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09-24-2011, 07:45 PM   #1
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Auto focus/fungus problem?

So I got a sigma 28-200 uc auto focus lens. It wound up having fungus all across one element towards the rear. They were spaced out tiny dots, that kind of looked like dust, till I took pictures and zoomed in on them, then I knew it was fungus. Image quality wasn't great but it wasn't terrible.
So I partially cleaned the fungus. I basically just wiped the bulk of it away but didn't really clean it up (problems with dis assembly). I plan on cleaning it better later.

So now on the long and middle end it seems to be pretty good, but on the wide end its pretty bad. I got the feeling there was a focus issue, and tested. It does in fact have a focus issue. I auto focused and took a shot, then switched to manual focus and took several shots, slightly changing the focus each time. I did it twice, once for each direction. It was defiantly not focusing right. I think the image problems are a combination of the glass issues and being out of focus.

Could the fungus issues be causing auto focus problems? If this problem is independent of the fungus, is there any way to test, diagnose and or fix a focus issue like that?

09-24-2011, 09:17 PM   #2
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Try testing the lens under a very bright light and at a simple contrast target. So forget real situations... boil it down to... if it fails even at dead simple.
Target could be news print and lit by two 100watts.
09-24-2011, 10:01 PM   #3
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I haven't tried bright light with a test target yet (gotta work in the morning so I'm up too late already). I just used a tripod, timer and longer exposures. Where I ran into problems with newsprint is that at 28mm and closest focus distance its not easy to manually focus on for comparison. I did try it though and tried some larger targets that were easier to manually focus on (still haven't found a great target for the situation yet). As best as I can tell, it is auto focusing wrong but only at wide ranges. I even tried manually focusing several times, then switching to auto focus to see if it would change focus. Every time it changed focus from where I thought it was right. There is something else I discovered. I'm not sure if that is just the way this lens is or if it is an effect of the fungus damage, but image quality drastically improved stopped down. I have tried with fastest aperture as well as several others. Stopping it down a lot makes a huge improvement. Shooting both fast and stopped down the focus issue seems to be there. I'll know better once I find a better target and or test some more but it seems to be consistently mis auto focusing at the widest ranges.

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