Posterization is a photographic and image processing technique that reduces the continuous, smoothly graduated tonal scale of a photographic image to a limited number of discrete, flat tonal steps or zones, replacing the subtle, imperceptible transitions between adjacent tones that characterise a conventional photographic image with abrupt, clearly defined boundaries between a small number of uniform tone areas. The resulting image has a graphic, bold, and simplified visual character reminiscent of a screen printed poster or lithographic print - an aesthetic quality that gives the technique its name and that has been widely exploited for creative and commercial effect in fine art, advertising, and editorial photography.
In traditional darkroom practice, posterization is achieved through a multi-stage process involving the production of tone separated negatives - a series of high contrast lith film negatives each recording a different tonal zone of the original continuous tone image. To produce a set of tone separated negatives, the original negative or transparency is contact printed or enlarged onto high contrast lith film at a series of different exposure levels, each exposure level causing a different range of tones from the original to be recorded as either solid black or clear on the resulting lith film separation. A short exposure records only the densest shadow areas of the original as black, leaving all lighter tones clear. A medium exposure records the mid-tones as black in addition to the shadows. A longer exposure records highlights, mid-tones, and shadows alike as black, leaving only the brightest highlight areas clear. Each of the resulting lith film separations therefore contains a different subset of the original image's tonal information, represented as a simple black and clear binary image with no intermediate tones.
Once a set of tone separated negatives has been produced, they are printed sequentially in careful register onto a single sheet of photographic paper or lith film, with each separation contributing its specific tonal zone to the composite image. The number of separations used determines the number of discrete tonal steps in the final posterized image - a three separation posterization produces an image with three distinct tonal zones, while four or five separations produce a more complex but still graphically simplified tonal structure. By using different coloured inks, dyes, or printing papers for each separation, multi-coloured posterizations can be produced in which each tonal zone is rendered in a different colour, creating richly graphic composite images with bold, flat colour areas separated by sharp tonal boundaries.
The registration of the multiple separations during printing is a critical and technically demanding aspect of the traditional darkroom posterization process, as any misalignment between the separations will result in tonal boundaries that do not coincide correctly, producing fringing, halos, or double edge artefacts at the transitions between tonal zones that compromise the clean, graphic quality of the finished image. Specialist registration pin systems and carefully designed registration carriers for the enlarger are used to ensure precise alignment between all separation negatives throughout the printing process.
Posterization is sometimes incorrectly referred to as solarization in popular usage, reflecting a confusion between two distinct but superficially similar graphic photographic effects. Solarization - more precisely known as the Sabattier effect - is produced by briefly re-exposing a partially developed photographic image to light during development, causing a partial reversal of tones in the affected areas and the formation of characteristic Mackie lines at tonal boundaries, resulting in an image that combines both positive and negative tonal areas in a way that has some visual similarities to posterization but is produced by an entirely different mechanism and has a distinctly different aesthetic character. The two techniques should not be confused or conflated, as they involve quite different technical processes and produce results with their own distinctive visual qualities.
In digital image editing, posterization is achieved with great ease and precision using dedicated posterization tools available in applications such as Adobe Photoshop, which allow the number of tonal steps to be set to any desired value and applied to the image immediately with complete control and reversibility. Digital posterization can be applied selectively to individual colour channels as well as to the overall luminosity of the image, allowing sophisticated multi-coloured posterization effects to be achieved that would require complex and time consuming multi-stage processes in traditional darkroom practice. The immediacy, controllability, and reversibility of digital posterization have made it a widely accessible creative tool, though the laborious and craft intensive nature of traditional darkroom posterization continues to be appreciated by photographers who value the distinctive aesthetic qualities and the creative engagement of the traditional process.