So I've been reading through some threads on here and it has become apparent that my Sigma 70-200mm is suffering from back focus (I think). More often than not, my focus object is not in focus, but objects behind it are. For example, I have noticed it when taking a shot of a bird - the bird is blurred but the feeder it is sitting on (which is behind it) is not. Initially, I thought it was me just being a noob and I'd tried different things such as backing off the focal length and closing the aperture, but nothing really seemed to make a difference. It is more noticeable on smaller objects. I was beginning to think the lens was dodgy, I hope not. I have read about calibration, but my Nikon D5200 doesn't have an AF adjustment option. So, how do I correct this?? Thanks.