Sigma 10-20 3.5 AF infinity

[Peter69]

I just bought a Sigma 10-20 and found although it focuses on items it doesn't focus on an empty sky.

Is this normal for this lens?

Also there is a mark on the lens. it can only be seen when you breath on it. Do I need to worry about it? It won't clean off.

 

[minnnt]

What is it meant to focus on if the sky is empty??

 

[viewfromthenorth]

Lenses tend to need an area of contrast to focus on, such as an edge, an empty skies don't normally have too many of those.

Have a read of this http://www.cambridgeincolour.com/tutorials/camera-autofocus.htm

Regards the mark - does it show up on any pictures? If not don't worry about it.

 

[Joeturner11]

I don't any lens that will AF on a empty sky or will at least struggle! Try focusing on something distant like a tree at the end of the garden or something, and as Andy says if its a little mark and doesn't show on the pics don't worry about it

 

[Peter69]

Thanks guys.

The mark is not visible on pictures (I tried clear sky with various exposures.) The lens smells like grandmas sock drawer as does the case. I thought it maybe the beginnings of mould.

 

[pingu666]

some lens/camera default to infinity or guess at infity if they cant lock..