All down to aperture on the lens. Basically, the smaller the f-number, the shorter the "depth of field" and therefore higher blurring of anything out of focus. The big boys (Grand Fromage included) have some truly super lenses (these big chaps that are about an arms length!) which can produce really pin sharp objects whilst the background looks super blurry.
My utility lens is great in the pit lane at this but when out trackside (and hence telephoto at full stretch), I struggle to really blur the background with head on stuff. Obviously no problem when panning as the blurring is done by a longer shutter speed (I use anything from 1/80 on the slower corners such as Arnage to around 1/125 at other parts of the track).
Below is an example of good depth of field in the pit lane.
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