Confused

A new toy arrived today - a fisheye lens filter. It's confused me. When I put the filter on, the lens won't focus. The focal point is somewhere beyond the infinity point. I feel like I should know the technical reason for this, but I can't think! Even setting the camera at its narrowest aperture didn't help, and nothing was sharp - putting any foreground interest in the frame would be pointless. Example here - thought the composition worked, but that's it.

So, the toy will have to go back. It won't work with my lens. I ordered two different types, so I've got another one on the way - hopefully it will be better!

Incidentally, I finished work today for seven days off. Great stuff!