09-04-2013, 02:42 AM
dpreview's "typical example" is a low contrast target (a white eye on a white statue) which is also tiny relative to the scene. I don't think that this is a good example.
On the other hand, like a lot of wide-angle autofocus lenses the focus throw on this lens is uselessly short. From 2m to Infinity is only a degree or so of focus-throw: http://www.dpreview.com/lensreviews/sigm...ndview.jpg. The dock or calibration won't help autofocus much in this range if the focus throw is so short!
It's an inherent flaw in the Sigma and a lot of lenses and has little to do with AF accuracy. Anything in this range is basically an autofocus crap-shoot or just defaults to infinity.
My Pentax DA14mm, DA15mm both had/have a frustrating short throw close to infinity, as did my Sigma 28/1.8. I have had trouble focusing them accurately even manually sometimes in that rather large range from ~2m to Infinity. My Pentax 18-135 also does not autofocus accurately at wide-angle. With all these lenses, it's noticeable at f/2.8~f/4 and only gets worse at f/1.8.
(Weirdly, my Zeiss 25mm Distagon also has a stupid AF-like short throw from 2m to Infinity, despite having 360° of focus throw in it's range but never being available with AF).
On the other hand, like a lot of wide-angle autofocus lenses the focus throw on this lens is uselessly short. From 2m to Infinity is only a degree or so of focus-throw: http://www.dpreview.com/lensreviews/sigm...ndview.jpg. The dock or calibration won't help autofocus much in this range if the focus throw is so short!
It's an inherent flaw in the Sigma and a lot of lenses and has little to do with AF accuracy. Anything in this range is basically an autofocus crap-shoot or just defaults to infinity.
My Pentax DA14mm, DA15mm both had/have a frustrating short throw close to infinity, as did my Sigma 28/1.8. I have had trouble focusing them accurately even manually sometimes in that rather large range from ~2m to Infinity. My Pentax 18-135 also does not autofocus accurately at wide-angle. With all these lenses, it's noticeable at f/2.8~f/4 and only gets worse at f/1.8.
(Weirdly, my Zeiss 25mm Distagon also has a stupid AF-like short throw from 2m to Infinity, despite having 360° of focus throw in it's range but never being available with AF).