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08-29-2011, 09:26 PM   #1
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Question if lens needs service

I have a Sigma 50-500mm the non HSM version that is only a few months old. Found a new old stock unit.

Anyway on a recent trip I noticed the lens just couldn't seem to focus or lock in on anything past 300-350mm focal length. It seemed to lock right in on any focal length up to the above mentioned range. I did have a circular polarizer on it since it was pretty bright outside and only attempted to use it outside on clear sunny blue skies type of days. It would just to seem to hunt back and forth.

Is this a problem that I need to contact Sigma about or do I need to make an adjustment to the camera? By the way using it on the K-5.
I've heard of this type of problem in dark situations but this was not the case.

Any thoughts or suggestions would be greatly appreciated.

08-29-2011, 11:14 PM   #2
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Personally I would try it under different shooting conditions before I would go spending money to get it serviced. While circular polarizers are supposed to work fine with auto focus, they do reduce light. I would try it without the polarizer too. There are a lot of things that can cause issues with auto focus. Try focusing with any lens on a plain white wall and you will see what I mean. Out of curosity, did you have the camera set for single or multiple point auto focus?
08-30-2011, 04:59 AM   #3
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Run a few tests without the polarizer. That filter removes a stop of light, and the lens is quite slow to begin with, so maybe there just isn't enough available light for your AF system to lock. Run a few controlled tests, note the parameters, use a light meter if possible (or if you have an iPod/iPhone, get a free light meter application) and THEN, if things are still bad, contact Sigma with your documented problem.

I find that with Sigma, you have to "prove" your problem before they will fix it. That's sad but that's how it was with me.
08-30-2011, 05:12 AM   #4
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I also have the Sigma 50-500 non-HSM and when the light is low it will definitely have a tendency to hunt for focus at the long end of the zoom range where the max F-Stop is 6.3 especially if I'm not shooting at a very bright object. Even on a bright sunny day if you add a teleconverter or a polarizer to the lens you are effectively reducing the light reaching the sensor by another 1-2 stops. I know that when I do that is sometimes very hard to get the Bigma to lock on focus at 350+.

As Ripit said it could be any number of things, try it without the polarizer filter and check your focus mode to see if that makes a difference. I notice that you have a K-5 and a K200D does it do the same thing on both of those cameras? I use a K10 and K20, neither of which is great at focusing in low light, and they both have the same issue with the Bigma.

08-30-2011, 07:39 PM   #5
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QuoteOriginally posted by ripit Quote
Personally I would try it under different shooting conditions before I would go spending money to get it serviced. While circular polarizers are supposed to work fine with auto focus, they do reduce light. I would try it without the polarizer too. There are a lot of things that can cause issues with auto focus. Try focusing with any lens on a plain white wall and you will see what I mean. Out of curosity, did you have the camera set for single or multiple point auto focus?
Single point on auto focus...will try it out without the CP before contact Sigma
08-30-2011, 07:40 PM   #6
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QuoteOriginally posted by bdery Quote
Run a few tests without the polarizer. That filter removes a stop of light, and the lens is quite slow to begin with, so maybe there just isn't enough available light for your AF system to lock. Run a few controlled tests, note the parameters, use a light meter if possible (or if you have an iPod/iPhone, get a free light meter application) and THEN, if things are still bad, contact Sigma with your documented problem.

I find that with Sigma, you have to "prove" your problem before they will fix it. That's sad but that's how it was with me.
Very true with regards to Sigma Service...had to deal with them on a TC...it was not a simple process.

thanks for the suggestion...
08-30-2011, 07:44 PM   #7
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QuoteOriginally posted by Odinz Quote
I also have the Sigma 50-500 non-HSM and when the light is low it will definitely have a tendency to hunt for focus at the long end of the zoom range where the max F-Stop is 6.3 especially if I'm not shooting at a very bright object. Even on a bright sunny day if you add a teleconverter or a polarizer to the lens you are effectively reducing the light reaching the sensor by another 1-2 stops. I know that when I do that is sometimes very hard to get the Bigma to lock on focus at 350+.

As Ripit said it could be any number of things, try it without the polarizer filter and check your focus mode to see if that makes a difference. I notice that you have a K-5 and a K200D does it do the same thing on both of those cameras? I use a K10 and K20, neither of which is great at focusing in low light, and they both have the same issue with the Bigma.
The day was very bright and was focusing in on Mount Rushmore which I wouldn't consider to be a dark subject matter.

I know this lens can be a bit slow but overall I'm quite pleased with it.

It was only used on the K-5...the K200D was busy with my son shooting away.
08-30-2011, 10:27 PM   #8
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QuoteOriginally posted by stl09 Quote
Single point on auto focus...will try it out without the CP before contact Sigma
I'm far from an expert here so someone correct me if I am wrong, But so far as I understand it, auto focus works on contrast. Imagine a black box in the middle of a white page. When it is out of focus, the edges of the black box are spread out and blurred. When it is in focus, the edges are sharp and defined. Your auto focus is measuring that edge where it transitions from black to white. When the mixed blurred grey area becomes smallest, its in focus, or something along those lines anyway. If you shoot at a white wall, there is no contrast so your camera has a hard time auto focusing. If it does auto focus it probably found some imperfection in the wall where there was a little contrast and locked on it. Focusing on Mount Rushmore, which is basically rock of similar color, there are less areas of significant contrast so it is harder to auto focus. I'm just guessing but if the camera is trying several point instead of one it might have an easier time finding a good point of contrast to lock onto.

This is of course just one rough theory of one of the many possibilities. One way to test would be to print a page that is half black and half white, and see if you can focus on it. Maybe even better a page with several sharp wide lines. Of course with a long lens like that you will probably have to mount it on a wall a ways away or something.

I just googled focus test chart thinking I could find a page you could print and stumbled on this (a much better explanation than mine).
Nikon D70 Focus Chart

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