So I've had my Sigma APO 120-400mm f4.5-5.6 DG OS HSM for 6months now after purchasing it from an online vendor. I shoot mostly motorsport and it has been my firm favourite lens ever since I received it. The images were nice and crisp at all focal lengths and I was generally really happy with how well it focussed exactly where I needed it to.
I went away for a couple of weeks and when I tried to shoot some motorbike racing on the weekend I had incredible difficulty getting a sharp focus on any of the bikes whilst panning. At first I thought I was just rusty and persevered with it, but to no avail. Eventually I tested on a stationary subject and this is the result:
Crop of above image:
This image was taken at 400mm, f5.6, 1/500, ISO 100 using the central focus point in AF mode. I've had a look at both ends of the lens and it doesn't appear to be an issue with dirt on the lens. I've tested my 24-70L which seems to be fine so I'm pretty sure it's a problem with the lens itself.
Does anyone have any suggestions as to what tests I could run to isolate the problem? Or should I just send it back and get it fixed? It looks like it's front-focussing to me, but I have no idea what could have caused it to do that so suddenly! (I treat all my gear like it's made of crystal)
I am so gutted!