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02-23-2014, 09:49 PM   #1
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AF inaccuracy?

Hello

I recently purchased a k-3, which was quite a substantive upgrade from my previous k-r. Part of why I bought it was because I read that the AF system was very accurate, especially in low light. After the initial honeymoon period I seem to have run into a problem with the AF system that I hope I can get some help with.

I've noticed that with closeups sometimes the AF will just be off. The subject will not be in focus, despite the AF locking. I have taken to testing the camera by taking pictures of flat surfaces at oblique angles, and I have found that sometimes the in-focus area will be just a little short or far of where I pointed the focus.

Over the weekend I made use of the K-3's AF fine adjustment. I used a focusing chart, set the camera up on a tripod, and adjusted each of the lenses until the in-focus area was consistently in the centre. This was kind of a makeshift project; I simply just displayed the chart on my laptop screen and tilted it 45 degrees. On most of my lenses I needed to add adjustment 'points', but I still seem to be getting the problem.

This is worst for my wide aperture primes (31mm f1.8, 50mm f1.8). It's gotten to the point where I'm afraid to shoot wide open for fear of losing the shot. I never noticed this on my 18-135, which unlike the primes, uses a DC motor (and also has a max aperture of f3.5).

Does anybody have any suggestions or similar experiences? Is it possible that I just got a defective camera or am not using it right?

02-23-2014, 10:48 PM   #2
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Hi! Welcome to the Pentax Forums!

The first question I always ask in regards to this sort of thread is whether you have tested the AF against a simple flat target with the camera tripod mounted. A sheet of newsprint taped to a wall at about 50X the lens focal length distance is a good choice. A resolution target works well too. Do your test with fixed center AF focus point. Use natural daylight if possible.

1. Do your best-effort try using focus peaking in Live view. This is your standard.

2. Using focus confirm (green hexagon) and manual focus mode you can see if the PDAF system matches what you determined using focus peaking.

The above is a rude version of a more involved flow detailed in the video below. Note that all testing is done using a flat target and single focus point. You are testing and calibrating the bias in the AF system, not the ability of the system to magically pick a point out of space to focus on.



Steve

---------- Post added 02-23-14 at 09:57 PM ----------

QuoteOriginally posted by "Student" Quote
Does anybody have any suggestions or similar experiences? Is it possible that I just got a defective camera or am not using it right?
A defective camera is always a possibility. If your camera cannot consistently acquire focus on a flat stationary subject in a timely manner or if precision is poor when attempting to calibrate, it is probably time to talk with the dealer to arrange a replacement unit.

Remember that the industry standard for serious defect with dSLR cameras is about 3-4% of all makes and models. Yep, more than one out of every 30 cameras that go out the door need to go back.


Steve

Last edited by stevebrot; 02-23-2014 at 10:58 PM.
02-24-2014, 12:37 AM   #3
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QuoteOriginally posted by "Student" Quote
Hello

I recently purchased a k-3, which was quite a substantive upgrade from my previous k-r. Part of why I bought it was because I read that the AF system was very accurate, especially in low light. After the initial honeymoon period I seem to have run into a problem with the AF system that I hope I can get some help with.

I've noticed that with closeups sometimes the AF will just be off. The subject will not be in focus, despite the AF locking. I have taken to testing the camera by taking pictures of flat surfaces at oblique angles, and I have found that sometimes the in-focus area will be just a little short or far of where I pointed the focus.

Over the weekend I made use of the K-3's AF fine adjustment. I used a focusing chart, set the camera up on a tripod, and adjusted each of the lenses until the in-focus area was consistently in the centre. This was kind of a makeshift project; I simply just displayed the chart on my laptop screen and tilted it 45 degrees. On most of my lenses I needed to add adjustment 'points', but I still seem to be getting the problem.

This is worst for my wide aperture primes (31mm f1.8, 50mm f1.8). It's gotten to the point where I'm afraid to shoot wide open for fear of losing the shot. I never noticed this on my 18-135, which unlike the primes, uses a DC motor (and also has a max aperture of f3.5).

Does anybody have any suggestions or similar experiences? Is it possible that I just got a defective camera or am not using it right?
Hi
I have been running into the same problem (focus unreliability) with my K5IIs, especially when using the 40mm 2.8 ltd. and the 70mm 2.4 ltd. It is so bad that I don't use those lenses any more. No amount of in camera 'fine tuning' would make any difference. The same problem appeared in my K3 but to a lesser extend. The focus points in the K3 are smaller than in other Pentax cameras and focussing requires accurate technique. I find both camera excellent when they focus as intended but sometimes it's really frustrating when 50% of shots are mis focussed for no apparent reason.
02-24-2014, 02:36 AM   #4
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Hey there. AF is a complex beast, there are many threads about it. About which settings to use for which subjects, about the AF points being bigger than the overlay in the viewfinder, about AF adjust and other such options (using focus charts) and explaining why this happens (due to different tolerances in the specific individual lens and camera)
Other problem is that users often confuse lack of sharpness (due to motion blur, thin DoF, or other problems) for "wrong focus"

02-24-2014, 03:05 AM   #5
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QuoteOriginally posted by Na Horuk Quote
Hey there. AF is a complex beast, there are many threads about it. About which settings to use for which subjects, about the AF points being bigger than the overlay in the viewfinder, about AF adjust and other such options (using focus charts) and explaining why this happens (due to different tolerances in the specific individual lens and camera)
Other problem is that users often confuse lack of sharpness (due to motion blur, thin DoF, or other problems) for "wrong focus"
Yes, mis focussing can be subject to several reasons, even a spec of dust on the mirror can cause focus problems, but for my part I can distinguish motion blur from focus error. I am too afraid to send my K5IIs to Pentax Australia. It took them 1 month to replace a memory card door. To let them check the focussing module is like saying 'Good Bye camera'. Cheers
02-24-2014, 04:03 AM   #6
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QuoteOriginally posted by "Student" Quote
Hello

I recently purchased a k-3, which was quite a substantive upgrade from my previous k-r. Part of why I bought it was because I read that the AF system was very accurate, especially in low light. After the initial honeymoon period I seem to have run into a problem with the AF system that I hope I can get some help with.

I've noticed that with closeups sometimes the AF will just be off. The subject will not be in focus, despite the AF locking. I have taken to testing the camera by taking pictures of flat surfaces at oblique angles, and I have found that sometimes the in-focus area will be just a little short or far of where I pointed the focus.

Over the weekend I made use of the K-3's AF fine adjustment. I used a focusing chart, set the camera up on a tripod, and adjusted each of the lenses until the in-focus area was consistently in the centre. This was kind of a makeshift project; I simply just displayed the chart on my laptop screen and tilted it 45 degrees. On most of my lenses I needed to add adjustment 'points', but I still seem to be getting the problem.

This is worst for my wide aperture primes (31mm f1.8, 50mm f1.8). It's gotten to the point where I'm afraid to shoot wide open for fear of losing the shot. I never noticed this on my 18-135, which unlike the primes, uses a DC motor (and also has a max aperture of f3.5).

Does anybody have any suggestions or similar experiences? Is it possible that I just got a defective camera or am not using it right?
Had similar thoughts about the focusing being slightly out, like you and spent ages experimenting with the camera AF fine tuning and taking pics and then viewing them on my computer monitor. You are not alone. At least with the high end cameras we can make fine adjustments to get the best out of our lenses.
02-24-2014, 06:44 AM   #7
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i just ask for the obvious : are you sure you are not too close of the subject (shooting closer than the minimum distance of the lens) ?
02-24-2014, 07:49 AM   #8
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take the following with a grain of salt. I too had AF inconsistencies with all my lenses. I set up AF fine tuning for each individual lens but it still would occasionally miss the target by a long margin. Basically I was on the verge of thinking this camera was simply unreliable. On a lark I decided to add +10 globally and suddenly, all my lenses are now spot on and more importantly, they appear to be consistent, which is more important to me.

How could this possibly be? Well, the global adjustment would affect the position of the AF module itself independently of what lens was attached. This could put the system into a "sweet spot" where the AF algorithms finally can do their job correctly.

Anyway, I say give it a try. It seems to have worked for me and I am back to enjoying this camera again.

Michael

02-24-2014, 08:29 AM   #9
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QuoteOriginally posted by "Student" Quote
Hello

I recently purchased a k-3, which was quite a substantive upgrade from my previous k-r. Part of why I bought it was because I read that the AF system was very accurate, especially in low light. After the initial honeymoon period I seem to have run into a problem with the AF system that I hope I can get some help with.

I've noticed that with closeups sometimes the AF will just be off. The subject will not be in focus, despite the AF locking. I have taken to testing the camera by taking pictures of flat surfaces at oblique angles, and I have found that sometimes the in-focus area will be just a little short or far of where I pointed the focus.

Over the weekend I made use of the K-3's AF fine adjustment. I used a focusing chart, set the camera up on a tripod, and adjusted each of the lenses until the in-focus area was consistently in the centre. This was kind of a makeshift project; I simply just displayed the chart on my laptop screen and tilted it 45 degrees. On most of my lenses I needed to add adjustment 'points', but I still seem to be getting the problem.

This is worst for my wide aperture primes (31mm f1.8, 50mm f1.8). It's gotten to the point where I'm afraid to shoot wide open for fear of losing the shot. I never noticed this on my 18-135, which unlike the primes, uses a DC motor (and also has a max aperture of f3.5).

Does anybody have any suggestions or similar experiences? Is it possible that I just got a defective camera or am not using it right?
Me too to some extent. Despite the hype I'm getting more shots out of focus after the confirmation bleep and confirmed as the right point (centre spot usually) than I did with my K30. Stupendous when it is right but telling me it is in focus when it turns out not to be is not a good scenario.
02-24-2014, 09:18 AM   #10
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I have only had a problem when it concerns my 16-50
02-24-2014, 10:23 AM   #11
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QuoteOriginally posted by MJSfoto1956 Quote
take the following with a grain of salt. On a lark I decided to add +10 globally and suddenly, all my lenses are now spot on and more importantly, they appear to be consistent, which is more important to me.
How could this possibly be? Well, the global adjustment would affect the position of the AF module itself independently of what lens was attached. This could put the system into a "sweet spot" where the AF algorithms finally can do their job correctly.
Michael

Hmnn? Indeed, how can this possibly be? I tried this as a lark with one lens that need + 8 and rest between -3 and +4. My + 8 lens was OKish at +10 global adjustment the others were all heavily front focused (as you might expect)

QuoteOriginally posted by Daedbird Quote
I have only had a problem when it concerns my 16-50

I sometimes get severe back focus issues with my DA*16-50. This is somewhat complicated by needing the following AFA adjustments , -1 @ 16mm, +2 @ 20mm and 28mm, +3 @ 35mm and +8 @ 50mm. However this random back focus occurs at 35mm when the AFA is set at +3 almost as often as it does with other focal lengths also set at +3.
02-24-2014, 10:50 AM   #12
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QuoteOriginally posted by unkipunki Quote
Hmnn? Indeed, how can this possibly be? I tried this as a lark with one lens that need + 8 and rest between -3 and +4. My + 8 lens was OKish at +10 global adjustment the others were all heavily front focused (as you might expect)
one must make the following assumption when dealing with observed behaviors between camera bodies: some cameras need global adjustment and others do not.
The anecdotal evidence suggests mine needed it.
Likewise, the anecdotal evidence suggests yours does not.

YMMV

Michael
02-24-2014, 10:59 AM   #13
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QuoteOriginally posted by MJSfoto1956 Quote
one must make the following assumption when dealing with observed behaviors between camera bodies: some cameras need global adjustment and others do not.
The anecdotal evidence suggests mine needed it.
Likewise, the anecdotal evidence suggests yours does not.

YMMV

Michael
Yes Michael that's very true (anecdotally). I wasn't thinking your solution wasn't working for you, just trying to get my head round it. Still can't unless, by some miracle, all your lenses require the same adjustment. My brain isn't the arbiter of effective solutions for all and I think it will retire defeated on this issue.
02-24-2014, 11:50 AM   #14
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QuoteOriginally posted by unkipunki Quote
Yes Michael that's very true (anecdotally). I wasn't thinking your solution wasn't working for you, just trying to get my head round it. Still can't unless, by some miracle, all your lenses require the same adjustment. My brain isn't the arbiter of effective solutions for all and I think it will retire defeated on this issue.
actually I think it is relatively simple: none* of my lenses actually "needed" any adjustment. The module itself appears to be somewhat "out of spec" and by choosing a global setting things magically "work".

Now here comes a total guess: I am going to bet that the AF lens adjustment is additive to the global AF adjustment. I don't have any proof, but only some (anecdotal again!) evidence. If true, then one can actually dial in as much as +20 by specifying +10 global and then +10 for any particular lens.

I'm not saying why it seems to work (as a software architect, I can imagine all sorts of edge cases that Ricoh engineering never tested for) -- I am only saying that setting individual AF lens adjustment did not produce reliable AF behavior, whereas setting global AF did the trick for my particular camera body. I have no doubt that another body would behave differently.

M

* actually, one of my lenses needed another +5 on top of the global +10 to give consistent results
02-24-2014, 12:00 PM   #15
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QuoteOriginally posted by MJSfoto1956 Quote
Now here comes a total guess: I am going to bet that the AF lens adjustment is additive to the global AF adjustment. I don't have any proof, but only some (anecdotal again!) evidence. If true, then one can actually dial in as much as +20 by specifying +10 global and then +10 for any particular lens.

My brain just started again! Now that is a really interesting possibility. It was my understanding that 'Apply All' AF adjustment does not have a cumulative effect with 'Apply One', just that you could use one OR the other. Your experience seems to indicate that this is not the case. Did you set any previous 'Apply One ' adjustments to zero and then set to @apply All' +10? Or did you carry through previous settings and then add a further +5 for troublesome lens? I won't be able to sleep tonight unless I can figure this out
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