Need some help understanding focus issue (Sigma 17-70 f/2.8-4.5 & 50D)

[Wojciech Drzazgowski]

Hello!


I am looking for some help understanding issues I'm having with my combo getting a proper auto-focus. I've not been very happy about the sharpness of some of the shots taken with the abovementioned and so I started to play with the microadjustment function.The confusing conclusion from the tests I've run so far is that the autofocus behaves differently depending on the distance from the object I try to focus on. With objects very close to the lens I need to set the ma to something about -10 to get the focus spot on. On the other hand when shooting objects further away ( > 10 meters) I need to set the ma to +7 to get sharpest shots.


All the tests were done on the longest end of the lens, fully opened (70mm f/4.5). Obviously with tripod, remote release, mirror lockup etc.


Can anyone shed any light on the issue? Is it a normal behaviour? Or maybe there's something wrong with the tests I'm doing? I'd expect the focus to behave differently depending on the focal length but not on the distance from the object...


I also ran a few tests using the same camera body but Canon's 70-200 f/4L lens and in this case I'm not sure about the results. Sometimes I seemed to be getting sharper shots with the ma correction, sometimes not.


I'll be greateful for any insight on this.





Best regards,





Wojtek

[neuroanatomist]

HiWojtek, and welcome to the TDP Forums!


I can't speak for this lens specifically, but I can say that a similar issue to what you describe is reportedly 'normal' for the Sigma 50mm f/1.4. Roger of Lensrentals.com sums it up nicely:

On full frame cameras and to a slight extent on crop frame cameras the lens exhibits what I will term ‘schizophrenic autofocus’: Closer than 5 feet it will front focus, further than 20 feet it will backfocus. This is not a calibration issue, its just how it is.

What Roger describes for the Sigma 50mm f/1.4 is something that AF Microadjustment will not correct.

[Wojciech Drzazgowski]

That makes too much sense...unfortunately. Well, will have to live with it then. Or get me a proper Canon lens instead Thank you!

[Sheiky]

Welcome!

Well, will have to live with it then

Yea, unless you've bought it recently and you can return it. Perhaps it's under warranty? So you can send it back for re-calibrating by Sigma, which might help a lot.


Although I have a very good operating 50mm 1.4 from Sigma I must say that I get more and more convinced by others that Sigma's AF system/control/calibration really has major issues like 70%? of time...too often at least. And Sigma gets less credit from me, however if you do manage to get a good copy, you can be a very happy owner.

Or get me a proper Canon lens instead

If you're looking in approximately the same budget, take a look at the Tamron 17-50(the one without vibration control/image stabilization, VC) The Canon alternatives are somewhat more expensive [:@]


Jan

[Wojciech Drzazgowski]

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Unfortunately the lens is way after the warranty period. But I did fire an email to Sigma describing the problem and maybe they'll suggest something.


I guess I will review my options after (if?) I hear back from them. Tamron seems like an interesting solution. A few of my Nikon friends use it and are quite happy with it.
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[Sheiky]

Unfortunately the lens is way after the warranty period. But I did fire an email to Sigma describing the problem and maybe they'll suggest something.

I'm curious if they have a cure. You are aware that Sigma offers like 4-5 years warranty? Not the 1 year Canon standard.

I guess I will review my options after (if?) I hear back from them. Tamron seems like an interesting solution. A few of my Nikon friends use it and are quite happy with it.

It's truly a great lens, I enjoyed my copy at the time. However perhaps your lens is still fixable or something. Let's not give up hope so soon []