Resolution: The ability of an imaging system to register separate images of two closelysituated objects.

Unsharpness: The ability of an imaging system to reproduce a sharp edge.

The resolving power of the radiological image depends on the combined effect of different kinds of unsharpness (see below), the number of X-ray photons to produce the image and the contrast. In other words, the resolving power depends on the MTF of the imaging system, the quantum noise and the contrast.

The resolution of the imaging system is usually measured using a test object with alternating transparent and absorbing lines. The resolution power is then usually given in line pairs per millimeter (lp/mm), i.e., gives the number of "tops" and "valleys" that can be detected per millimeter.