# Final image render produces blurrier result than 3D Viewport's render

I'm using Cycles and finding that the image I see in the 3D Viewport (when using rendered shading) is far crisper than the final render produced with Render Image (F12). I can't work out why.

First, the render as seen in the 3D Viewport (you can see the 3D Cursor) etc.:

And now the dramatically blurrier Render Image result:

I'm guessing this bluriness is somehow supposed to be truer? I.e. the crispness seen in the 3D Viewport is a result of not being such a true renderer, i.e. various short cuts (in addition to fewer passes) are at play here. Other differences, I can see are e.g. that the final render includes a definite shadow (that for some reason is not visible in the 3D Viewport view).

I've looked at the Compositor and Shader nodes and I see no obvious reason for the bluriness.

This sphere has been added into an environment created by tracking. I.e. I shot the background footage, solved for the camera motion and then let Blender set up the tracking scene which resulted in a cube (which I replace with a reflective sphere), a shadow-catchter plane and the following Compositor nodes:

And then I added my own HDRI (which you see reflected in the sphere) with the following Shader nodes:

The .hdr file, the .blend file and the background .mkv can be found here:

Any insight into what's going on here would be much appreciated.