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10-07-2013, 09:18 AM   #1
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Troubleshooting Sigma 28-70 EX DF

Hi All,

I just bought a lens that has an issue. It is the Sigma 28-70 EX DF. It is a dream in every aspect except I have found that at f2.8 the actual point of focus shifts from front focus to back focus as it moves through the zoom range (actually, as I adjusted the focul point in my camera it was probably a back focus to a worse back focus problem). At the minimum focul range the actual focus point appears to move about an inch or possibly two. I checked my other zoom f2.8 lens and it remains in the same point throughout the zoom range. Anyone have success making a lens like this usable? Sharp focus tough to attain with certainty. Should I send this lens back to the seller?

Jerrod

10-07-2013, 10:17 AM   #2
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Parfocal. As I remember that is name for lenses which doesn't change focus distance during zooming.
I don't think that particular sigma is parfocal lens. So is normal that zooming change focus distance.
http://en.wikipedia.org/wiki/Parfocal_lens
10-07-2013, 10:46 AM   #3
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Original Poster
Thanks for the input. I don't think that I have an issue with it not being a 'parfocul' lens. If this were the problem I'd be able to re-focus after changing the zoom length and nail the focus. The problem I'm describing is that I need to apply a +2 fine focus for 70mm in the camera body and a +6 at 28mm (or vice versa) in order to get sharp focus.
10-07-2013, 11:37 AM   #4
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aha
same thing i had with tamron 17-50 that works perfectly on older bodies but on K-5 behave like that.
In service explained to me that electronic inside need to be replaced to work with new body so I did that and than worked perfectly. I don't know is that case with that particular sigma but can be because 28-70 is pretty old model.

10-07-2013, 11:43 AM   #5
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QuoteOriginally posted by Barbarosa Quote
aha
same thing i had with tamron 17-50 that works perfectly on older bodies but on K-5 behave like that.
In service explained to me that electronic inside need to be replaced to work with new body so I did that and than worked perfectly. I don't know is that case with that particular sigma but can be because 28-70 is pretty old model.
Thanks...that helps. I have the K-5 as well and the 28-70 EX DF has to be 10 years old if not 15. I don't think the lens is worth working on as I bought it for less than $175 shipped. It would have been a true gem without that issue.
10-08-2013, 07:31 PM   #6
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I have this lens, bought it new about 6 years ago from a seller in Australia. I also have a K-5, though I haven't shot much lately with the lens on the K-5. When I get a chance I'll test it out and see if I experience any focus issues.
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