Contrast in Film

INTRODUCTION AND OVERVIEW

CONTENTS

Contrast is perhaps the most significant characteristic of an image recorded on film. Contrast is the variation in film density (shades of gray) that actually forms the image. Without contrast there is no image. The amount of contrast in an image depends on a number of factors, including the ability of the particular film to record contrast.    Film can be considered as a contrast converter. One of its functions is to convert differences in exposure (subject contrast) into film contrast (differences in density), as shown below. The amount of film contrast resulting from a specific exposure difference can vary considerably. The General Relationship between Exposure Contrast and Film Contrast    The exposure contrast between two areas can be expressed as a ratio or percentage difference, as illustrated above. The film contrast between two areas is expressed as the difference between the density values. The ability of the film to convert exposure contrast into film contrast can be expressed in terms of the contrast factor. The value of the contrast factor is the amount of film contrast resulting from an exposure contrast of 50%. The amount of contrast produced by medical imaging films depends on four basic factors: (1) type of emulsion, (2) amount of exposure, (3) processing, (4) fog.    In this section we consider the basic contrast characteristics of film, how these characteristics are affected by the factors listed above, and how contrast characteristics relate to clinical applications.