The Formation of Analog and Digital Images

Printing-out prints

Printing-out prints are made using a solution of sodium chloride and an excess of silver nitrate. Mostly employed in the 19^th century, these processes weren’t very sensitive to light and were therefore essentially used for contact printing: the paper is brought into contact with the negative in a press frame, then exposed to sunlight. The dimensions of the prints produced are therefore identical to those of the negative. The image has warm tones and a very fine grain. Examples of printing-out processes are salt prints, albumen prints and aristotypes.

Developing-out prints

Developing-out papers are more sensitive to light than printing-out papers. They became common by the end of the 19^th century. The latent image produced by exposing the photosensitive paper to the light of an enlarger is revealed in a developing bath, before being fixed. The resulting prints have neutral black tones that are warmer or cooler depending on the reagent used, and. can be modified 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® and Fuji Instax®, chromogenic slides and prints, dye destruction prints (Cibachrome® or Ilfochrome®).

Digital Photography

The formation of digital photographic images is based on the principle of photosensitive sensors, which transform the optical image formed in the camera (shapes, colours and light values) into electrical impulses coded in a computer file as a sequence of numbers associated with quantities (hence the expression ‘digital’ in English). Unlike analog photography, there is no direct resemblance between the subject photographed and its computer coding. The storage media for these files can be electronic (flash cards), optical (CDs) or magnetic (hard drive).

Taking digital pictures

The traditional photochemical medium is replaced here by a photosensitive electronic component capable of converting light into an electrical signal. This signal is converted into a digital value saved on a memory card inside the camera to form the computer file. The sensors inside a digital camera are a grid of individual photosensors sensitive to the entire spectrum of visible light. Colour recording, based on the principle of trichromy (RGB system), is carried out using a network of colour filters: each photosensor is associated with a single colour filter so that it only receives information regarding one of three primaries optical colours —red (R), green (G) or blue (B). The camera, or a software for processing the original “RAW” files, renders the colours of the subject by electronically converting the colour 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” (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.

Proof and print

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. The computer file can also be printed on traditional photosensitive paper, gelatin silver, chromogenic, dye destruction, thanks to digital enlargers such as the Durst Lambda®, Océ Lightjet® or Fuji Frontier®. The images produced in this way are often referred by their authors to 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, for example by using traditional processes. The whole range of historical processes then becomes available, contributing to the current revival of many alternative processes.

Glossary