Digital photography

I bought a lens from someone on here a few months ago but until now I've not really had much of an opportunity to play around with it. However now I'm looking at the pictures it seems that at shallow DoF the pictures are consistently slightly out of focus.

For example here:

Full size here, f1.8, 1/200, ISO 100, Flash.

In this case I used auto focus and a single focusing point directed at the subject's left eye. As you can see however, the eyes are not sharp while the plane of focus is a couple of cms further away from the camera. I haven't yet had an opportunity take photos of a ruler with the camera on a tripod but the same effect seems present whenever I shoot with a shallow DoF, and I've never noticed it with any of my other lenses.

What are the likely sources of this error and what can I do to try and diagnose the problem?

My first suspicion is whether I've simply bought an incompatible lens, the camera body is DX while I believe the lens is FX compatible (Camera: Nikon D7000, lens: Nikon AF NIKKOR 50mm 1.8D). Might this be the problem?

Alternatively, is the fault more likely to lie in the auto-focus mechanism of the camera body or somewhere in the lens and how do I find out?

I should add that I've not yet discussed this with the seller (although they may read this) and I'm not accusing them of selling me a dud (at least not intentionally), but rather I'm just trying to identify whether a problem exists and whether or not it's caused by my own ineptitude.

Hi Ewan. You bought that from me! I never had a problem with that or any fast lens which wasn't user error. At f/1.8 the dof is very thin, so even a small movement in the subject puts the desired in-focus area out just slightly. Of course, you know all this already.

How are you focusing? Do you know the back button focus/ AF-C technique? I stopped using the shutter button to focus years ago as I found it a much more reliable to be in continuous AF using the AF-ON button, using the shutter button only to trip the shutter. On a D7000 I don't think you have a dedicated AF-ON button, but you can configure the AE/AF lock button to act this way. For portraits using a fast lens wide open, that would be my choice of focusing.

As someone else said, there is also a possibility that the lens is slightly front- or back-focussing with your camera body; you can adjust for this, but I think it is much more likely to be your technique (not a criticism!).