THE FORMATION OF ANALOG AND DIGITAL IMAGES

Les images photographiques actuelles peuvent être de nature analogique ou de nature numérique. La formation des images analogiques repose sur le principe du noircissement à la lumière de composés photosensibles (argentiques ou non argentiques) appliqués sur un support. L’image obtenue peut être négative ou positive. Les tirages argentiques sont obtenus soit par noircissement direct à la lumière, soit par développement d’une image latente.

ANALOG PHOTOGRAPHY

In traditional, analog photography an image can be recorded with or without a camera or a lens. Examples of the ways photographs can be made without a camera are by placing an object or chemicals directly on a photosensitive surface (photogenic drawing, photogram, chemigram). Photographs are taken without a lens when pinhole techniques are used.

Negative
A negative is a matrix used to print positive images. The visual content is reversed from left to right, and the light areas of the negative correspond to the dark areas of the subject photographed and vice versa.

Direct positive
A direct positive is an image created directly on the light-sensitive surface without using a negative. Like a negative, in a sense, a direct positive is a unique image, not a multiple. The daguerreotype and the ferrotype are examples of direct positives, but despite a common misconception the ambrotype is not: it is a collodion-on-glass negative that is placed on a black background to invert its values and turn it into a positive image.

Printing-out prints
Printing-out prints are made using a solution of sodium chloride and an excess of silver nitrate. Mostly employed in the nineteenth century, these processes weren’t very sensitive to light and were therefore essentially used for contact printing—thus providing prints that were the same size as the negative. The image had warm tones and a very fine grain. Examples of printing-out processes are salt prints, albumen prints and Aristotypes (using gelatin chloride or collodion chloride).

Developing-out prints
Developing-out processes are more sensitive to light than printing-out processes. They became common starting at the end of the nineteenth century. The latent image produced by exposure to light is made visible in a development bath, then fixed. The prints obtained in this way have neutral black colors that can be influenced by toning. Examples of developing-out processes are Blanquart-Evrard’s silver iodide technique, gelatin silver chloride papers, gelatin silver bromide or chlorobromide papers, dye diffusion instant photography processes like Polaroid, chromogenic prints, dye destruction prints, and dye-transfer prints.

DIGITAL PHOTOGRAPHY

Taking digital pictures
In digital photography, the traditional photochemical surface is replaced by a photosensitive electronic component that translates light into an electric signal. The signal is converted into a series of numeric (or digital) values recorded on the memory card in the camera to create an image file.

The electronic sensor inside a digital camera is in fact a grid of individual photosensors sensitive to the entire spectrum of visible light. Color is recorded thanks to tricolor techniques, using a screen of colored filters: each photosensor is associated with a filter of a single color so that it only receives information regarding one of three primaries—red (R), green (G) or blue (B). The camera or software for processing the original “Raw” files renders the colors of the subject by electronically converting the color data recorded by each group of RGB photosensors.

The total surface area and number of individual photosensors composing a camera’s electronic sensor are crucial to the quality of the images recorded. The “pixel” (a contraction of “picture element”) is the smallest element by which a digital image’s definition can be measured. It is also the unit used to describe the definition of digital sensors and of image display screens.

Digital prints
Digital images can be rendered using many types of media. They include prints on various materials (paper, fabric, metal, etc.) using an ink-jet or a thermal printer. It is also possible to employ traditional photosensitive processes (gelatin silver, chromogenic, dye destruction, etc.) thanks to digital enlargers such as the Durst Lambda, the Océ Lightjet or the Fuji Frontier. The images produced in this way are referred to as digital prints, and their authors often specify the brand of the enlarger used (“Lambda print,” “Lightjet print,” etc.). Another possibility is to print the image on a transparent surface to obtain a negative that can be used to make photographic contact prints. The whole range of historical processes then becomes available, contributing to the current revival of many alternative processes.

Digital prints
Digital images can be rendered using many types of media. They include prints on various materials (paper, fabric, metal, etc.) using an ink-jet or a thermal printer. It is also possible to employ traditional photosensitive processes (gelatin silver, chromogenic, dye destruction, etc.) thanks to digital enlargers such as the Durst Lambda, the Océ Lightjet or the Fuji Frontier. The images produced in this way are referred to as digital prints, and their authors often specify the brand of the enlarger used (“Lambda print,” “Lightjet print,” etc.). Another possibility is to print the image on a transparent surface to obtain a negative that can be used to make photographic contact prints. The whole range of historical processes then becomes available, contributing to the current revival of many alternative processes.

Images from the top:
Tirage à l’huile (oléotypie) sur papier aquarelle d’après négatif numérique jet d’encre sur support polyester © Constance Asseman / 2015

Négatif numérique sur support polyester lumière transmise sur table lumineuse © ARCP/Mairie de Paris, Constance Asseman / 2015

Impression d’un négatif numérique jet d’encre sur support polyester © ARCP/Mairie de Paris, Constance Asseman / 2015

GLOSSARY

CHROMOGENIC PRINTS

DYE DESTRUCTION PRINT

CARBON PRINT

AUTOCHROME

PHOTOGENIC DRAWING

TINTYPE

GELATIN SILVER BROMIDE PRINT

AMBROTYPE

ARISTOTYPE

GELATIN SILVER CHLORIDE PRINT

PLATINOTYPE

DYE DIFFUSION PRINT

PRINT

FRESSON QUADRICHROMY PRINT

PHOTOCHROME

SALTED PAPER PRINT

NEGATIVE PAPER

PIGMENT INKJET PRINTS

CYANOTYPE

GUM BICHROMATE PRINT

THE FORMATION OF ANALOG AND DIGITAL IMAGES