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06-20-2013, 10:06 AM   #1
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CDAF is less accurate than PDAF

I was just doing test shots when I noticed that AF accuracy using Liveview is much much worse.

The light is good and I had, from various sources, the impression that CDAF would be most accurate, and can therefore also be used for solid comparisons - what does it mean in my case? Shouldn't CDAF be at least as accurate as PDAF provided the light and subject are optimal?

The focus is very off in both 70 and 200mm (Sigma 70-200mm) ends of the lens (using LV).

I've tried different subjects - micro adjusting can't bring it close enough.

Edit: Btw, the camera used was a K-30


Last edited by Zafar Iqbal; 06-20-2013 at 10:11 AM.
06-20-2013, 10:09 AM   #2
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My k-01 can't focus with my Tamron 70-200. My k-x does the same in live view. In PDAF, it's mostly good. During CDAF, it focuses and confirms it is in focus, but it is clearly not in focus at all. I have no idea what the issue is. My Sigma 50 F2.8 focuses perfectly on my k-01, even at 1:1 macro. Same for my DA 21 Ltd. So reading your post, I think it is a similar issue.
06-20-2013, 10:18 AM   #3
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QuoteOriginally posted by Zafar Iqbal Quote
I was just doing test shots when I noticed that AF accuracy using Liveview is much much worse.

The light is good and I had, from various sources, the impression that CDAF would be most accurate, and can therefore also be used for solid comparisons - what does it mean in my case? Shouldn't CDAF be at least as accurate as PDAF provided the light and subject are optimal?

The focus is very off in both 70 and 200mm (Sigma 70-200mm) ends of the lens (using LV).

I've tried different subjects - micro adjusting can't bring it close enough.

Edit: Btw, the camera used was a K-30
CDAF is only accurate if (a) it is implemented properly and (b) there is sufficient contrast in the AF area. The K-30's implementation is pretty good, though it can miss completely at times.

If the AF area contains objects are varying distances from the camera, the camera may focus on the background or on the foreground, depending on which is more significant and depending on the implementation.

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06-20-2013, 10:49 AM   #4
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When I try my Tamron 70-200 in Live View (K-30) in my neighborhood in afternoon sunlight, the focus just racks back and forth. PDAF works fine, though.

06-20-2013, 11:00 AM   #5
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Adam: Yeah, I'm aware of that and tried different subjects.

I just tried using my second K-30 body and got the exact same thing going on.

I then dusted off my K-x (haven't touched it for a year) and gave it a go. The K-x performed like K-30 PDAF.

I thought the Sigma 70-200 needed calibration which is why I initially did the tests - instead of sending it in, wasting time and money on nothing. Now I'm confused.
06-20-2013, 11:01 AM   #6
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For the newbies who also want to follow this thread:

CDAF = Contrast Detection Autofocus.
PDAF = Phase Detection Autofocus.
Autofocus - Wikipedia, the free encyclopedia
06-20-2013, 11:04 AM   #7
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It could be that the lens is not properly optimized for CDAF use... I've heard that before.

In addition, my Sigma 17-70 refuses to even try to focus in live-view mode. Another user on this forum sent this to me:

"I saw in one of the posts here that you have a the 2nd generation Sigma 17-70 lens and that yours does not work with your K-01. I too had this problem. Since my lens was still under warranty, I sent it back to Sigma to get it fixed . They fixed it and told me that the GMR sensor was not working and was repaired. I got back the lens in about a week. You have to download the repair form from their website. It only cost me shipping and insurance (optional). It now works with my K-01. It is moderately fast and now I have a lens that you can have optical stabllzation with since SR in the K-01 is disabled in video mode. Hopefully you can use this information and pass it on to someone who can benefit from it."

So I'd be hesitant to think it's a body issue..
06-20-2013, 11:36 AM   #8
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You might be correct.

I tried some more using Tamron 90mm Macro, Pentax 70mm Limited and Sigma 17-50mm @ 50mm and it stands clear that the Sigma 70-200mm is way off. So much so that it's clearly noticeable on camera display even before zooming in.

06-20-2013, 11:44 AM   #9
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I've noticed the same issue with my K-5 and various lenses. Some of my lenses, like my Sigma 70 Macro, seem to work fine with CDAF with consistent results, but others do not. I tested 4 different lenses under the same test conditions with a high contrast target, and I had more misses with CDAF as compared to PDAF. I tested my Sigma 50-150 after I sent it to Sigma to enable Live View and for calibration. I get a focus lock very quickly, but many of the shots are not in focus. At first, I thought it was just the lens, until I started testing my other lenses. CDAF may work under certain conditions with certain lenses, but I'm not going to risk losing a good shot by relying on it when I can use PDAF and quickly take several shots with a much higher probability of getting a good shot.
06-20-2013, 11:55 AM   #10
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QuoteOriginally posted by JinDesu Quote
It could be that the lens is not properly optimized for CDAF use... I've heard that before.

In addition, my Sigma 17-70 refuses to even try to focus in live-view mode. Another user on this forum sent this to me:

"I saw in one of the posts here that you have a the 2nd generation Sigma 17-70 lens and that yours does not work with your K-01. I too had this problem. Since my lens was still under warranty, I sent it back to Sigma to get it fixed . They fixed it and told me that the GMR sensor was not working and was repaired. I got back the lens in about a week. You have to download the repair form from their website. It only cost me shipping and insurance (optional). It now works with my K-01. It is moderately fast and now I have a lens that you can have optical stabllzation with since SR in the K-01 is disabled in video mode. Hopefully you can use this information and pass it on to someone who can benefit from it."

So I'd be hesitant to think it's a body issue..
Pentax needed to do a firmware update to get cdaf to work with their 560mm. So it could well be the case that it's simply not optimized for the sigma at 200mm, as it is also a fairly long long focal length.

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06-20-2013, 12:10 PM   #11
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QuoteOriginally posted by Adam Quote
Pentax needed to do a firmware update to get cdaf to work with their 560mm. So it could well be the case that it's simply not optimized for the sigma at 200mm, as it is also a fairly long long focal length.
For my Tamron 70-200, it doesn't work at any focal length. It'll lock confirm, but looking at the screen you can instantly see everything is out of focus. This was true on my k-x and also on my k-01.
06-20-2013, 02:06 PM   #12
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FWIW my Sigma 70-200 F2.8 EX (non macro, older version) works perfectly with CDAF and my K-30

Indoors its pretty slow to find a lock, might hunt but when it does its in focus.
06-21-2013, 12:12 AM   #13
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QuoteOriginally posted by FruitLooPs Quote
FWIW my Sigma 70-200 F2.8 EX (non macro, older version) works perfectly with CDAF and my K-30
Damn you
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