified with close focus distances or narrow depth of field shooting. Typically if you use the center point to focus on a subjects face or eye, then you aim the camera down to compose, you will get a back-focused image. Here is an explanation with pictures: The Problem With The Focus-Recompose Method - Digital Photography School

12-01-2014, 07:19 AM	#15
sinus007 Site Supporter Join Date: Jul 2008 Location: Cleveland, OH, USA Photos: Gallery Posts: 292	Originally posted by devorama The problem with the focus/recompose method is that recomposing the picture can change the distance between your intended focus target and the imaging sensor. This effect is amplified with close focus distances or narrow depth of field shooting. Typically if you use the center point to focus on a subjects face or eye, then you aim the camera down to compose, you will get a back-focused image. Here is an explanation with pictures: The Problem With The Focus-Recompose Method - Digital Photography School Devorama, If your intended focus target is the face, not an object behind it, it won't matter if you use a side cross to focus without turning the cam or, if you use center cross to focus and then recompose to have the face and the background object inside the frame - the distance to the face will be the same. Therefore the face will be in focus and the object behind will be blurry. If, OTOH, you want both the face and the object behind to be in focus, then you need to find a "common ground": focus on something in between so both are within DOF or change the aperture. Thanks for the article, BTW. It's funny that the author, at the end, even provided a proof that hypotenuse is larger than cathetus.