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12-24-2011, 01:06 AM   #1
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Sigma 28-70mm EX F2.8 DG

Hi all
I am so struggling with this lens on the K5..........the focus seems very inconsistent on it........when I had it on the little km/k2000 there was no issues at all.........but now I am struggling to get a 10% keeper rate with it.......what can I be doing wrong? These are sooc from lightroom.....and in theory should have been tack sharp....one looks ok and the other is cr*p........I have left them large and hope the exif is still in tact. Any advice?
cheers
Jan

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View Picture EXIF
PENTAX K-5  Photo 

Last edited by janstew; 12-24-2011 at 01:34 AM.
12-24-2011, 01:14 AM   #2
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Seeing the exif data would help (if you attach the photos or post them in an album, the exif will be read), but one possibility could be program line. Remember that the K-m has autopict whereas the K-5 does not, so you have to be a little bit more mindful of what settings you use.

Auto program line plus an iso of 400-800 should deliver some pretty good indoor results, and if needed, you can adjust the aperture via hyper program so you get more DOF.

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12-24-2011, 01:33 AM   #3
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Actually, I have exactly the same problem with my copy. It works a treat on my KM, but on the K5, well disappointing is the word I would use. It is very hit and miss and yes I have tried to fine tune the focus. I find that depending on what end the focus comes from dictates it's accuracy, if it starts from close focus then the autofocus works most of the time, but if it is coming from the infinity end then there is a snowballs chance in hell of it getting an accurate focus.

I originally purchased this for my walk around lens, but now in the process of shifting to primes (I will keep it on the KM for the moment, who knows one of my boys may get the bug).
12-24-2011, 01:33 AM   #4
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Thanks Adam I have attached the images with the exif............I am not sure about the program line and will check what it is set to
cheers
Jan

12-24-2011, 01:43 AM   #5
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So what is your set to Bruce? I just checked and my program line was on auto.......I changed it to normal.........bear in mind I only shoot in Manual.........would it make any difference to manual settings?
cheers
Jan
12-24-2011, 01:48 AM   #6
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I usually set the program line to Auto and use AV mode most of the time with this particular lens.
12-24-2011, 01:59 AM   #7
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I just played with it and the closer I am to 70mm the blurrier it is? At the shorter end it is less blurry? Doesn't make any sense to me?
cheers
Jan
12-24-2011, 06:01 AM   #8
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You need to work out at what end you shoot most and fine tune the AF to around that focal length. Zooms have a sweet spot, you just have to find it (generally it won't be 'as sharp' through the whole range. Fine tune the focus to the sweet spot and bingo! My copy is best around the 50mm mark and that is where I have adjusted the focus to. So at 50mm f2.8 it is sharp and either side of the 50mm mark I just widen the DOF by adjusting the aperture to give a deeper in focus area to make up for the softness (hence the use of AV mode).

12-24-2011, 12:25 PM   #9
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trouble is I shoot at both ends........hence being annoyed. I wonder if it is still under warranty? Does Sigma give a 2 yr warranty? as its just over 12 months old...........and is no good to me if I have to fuss this much with it. I bought it for this focal length in a home studio setting. Phooey
Jan
12-24-2011, 06:58 PM   #10
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Have a chat to Kennedy's and see what they will do for you (after all they are the 'exclusive' importers of the Sigma gear as well as pentax).
12-29-2011, 12:46 PM   #11
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I had some AF issues with the same lens and I sent it back to sigma. They replaced some sensor (under warranty) and it has been working great.
12-29-2011, 11:58 PM   #12
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thanks guys.........seems the issue is with the lens therefore I will ring Kennedy's on the 3rd of January.......OMG I cant believe a new year is starting so soon!
cheers
Jan
12-30-2011, 12:03 AM   #13
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I wouldn't expect immediate satisfaction though as one of my K-5's went off to Melbourne yesterday to have the Focus Mode Selector switch fixed and I was told it may be 4 weeks before I see it again.
12-30-2011, 06:27 AM   #14
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Hi Jan,
Looking at your shots, i am not sure the problem is with your lens. The exif file helps to explain a few things.

Your photos might have looked different if you have used Av or TAv. Check to see if your ISO is on Auto. In the type of lighting in photo 1, i am surprised the ISO is still 200 with a Shutter Speed of 1/160 and aperture f4. My guess is the correct exposure for photo 1 with aperture f4 is a shutter speed of 1/60 or 1/80 and ISO 400-1250 or 2-3 stops more than your current exposure.
Photo 1 is sharp from middle subject to the back. The front subjects are out of focus.
You can try using onboard flash but dialled flash output to -1.7 or -1.3.

K5 is an amazing camera. It can do more than what other brands have to offer with high ISO performance etc... Sigma is a fantastic lens as well..
Just my 0.02!
01-01-2012, 03:36 PM   #15
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Hi Bestzoom.......those pics actually were using flash.......I had a sb28 onboard.....and I was using all manual......so i took some very contoled pics today to see if I could come up with anything............all the same values , natural light, on a tripod with sr turned off.........I still don't know if I have done anything to prove my point l. Each image is enlarged to 100% and the long side is 1024.....any ideas anyone?
cheers
Jan
Exif......... iso 400 F8 1/15.....top left 70mm, bottom left 50mm, top right 34mm, bottom right 28mm
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Last edited by janstew; 01-01-2012 at 03:42 PM. Reason: so where did the exif go?
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