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03-18-2009, 05:54 PM   #1
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K200D Focusing (Sigma 24-60 EX/DG)

Hello all,

I have a 50mm f/1.7 and a CHEAP tamron 70-300mm f4-5.6 zoom. Also have played with the A series 28mm a bit.

Anyway, I have a complaint. My walk around, standard lens is the Sigma 24-60 EX DG f/2.8. And first of all, it front focuses a bit. Why is it that ALL Sigma lenses front focus? They seem to have a serious QC problem compared to Tamron. In addition to this, the lens or camera (?) screw up the focus ALOT! Its not that its not focusing quickly, although indoors that CAN be a problem, its that the focus is not LOCKED IN...

So, I was wondering two things... One, is this a known problem with the K200? inaccurate or slow autofocus? Or a known problem with the Sigma lens? It *SEEMS* to occur more with my Sigma lens than the others. But I also shoot more with the Sigma.

IF we do think it is the lens. What lens would you recommend to replace it? How does the 31mm compare VS the 21mm, limited's that is as a walkaround combo with my 50mm... Or are there any zoom lenses that focus quickly/accurately? Tamron 28-75mm f/2.8, maybe?

The rough part is the Sigma's glass seems sharp... but the images are terribly inconsistent.

03-19-2009, 09:46 AM   #2
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What do you mean by focus is not "locked in"? That implies to me it focuses first on half press, then changes its mind by the time you finish pressing the shutter. Is that what you mean? Could you post pictures that demonstrate what you are talking about?

Inaccurate focus - in my experience 99% of reports of this are simply cases of the camera choosing to focus on a different subejct than you intended. Are you selecting th focus point, or are you letting the camera do it Also, do you realize how much larger the AF sensors are than the little red squares in the viewfinder and how this affects focus?

As for slow, yes, pentax in general is not known for fast AF, and it gets worse the bigger and heavier the lens is, since the relatively weak motor is having to turn a greater mass. To a tough approximation, you can guess the focus speed of a lens by its weight, although focus speed is helped by large maximum apertures (which generally adds weight), as well as sharpness. I don't of any zooms that manage to find a tradeoff between betwen weight & maximum aperture that allows them to focus as quickly as a typical prime, but I haven't really compared, either.
03-19-2009, 08:03 PM   #3
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Seeing as you mention only the one Sigma lens in your post, it's not clear to me upon what your are basing your general impression of Sigma lenses as having front-focus problems. My copy of the Sigma 24-60mm f/2.8 EX DG has become my most-used lens, and you can trust that I did not make it so by ignoring a front-focus problem. I've seen quite a bit of praise for the 24-60 around the Interwebs, this forum included, and yours has been the first report of a front focus issue that I've seen.

I also own the 17-35mm f/2.8-4 EX DG and it too is free from any focus-related defects.

Assuming you aren't able to identify another source of your problem, I'd recommend getting the 24-60mm serviced as it's generally considered to be well-worth owning.

-XM
03-19-2009, 09:58 PM   #4
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Ok... I'll upload to flickr. I have 100 of the bad and like 10 of the good. Keep in mind.

And, yes, I have looked at it A LOT and think its the focus rather than the shutter speed. I have had some similar problems at LOWER shutter speeds.

I think the issue might be that it focuses on the CLOSEST image in the foreground rather than the MAIN image in the foreground. No? I don't mind fast/slow autofocusing. I just want accurate. Which is why I'm thinking of ditching the lens and getting a pentax lens (21LTD?) with manual focus override ON the lens.

In any event, the shots are 1/2000 ~f/3.5 ISO either 100 or 200. No image stabilization on. It was a moving target but not a nascar; a skateboarder. 1/2000 REALLY should be enough to freeze it.

You'll see some of the pictures are frozen nicely. They fricken POP. THATS what i mean by having the focus "locked in"

Anyway... See links below.

Good: http://farm4.static.flickr.com/3424/3364238126_a15631ea64_b.jpg
Flickr Photo Download: Stall
http://farm4.static.flickr.com/3121/3369033049_e9754c0065_b.jpg

Bad:
http://farm4.static.flickr.com/3574/3369857374_79940c7953_b.jpg
Flickr Photo Download: Crooked

The inbetween, and the picture I'm happy with but also annoyed with:
http://farm4.static.flickr.com/3471/3363425511_01f398e480_b.jpg

To be fair, the lens Image quality is AMAZING for the price. Its clearly a great value even with the possible criticism I am giving it... But let me know. Many of the pictures on my FLICKR page have come from it. Flickr: pm650x's Photostream
I will be deleting all but the "good" pictures following this thread.

03-19-2009, 10:01 PM   #5
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QuoteOriginally posted by XMACHINA Quote
Seeing as you mention only the one Sigma lens in your post, it's not clear to me upon what your are basing your general impression of Sigma lenses as having front-focus problems. My copy of the Sigma 24-60mm f/2.8 EX DG has become my most-used lens, and you can trust that I did not make it so by ignoring a front-focus problem. I've seen quite a bit of praise for the 24-60 around the Interwebs, this forum included, and yours has been the first report of a front focus issue that I've seen.

I also own the 17-35mm f/2.8-4 EX DG and it too is free from any focus-related defects.

Assuming you aren't able to identify another source of your problem, I'd recommend getting the 24-60mm serviced as it's generally considered to be well-worth owning.

-XM
Impressions from prior Sigma lenses with Film cameras as well as family's dslr cameras. And that I have never had similar problems with Tamron. Needless to say I am merely one person, and therefore biased (from my experiences). If I were given other peoples accounts of more problems with tamron i would be swayed... I have not heard either way.
03-19-2009, 10:13 PM   #6
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One last thing too, as I do appreciate the responses. Its (the focusing problems) not just from the skate pictures being from different perspectives (though, i do appreciate, that, that plays a role)

VERY similar issues have occurred with graduation pictures and a few portrait pictures. I deleted those though and no longer have them.

I am NOT overly angry... but am just wondering if anyone has a feel in terms of what I am doing to cause this, it being the lens, or just a function of the body, or my photographing.
03-19-2009, 10:48 PM   #7
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My Sigma 24-60 front focuses severely, to the extent I have to set my K20D focus correction at -6 to compensate. This does not occur with any of my other lenses/ex-lenses like Sigma 17-70, Tamron 70-300, Tamron 18-250, DA21 and a few FAs.
03-19-2009, 11:23 PM   #8
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QuoteOriginally posted by jk333 Quote
I think the issue might be that it focuses on the CLOSEST image in the foreground rather than the MAIN image in the foreground.
The camera decided what to focus on, not the lens. And you have relatively little control over what it decides. You can, of course, select which focus point is used - did you do that? - but the AF points are fairly large areas, so it's still difficult to pinpoint exactly what the camera will focus on. That's not front focis - that's just the camera failing to read you mind. Front focus would be visible in a situation where you can prove the camera definitely tried to focus on a given subject, but missed. In the picture you posted, there is basically no way to even guess what the camera might have tried to focus on.

QuoteQuote:
You'll see some of the pictures are frozen nicely. They fricken POP. THATS what i mean by having the focus "locked in"
OK; that's not what the term actually means, but at least now I know what you meant by it. Better to simply say, the image was not sharp, not that the lens failed to lock focus. If it had failed to lock focus, you would have no picture at all - the shutter would not have fired.

03-19-2009, 11:52 PM   #9
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I have that lens and find that it works very well particularly in decent light like what you are shooting. But... first make sure you are on spot focus and what you want to be in focus is where you need to have the focus point on. use your up/down/side arrows to move your focus point if you feel you have to use auto focus. But don't focus on solid colors like a dark t-shirt... focus on an area that has lots of contrast, like their hair line, or the line of skin vs t-shirt.

Also, your subject is moving pretty fast, particularly when you are somewhat close to your subject, so make sure you're using af-c, and anticipate where you are going to want to take your shot and consider using manual focus like some sports photogs do.

Lastly, crank up your aperture. Your shot labeled "crooked" was taken with an aperture of 2.8 w/ shutter speed at 1/2000 w/ iso @100. If you boost your aperture to around 8, your area that will be in focus (depth of field) will be considerably larger.

For example... if you're shooting at f/2.8 from 5 feet at 24mm (like your shot "crooked"), your area in focus is 1.5 feet
Subject distance 5 ft

Depth of field
Near limit 4.36 ft
Far limit 5.86 ft
Total 1.51 ft

BUT... if you get you're at f/8 w/ all else being the same you get:
Subject distance 5 ft

Depth of field
Near limit 3.53 ft
Far limit 8.57 ft
Total 5.04 ft

HUGE difference between 1.5 and 5 feet! More of your subject will be in focus and manually focusing becomes super easy. In fact all you have to do is point and as long as your subject is more than 3.5 feet away, it'll be in focus. No more lockups where the camera won't fire because it's not sure if it's in focus, it'll just shoot. No more thinking about where your focus point is, or where the high contrast area is so you can get a faster focus lock... all you have to do is concentrate on your framing. Practice manual focusing... it takes time to get right, but doing what you're doing with fast action, it can be invaluable.

BUT... at f/8, your shutter speed will have to be a lot lower to get the same exposure unless you crank up your ISO. SO... you'll want to have shutterspeeds of at least 1/250th's, so whatever that is given the lighting conditions... perhaps use TAv on your camera setting so you can lock in shutterspeed and aperture, then set your ISO on auto w/ a range of 100-1200, and see if things improve.

use this site to calculate your depth of field (it's worth a lot!!!):
Online Depth of Field Calculator
Get to understand the relationship between aperture, focal length and shooting distance to know how much wiggle room you've got.

I have that sigma 24/60 f2.8 among many other lenses, and it can focus fast in lower light, but use some of these other tips to maximize your chances. My copy front focuses a bit too on charts and brick walls, but in the real world, it's plenty fine. Technique will improve your odds of getting a good capture and your shots that are in focus are fine so it's not the lens. Good luck!

ps: your "mystic river" and "sunset blacks" pics kick arse!

Last edited by vagrant10; 03-19-2009 at 11:59 PM.
03-20-2009, 12:17 AM   #10
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The Sigma 24-60 (while also being one of the best lenses ever created) has a severe front focusing issue mainly with it's Canon mounts. It's actually been documented as being the worst occurence in a lens. You can have Sigma repair your front focusing problem, they'll usually do it for free with this lens, even without any warranty on it.
03-20-2009, 10:37 AM   #11
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So the verdict is inaccurate autofocus, eh... Ok... I'll work on the spot focusing a bit more.

Thanks Vagrant, thats awesome stuff! I'm going to read up on that. I will work on it.
03-20-2009, 10:44 AM   #12
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QuoteOriginally posted by Marc Sabatella Quote
OK; that's not what the term actually means, but at least now I know what you meant by it. Better to simply say, the image was not sharp, not that the lens failed to lock focus. If it had failed to lock focus, you would have no picture at all - the shutter would not have fired.
I hear you. Will do. Which brings up another question? Why does it just NOT take a picture? (I get that its hunting for a locked focal point, but...)
Shouldn't it tell you it doesn't have a lock, BUT THEN, after say, 5 full seconds shoot a picture? Obviously If you are holding the button you want it to take a picture. At some point I would think the focus lock should just let you override and take a picture.

I notice this the most in low light trying to take pictures of a point that is in the distance, focused to infinity. I'm like, damn it, you stupid camera, default to infinity. Just seems like after X seconds it should take a picture, defaulting to infinity would be better than not taking one at all. No?

Maybe I should look into the pentax lenses with the manual override. Anyone love them or hate the feature?
03-20-2009, 01:33 PM   #13
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QuoteOriginally posted by jk333 Quote
I hear you. Will do. Which brings up another question? Why does it just NOT take a picture? (I get that its hunting for a locked focal point, but...)
Shouldn't it tell you it doesn't have a lock, BUT THEN, after say, 5 full seconds shoot a picture? Obviously If you are holding the button you want it to take a picture. At some point I would think the focus lock should just let you override and take a picture.

I notice this the most in low light trying to take pictures of a point that is in the distance, focused to infinity. I'm like, damn it, you stupid camera, default to infinity. Just seems like after X seconds it should take a picture, defaulting to infinity would be better than not taking one at all. No?

Maybe I should look into the pentax lenses with the manual override. Anyone love them or hate the feature?
You have a manual override on the camera body... flip the switch and it will fire. Sorry to keep harping on this but practice manual focusing and how to switch quickly from auto to manual so that it's routine and you don't have to look down to find the switch. Actually with time, this will come naturally, but if this is an area that needs improvement...
03-20-2009, 01:48 PM   #14
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QuoteOriginally posted by vagrant10 Quote
You have a manual override on the camera body... flip the switch and it will fire. Sorry to keep harping on this but practice manual focusing and how to switch quickly from auto to manual so that it's routine and you don't have to look down to find the switch. Actually with time, this will come naturally, but if this is an area that needs improvement...
Wait, there's a manual override on the body? HECK YES!!!! (off to grab my camera to start checking this out) Now we're talking.

As you can see, almost all of my photos have been landscapes with the dSLR. But now that I'm doing some sports, I'm probably gonna just be mostly manual focus. Back in the old days, with my ZX-M and A series lenses, that was the only option...

EDIT - Holy crap... thats a big button to miss. I wonder what else this camera has...

Last edited by jk333; 03-20-2009 at 01:54 PM.
03-21-2009, 11:45 PM   #15
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If by "manual override" you mean the AF/MF switch, tha will indeed do the job. But you can also program (via custom option) the OK button to temporarily disable AF, and I do this all the time. Sometimes to stop hunting, but more often to allow me to shoot a series of pictures after focus is achieved without the camera being tempted to refocus.
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