Dan: the centre being brighter than the rest can be attributed to vignetting. However, the vignetting on this photo isn’t from a lens or anything like that, I did it digitally in Photoshop when converting it from raw. There is a little slider that is used for correcting vignetting, but you can use it backwards to add vignetting.

Alternatively, you could do it in Photoshop using an adjustment layer and a radial layer mask.