Sigma 70-200 2.8 Focus Problem

[Andy The Gardene]

Evening all,

A few months ago I bought a Sigma 70-200 2.7 APO DG Macro HSM off ebay. I have noticed in the past a little bit that the focusing may be slightly off but then some, more than 50% of the shots were sharp etc.

But, today I went to a motocross track and have thought the focusing may be off again. I have the camera set on continuous focus, dynamic focus point and spot metering and yet it doesn't seem to nail the focus where I tell it to. Still though, it only tends to do it 50% of the time, some are spot on.

I have uploaded a picture of 2 riders and put a black dot where the focus point was, and as you'l agree it has focused further back than where i wanted it to (easier to see on the larger image if you click the link).



Uploaded with ImageShack.us

This was shot with a D5000 at 170mm, 1/800s F4 +0.3EV and ISO 400. Oh, and its straight out the camera other than being reduced 50% in size.

So, if i've made sense of that (im not great at putting what I mean into writing), does anyone know what this could be and presumably that if it needs to go in somewhere to be fixed, where could this be done (Im in Wigan, Lancashire).

Many thanks.
Andy.

 

[scottthehat]

sorry to tell you this but stuff like this you should use single point focus as there is to much stuff in the pic to confuse the af system, as you can see the 2nd rider is pin sharp.

 

[Andy The Gardene]

I've tried both, single point and the dynamic point focus and it still throws out shots like this. Switched to my basic Sigma 70-300 to check, and they were all focusing correctly.

 

[scottthehat]

well you should use single center point focus, and the lens sshould be plenty fast enough to manage with the speed,

 

[scottthehat]

also i wouldnt use spot metering for this kind of subject, use matrix.

 

[Andy The Gardene]

You'll have to forgive me if im getting confused, but, if i've got the following set

Focus Mode: Continuous AF
AF Area Mode: Single Point

Then I move the selected point to say top right the camera/lens should focus on the top right. But, it isn't on 50% of the shots, its focusing in the wrong place.

As I say, I took off the 70-200 and put on my 70-300 and used exactly the same settings and they were all bang on. So im presuming its a focusing error with the lens, and not my body or not my technique/settings.

 

[scottthehat]

yes afc is good and single point personally i would just use the center focus point.
what the lens like on stationory objects, using single focus mode afs

 

[Andy The Gardene]

On stationary (and slow moving) stuff it seems spot on. Ive only ever noticed a problem when it comes to photographing motosports. Although, i'll do a decent 'test' tomorrow night just to be sure with a tripod etc.

 

[scottthehat]

well i would be interested to see some test pics, and the lens should be fast enough to cope with the situation and should be better than your 70-300mm by miles.