Back focusing. Lens or body?

[Bigvin]

As the title, I've just returned to photography after work got in the way.

All of my shots appear to be back focusing by about 12-18 inches.
I fine tune the lens to -20 and any images taken within about 15 feet seem OK.
Any images shot further away the back focusing creeps back in.

How can I tell if it's my body or the lens to blame? My suspicions are the lens.
I'm using a Nikon D300S and a Sigma 120-300 2.8.

Cheers

 

[mart77]

try a different lens, any lens, if thats ok then its the lens. it will be the lens, sigma lenses do this often.

 

[pingu666]

ive heard of sigma lenses doing this before , not sure why, but u could send it into sigma to be fixed/checked

 

[Bigvin]

Tried an 18-70 the other day and it was OK.
Just tried it again and it was back focusing.
Took lens off and on again and it was OK for a few shots then seemed to be back focusing again.

I take it this indicates the camera being NG if it fluctuates between working and not working?

 

[Bigvin]

Both lenses seem to be working fine, for now!!!!!
Excuse to buy a new camera.