Penumbra (fuzziness or blurry images) is lack of sharpness of the film. It is a fuzzy, unclear area that surrounds a radiographic image and is affected by focal spot size(smaller the better), film composition(larger the size of crystals less sharp the image), and movement during the exposure.
Why is contrast important? A larger penumbra equals more unsharpness on the image and detail that the physician cannot to make a proper diagnosis of the affected organ.
The penumbra blur is related to the focal spot and magnification by using the following formula:
$ P = f ( m-1) $