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05-07-2011, 10:53 AM   #1
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K-x & Sigma 150-500 focusing problems

Earlier this week, my new Sigma 150-500mm lens arrived. I tried it out immediately sitting in my front yard taking pictures of birds in the tree, about 20-25 feet away. I was very pleased.

But now I'm discovering that with the lens set at 500mm it simply will not focus past about 40 feet. I can't believe that this is to be expected, but I can't think of anything else to try to get it to work.

Here's what I've tried:

1. Switched off image stabilization in the camera and activated it in the lens -- with the lens set in its #2 position, this seemed to help a bit, but it was still generally disinclined to let me take pictures.

2. I switched from AF.A to AF.C. This allowed me to take pictures but most of them were hopelessly (I'm not talking soft, I mean hopelessly) out of focus.

3. I start by manually adjusting to get the focus into the right vicinity before turning it over to the AF. Again, this occasionally lets me get a shot or two, but the camera quickly loses focus.

Unlike the behavior of my Pentax 55-300mm lens (that came with the camera), the Sigma seems to zero in on some narrow range of the focus depth and stays there, failing to focus. I can be pointing at something 500 feet away and it's trying to focus with the setting set at under 10 feet.

Am I just missing the boat? Or have I gotten myself a dud lens?

Any help appreciated.

Dave

05-07-2011, 12:23 PM   #2
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Hi Dave,
My situation isn't entirely the same (k-5 and old Sigma 170-500 I got about a month ago), but the behaviour is the same indeed. If the AF starts hunting, it won't start again once it has reached the close end of the focusing range.
I'm not quite sure why, but technically the f/6.3 (f/6.7 in real life) maximum aperture is smaller that what my camera's manual specifies for the AF to work (f/5.6), so maybe that plays a role. I've noticed that when I simply attach a 1.7x rear converter to my 70-300 mm f/4-5.6 zoom, the AF continues to work, and my guess is that is because the camera still reads this combo as f/5.6. But that is pure guessing on my part.
Another thing may be that once you focus on something quite far, it appears to fall between the AF sensor points. I just focused on a large building several hundreds of meters away, and that works without problems.
Essentially, I'm not sure if the problem lies with the camera or with the lens. I'm guessing the camera, given how my old version of the lens has no active components.

I have to say, most of the time I use the thing in MF mode anyway, simply because of the fact that my typical subjects tend to hide between twigs, which is AF nightmare. Only if something is close or rather big, with no obstacles between it and the camera I use AF. Invariably those pics are a tad sharper than when I use MF (maybe I simply need glasses), so I feel your pain .
05-07-2011, 01:39 PM   #3
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Ah yes, I hadn't thought about the implications of that f/6.3. I too have been using a 55-300mm lens with a 1.4x teleconverter. It will auto-focus, albeit slowly. It was the slow autofocus that I was hoping to overcome with this new lens. Seems I've spent a lot of money for a lens that doesn't deliver much for my particular camera body. I wonder what cameras will work with the Sigma. Off to investigate (although I'm sure the wife will balk at the idea I should buy another camera -- oh wait, my Sigma has the Pentax mount, so that limits the choices.

Dave
05-07-2011, 03:43 PM   #4
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I have the 170-500. I use the af.s and use the sr in the camera body. i use the sigma on a kr and
k7 camera without any trouble. the pentax forum might be to help you more. they have lens groups and one of the groups is the sigma 150-500.

05-07-2011, 04:16 PM   #5
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I'm sorry, I'm confused by your response. I thought this was the Pentax forum. Can you give me a link to the sigma 150-500 group?

Thanks,

Dave
05-08-2011, 10:47 AM   #6
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Looks like my lens is behaving the way that ThomasG reported here:

Sigma 150-500mm f/5-6.3 DG HSM APO Lens Reviews - Pentax Third-Party Lens Review Database

I see no follow-up from ThomasG, though. Did Sigma replace the lens? Fix it?

Dave
05-09-2011, 03:51 PM   #7
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Here's an interesting snippet from the Sigma Service page:

SigmaPhoto.com - Service & Support

4. AF Data Adjustment

Due to extreme sensitivity of the digital camera mirror box, more than 5 times as sensitive as that of a film camera, it may be necessary to adjust the AF data in your Sigma lens. At Sigma, we adjust all of our lenses to a neutral focus point during manufacturing. It may be necessary to have your Sigma lens adjusted to your own personal camera. This is a known problem with digital cameras and many companies have included Micro Focus Adjustment capabilities on their high end cameras to address this problem with their lenses. If you are, in fact, able to do this with your camera and lens combination there is no need to send your equipment in for adjustment.

So, does the K-x support this? I don't see a mention in the manual's index, so I expect not.

Looking around the web, I seem to be the only one (of a very small community) who is the least bit unhappy with the K-x/Sigma combination.
05-11-2011, 05:07 AM   #8
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Contacted Sigma technical support. They think they can tune the lens to my camera. I'm driving up to Ronkonkoma on Thursday. Will report back.

Dave

05-13-2011, 07:48 AM   #9
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Happy Ending

Drove up to Sigma's offices in Ronkonkoma yesterday where they swiftly adjusted my lens to work properly with my camera. Took a walk through a local state park to test the lens in the vicinity but I needn't have worried. It worked like a charm.
05-17-2011, 04:53 AM   #10
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Next time you adjust the af on your k-x yourself, here are the instructions:

RiceHigh's Pentax Blog: K-x Debug Mode Tutorial - AF Adjustment
05-20-2011, 05:29 PM   #11
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Curious to know if they adjusted the lens or the camera. The k-x can be adjusted as described in the previous post but this adjustment then applies to all lenses not just the one. Other cameras (k-5) allow settings for multiple lenses but the k-x does not.

Can you check to see if your other lenses still work correctly? If so then I would assume the lens itself was tuned.
05-21-2011, 03:53 AM   #12
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They adjusted the lens. Left me with the camera in the lobby while the took the lens into a back room. The process took only a few minutes. My wife also has a K-x, so a solution where the camera had to be adjusted would not have pleased me.

Dave
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