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05-01-2011, 01:47 AM   #1
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Why is autofocus not consistent on my K-X?

Hi, I was using a K-X and Tamron 18-250mm lens yesterday, well lit indoors (indirect sunlight) with fill flash. Two of my three shots (same subject, same distance, same FL on lens, same everything on the camera) were ever so slightly soft in focus - not OOF but not pinsharp and one shot was pinsharp. All three were autofocused, with AF beep confirmation and stabilisation locked in.

So how come the camera doesn't nail the focus every time?

I know this is not the first time that AF has not been absolutely spot on but with three identical images, it is now vividly clear that the camera does not do 100% consistent AF. Which is a huge irritation. Is this a common problem on the K-X?

05-01-2011, 03:40 AM   #2
hcc
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You need to provide more info:
* Did you use AF.S or AF.C ?
* Did you use center focus or multiple point focus?

I recommend that you use AF.S and center focus.

I have the 18-250mm. The low light conditions is not the strength of the lens, but I have had some very good results using the flash with AF.S and center focus.

Hope that the comment will help...
05-01-2011, 07:58 AM   #3
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Generally, it's not a question of *whether* the image is in focus, but *where* it is in focus. The camera can't read your mind as to whether you wanted focus on the eyes, the nose, the shirt, etc. Unless you choose which focus point to use, it could pick anything to focus on. And even if you choose the focus point, it's not really a "point" but rather a broad rectangle, and the camera might legitimately focus anywhere within it. And if you are checking for sharpness somewhere other than exactly where the camera focuses (say, the camera focused on the nose, but you are checking sharpness on the eyes), it can indeed come off as soft, depending on what aperture you shot at (larger apertures means less DOF).

You'd have to post images to say for sure, but 99% of the time when people ask questions like this, that is the explanation.
05-01-2011, 08:37 AM   #4
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Thanks for the ideas. Camera was set to AFS, single focus point middle of screen. Subject was child's face, eyes virtually in the middle of the image on each image. The focal area would be somewhere on the nose or bridge of the nose.

I think there was enough lighting even without the fill flash - I set the cam at 1/200s at 200 iso. FL was 180mm. My copy of the Tamron lens is sharp except right down at 18mm.

Aperture was f6.2 (as read on the Exif). I checked the images to see where they were actually in focus. The best one focused on the nose, the other two were slightly soft everywhere on the face, probably an inadvertent ideal portrait style . Subject was about 2 meters distance from cam. I would imagine the hyperfocal range would span a few inches so at least some part of the face would look focused but this was not the case.

I'm going to try out some more test shots. A face does not carry enough depth to determine whether the camera did a BF or FF on the slightly soft shots.

05-08-2011, 05:42 AM   #5
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QuoteOriginally posted by jlaubza Quote
Subject was child's face
That explains it, no child sits still. Try with an immobile target.
05-08-2011, 03:43 PM   #6
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QuoteOriginally posted by selar Quote
That explains it, no child sits still. Try with an immobile target.
+1 on this. Unless the child was sleeping
05-10-2011, 05:04 PM   #7
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Unless you are very young it was probably a combination of the fact that the shutter speed was too slow for that focal length (180mm X 1.5 crop factor equals 270mm so 1/250th would be minimum) and that you nullified the SR by activating the flash and that your camera was set at a shutter speed higher than it is capable of syncing at. Lastly, no auto-focus nails it everytime.

Just my 2 cents worth.

Ray
05-11-2011, 03:19 AM   #8
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Dear Sawtooth, thanks for the ideas - I used the camera on manual with shutter speed set to 1/180s and effective FL at 240mm so the speed was a tad slow.

You stated that using the flash nullifies the SR but with manual settings and an old auto flash, not a new P-TTL flash, is this still the case?

But I'm perturbed to read that AF does not nail it every time. Is it a DOF issue? My Panasonic P&S nails it every time so why not a fancy shmancy camera like the K-X?

Thanks

05-11-2011, 10:38 AM   #9
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No, your Panasonic does not "nail it" every single time. It is subject to the exact same factors as I described above. Meaning, *some point* is always in focus, but it is *not* necessarily the exact point you intended. Every single AF system ever invented or likely ever will be invented will be subject to those factors, at least until mind-reading technology is perfected. It's just that with a much smaller sensor, the much larger DOF on your P&S masks most of these discrepancies.
05-11-2011, 10:14 PM   #10
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Correct me if I'm wrong but I think that P&S cams use contrast detect instead of phase detect. From what I've read it's more accurate.

And plus one on the large DOF comment from Marc Sabatella above. They don't have to be accurate to the dot.
05-12-2011, 07:46 AM   #11
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The only thing more accurate about contrast detect as opposed to phase detect would be that there is no danger of the focus sensor being misaligned and thus causing *consistent* FF or BF. Inconsistent results are not the result of any weakness in phase detect; it's just a simple of matter of the factors I keep explaining, that affect *all* AF systems and always will until mind-reading technology is perfected.
05-12-2011, 09:13 AM   #12
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I'm not an expert by any means but shooting handheld even with SR is never perfect...
especially as you are 2 M (6 ft) away and at 180 to 240, and your subject is alive and moving...that is a really tight close up of a face....maybe try a similar shot on a stationary object with and without a tripod...see if you have the same issue....
05-12-2011, 02:47 PM   #13
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QuoteOriginally posted by jlaubza Quote
You stated that using the flash nullifies the SR but with manual settings and an old auto flash, not a new P-TTL flash, is this still the case?
No, it is only the case when you use a P-TTL or built-in flash, but with an older flash, triggered by a remote trigger, the shutter speed at which the flash syncs can be as low as 1/90th of a second depending on the brand of trigger. And heaven forbid that you are using an old flash with a high voltage trigger current on a DSLR. You can fry the electronics very easily.
05-12-2011, 03:27 PM   #14
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The auto focus on my kx is inconsistent as well. Stationary target, tripod mounted, remote shutter release, afs, single center point, good contrast on target, good daylight, and the kx does refocus and change the focus plane each time. I can hear the screw drive turning.
05-12-2011, 06:14 PM   #15
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Yes, for *exactly* the reasons I've described.
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