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10-16-2011, 06:01 PM   #1
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Hitting the aufofocus on the subject (help.)

I have had a few missed focus with my K-5, and it is most probably a user error on my part. I am looking for some tips from more experienced photographers.

A particular problematic scenario is a person/subject standing in front of a tree, anywhere from 1-10m from me. The face doesn't quite fill the frame or even the red autofocus square. I always use centre spot focus and recompose, but the autofocus mostly locks onto the trees.

This is not immediately obvious on the viewfinder or even the lcd. I tend to find out about the ruined image on my computer screen . I understand that the trees are a higher contrast subject and that the autofocus sensor covers a larger area than the red spot in the viewfinder.

So what can I do in these kinds of situations?

I can provide a couple of sample shots if the descriptions aren't clear.

The K-5 is my first DSLR, I came from the micro4/3 system. Those cameras have relatively fast contrast-detection and adjustable focus area size. The focus is apparent on the electronic viewfinder/lcd, so I can refocus immediately if it misses. Hence, I am probably spoiled in this area .

Any tips would be much appreciated.

10-16-2011, 09:30 PM   #2
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Hmmm....I can't imagine my K-5 (a full year now) missing a person withing 10m especially when going for the torso and then recomposing. When you say "center spot" I hope that means "center focus" and not "spot metering". Also, that problem might occur if set to AF.C rather than AF.S. Its worth checking. Sorry if you possess a high experience level and find these suggestions too noob but its all that comes to me right off.
10-16-2011, 11:25 PM   #3
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QuoteOriginally posted by hmpws Quote
I understand that the trees are a higher contrast subject and that the autofocus sensor covers a larger area than the red spot in the viewfinder.
That is usually the problem, you have to make sure that there isn't something else to focus on on a rather large area around the red spot. If the head of your subject isn't large enough, aim for the torso.

Last edited by Gimbal; 10-17-2011 at 05:05 AM.
10-17-2011, 02:36 AM   #4
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Centre focus point was indeed what I meant.

Merit of the shot aside. Here is an example, the subject may have been more than 10m away . I was trying to focus on the person right in the middle.

I guess I would try and aim for the torso next time.

Attached Images
 

Last edited by hmpws; 10-17-2011 at 02:43 AM.
10-17-2011, 02:12 PM   #5
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You could also give yourself some extra wiggle room around your focus target by boosting your DOF.

Further, if you made it so the hyperfocal distance was 5 metres or so then everything from there to infinity would be acceptably sharp.
10-17-2011, 07:03 PM   #6
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I'm not a pro by any means. I had an Olympus DSLR and the focus target seemed to be quite a bit smaller. I was able to focus on a bird through the branches with some success. The K5 focus target seems quite a bit larger. Someone else may have better eyes, but it almost looks like it caught the shoulder of the guy to the right and front focussed. I'm not certain what the solution is other than to pan a bit to see where the focus point is. I remember it took me 6-8 months with my Oly to get a reasonably good feel for it, I expect it to take at least as long with my K5.

You might want to peruse the thread about the S type focus screen. Part of the difficulty with the K5 is not being able to discern clearly what is in focus and a screen may help.
10-18-2011, 06:25 PM   #7
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Thanks. I will have a look at the focus screen some time. Hopefully I get to shoot more this weekend and see whether I get better results.
10-19-2011, 12:54 AM   #8
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If it is the guy behind the group on the right. My camera has a hard time with black not being contrast enough. My 7D is the same. Hard to get a good focus lock. The K-5 will lock without good focus for me. I try to go for the very contrast areas. Face whatever. It saves me a lot of mis focused shots with the K-5. Just something you might keep in mind and try.

10-19-2011, 03:15 AM   #9
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Any AF system will have trouble with a subject field that is either too rich or too bland. A blank sky is too bland; the picture above is too rich, with multiple targets at various focus distances. The picture looks like it was shot with a short tele at a moderately wide aperture giving fairly thin DOF. A small-sensor system like m4/3 generally uses shorter lenses with thicker DOF than APS-C and FF systems, which gives a bit more leeway for focusing. An APS-C's AF system working through a thinnish-DOF lens doesn't have that leeway.

How to deal with this? Use a shorter lens and/or a tighter aperture for more DOF. But that might not be feasible nor desired. So in a target-rich subject field, use spot metering, not center-weighted nor matrix. Still, with any AF technique, an AF system may decide to focus on something other than what you the photographer want. What then? MANUAL FOCUS!! Take control. Outsmart the camera. Focus on what YOU want, not on what IT wants.
10-19-2011, 05:00 PM   #10
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hmpws,

I would use one of the 11 points on the autofocus and lock it on the head of the subject. I would also set the camera to "AP.S" instead of "C". It is an adjustment on the camera body near the lens. In low light your focus might be off a bit. So just concentrate on getting the red dot to blink in your viewer as you are composing your image.

I use "C" for flying objects and "AP.S" for stationary objects.

I have been getting the sense that Pentax can just make some minor changes to the autofocus and it could really improve its capabilities. I still nail my subjects but not all of the time.

I have been wondering if Pentax could design a camera where the autofocus is in front of the shutter and not behind it. Perhaps they can place some autofocus hardware in the lens. I would be happy to supply Pentax with some ideas for thought if I come up with more.

Flickr: Andrew's Wildlife's Photostream
10-19-2011, 08:50 PM   #11
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If you have some more focal length available, zoom in first to the target figure larger in the VF and then lock focus on him. Then zoom out and shoot.

Dan.
10-19-2011, 09:03 PM   #12
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QuoteOriginally posted by imtheguy Quote
Also, that problem might occur if set to AF.C rather than AF.S.
I've frequently made that mistake when recomposing: forgetting that I had it on AF.C
10-19-2011, 09:06 PM   #13
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QuoteOriginally posted by RioRico Quote
So in a target-rich subject field, use spot metering, not center-weighted nor matrix.
Ok, I feel really stupid, but I didn't know the choice of metering method affected the AF accuracy. Wow. That really makes me wonder about a lot of my missed shots...
10-19-2011, 11:10 PM   #14
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QuoteOriginally posted by pentup Quote
I didn't know the choice of metering method affected the AF accuracy. Wow. That really makes me wonder about a lot of my missed shots...
Oops, I should have said focusing, not metering. Sorry -- I was sloppy there. I usually have my metering and focusing synced to the same method. I've set a [C] menu option [Link AF Point and AE] to ON so that in Matrix metering they're tied together. There are undoubtedly occasions where we might want to focus on one point and meter on another, but I haven't run into those. I default my AF SEL to the center point so it's the same as spot-metering. So I just think of metering+AF as a single function: matrix or spot for AF lenses, center-weighted for MF lenses or general AF. It's a mental shortcut for me. Sorry if I caused any confusion.
10-20-2011, 12:47 AM   #15
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QuoteOriginally posted by traderdrew Quote
So just concentrate on getting the red dot to blink in your viewer as you are composing your image.
But, as always, don't mistake the red dot for focus confirmation. It's the green hexagon you should wait for.
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