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Author Topic: AF problems  (Read 3754 times)

Offline kinigets

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AF problems
« on: July 18, 2013, 10:00:55 AM »
Hi,

I've been having some auto focus issues with my Sigma 17-70 lens. I bought it about a year ago, and while it has always been a bit touch and go, it now refuses to focus. Basically it goes back and forth trying to find a point to focus on, and then finally stops with the green square and the happy "beep beep" to say the image is in focus. The result is a totally out of focus image.  If I want to use the lens now, I have to put it in MF and focus myself.

Normally this would not be a problem, but as I bought this lens as my quick, multipurpose, walk around lens, getting those last minute captured shots takes a bit more time, and I often miss the shot. I do not have this issue with any of my prime lenses, or the kit lens that came with my original Rebel xsi.

Is there a fix for this issue? I already returned one lens because of this. The one I have, I tested in store and has been fine for the first 6 months. Doubt I will be able to return this one.

Thx.

Offline AndyCivil

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Re: AF problems
« Reply #1 on: July 18, 2013, 03:45:18 PM »
You talk about the "green square" which implies you're using live view autofocus; in this case it's the camera that decides when focus is achieved, not the lens. So if the lens is correctly hunting back and forth but a bad decision is made about what constitutes 'in focus' then it's the camera that's at fault.

Please:
  • If possible, try a different lens and tell us if that's any different
  • Try viewfinder (quick) autofocus and tell us if that's any different
  • Describe any differences in focus ability depending on the contrast level of what's in the box
  • Tell us how reliable is the failure (always, sometimes, etc.)

Offline kinigets

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Re: AF problems
« Reply #2 on: July 25, 2013, 05:47:42 AM »
Hi, I'm going to try replying again, as I just wrote a really long message, uploaded attachments and then the message disappeared. So here goes again...

I rarely use Live View (only when I'm doing an awkward shot where I have to hold my camera away from me to get the shot), and always shoot with the viewfinder. I actually just mentioned the green square and the "happy beep beep" to indicate that the camera had stopped it's attempt at auto focus. (To be honest, I don't remember if I've ever experienced the problem using Live View)

In answer to your questions...

This failure can at times happen very frequently (will go a month where every time I use the camera this error occurs) and then suddenly, like for the last week, it will be more or less ok.

It seems to be more of a problem when I am focusing on something in the foreground or taking a portrait of someone, but can occasionally happen with landscape images as well. (I once tried to photograph a house and it just wouldn't focus).

The problem will happen in all camera settings whether on Auto, P, TV, M, Macro, ... However, it never happens when in film mode. It occurs regardless of lighting, contrast and number of focal points. (I've tried changing those settings as well)

Generally, when the problem occurs the camera will attempt to find the focus for up to 10 seconds before settling on an image that is totally out of focus. In the process it will alight on the correct focus but keep searching.

Apart from occurring with a 15 year old kit lens that came with my first SLR, I don't have this problem with any of my others lens. (Most of them however are prime lenses). With all of them the camera is very quick to focus, regardless of lighting, contrast, proximity of the subject, etc.

A friend suggested I need to use the camera disc to re-install some settings related to the lens as not all cameras come with all lens info pre-installed. However, as I've misplaced my disc, I've been unable to try this.

I hope this gives you more info as to the nature of the problem. I've taken some photos but will upload in a separate message this time. The lens is now over a year old, so not sure a warranty would cover it anymore, although according to the Sigma site, the UK warranty period should be 2 years.

Thanks for your help
:)

Offline kinigets

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Re: AF problems
« Reply #3 on: July 25, 2013, 05:58:23 AM »
Here is a sample image - Dream catcher (subject) was a meter away. This is what the camera eventually settled on as the correct focus.

Offline kinigets

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Re: AF problems
« Reply #4 on: July 25, 2013, 05:59:52 AM »
This is roughly the same image taken on my Sigma 30mm 1.4f. No problems here.

Offline kinigets

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Re: AF problems
« Reply #5 on: July 25, 2013, 06:08:53 AM »
Also taken with the problem lens...this was shot in a brightly lit hallway (fluorescent lighting)

Offline AndyCivil

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Re: AF problems
« Reply #6 on: July 25, 2013, 11:42:24 PM »
Tough one.

There's something that doesn't make sense in your description. We have two methods of focusing, the quick mode that uses the sensor in the viewfinder and the slow mode that uses contrast in the image. The quick mode is a "go straight there" method that measures the distance to the subject and tells the lens to go to that focal length. The slow mode moves the focus and judges whether the image is sharper or blurrier and adjusts the focus more or less, accordingly. You describe that this problem doesn't happen in video mode - that implies that slow focusing is working (because video mode always uses slow focusing). You also describe that most of your lenses "just go there" on the quick focusing; as they should! But with your problem lens, you say that it's searching for a focus point... that's wrong: it shouldn't be searching, it should be 'just going there'. I can't think of any fault your camera would have, which would cause it to 'search' for a focus point while it's in viewfinder mode.

Please try the slow focusing method by pressing the live-view button, just so that you're familiar with what it feels like. Now - when you go back to viewfinder (quick) focusing, when it fails, does it 'feel' like the slow focusing method? And when it works, does it "just go there" or does it feel like the slow method, but it stops at the right point?

I was looking at the spec for your lens, and I notice that you can disable the autofocus by turning the manual focus ring. I've never owned one of these lenses so this might be stupid, but is there any possibility that you're unknowingly interrupting the focus process by touching the focus ring as it's trying to operate? This isn't a heavy lens, so I don't imagine you're doing that cradle thing where you hold the lens with your left hand... but you could be.

I don't think you need to download new software for the camera, but it looks like your lens can have its software updated (something I know little about) so that would be worth trying. Also, it might be worth clearing the settings in your camera just in case something is corrupted. You'd lose your preferences, so be ready for that. It's under the three-dot wrench/spanner menu and only appears in P mode or higher. If the memory card has the last photo you took on it while you do this, then when you take the first picture after clearing the settings, it should start counting image numbers onward from there, rather than starting from zero again.

Offline Ochotona

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Re: AF problems
« Reply #7 on: July 26, 2013, 09:43:08 AM »
I had a sigma lens in my film days that had a similar problem.  The lens body had loosened up (the threaded rings that hold the individual glass elements together) and the focus ring had slipped resulting in a messed up focus index.  This is difficult to explain, but when camera thought it was focused the focus ring was actually in the wrong place because of the slip.  I was able to loosen up the lens and set the focus index back to the correct position manually then tightened it back up and it worked better after that.  Not likely that you have the same problem, but you may want to make sure your lens body is still nice and tight.

Offline Fyrblade

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Re: AF problems
« Reply #8 on: July 26, 2013, 10:57:09 AM »
If you focus manually, do you get a clear image with it?
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