PHOTOGRAPHY-41

 PHOTOGRAPHY-41

FILM PROCESSING

To be honest, the most important step in photography for image quality is ‘film processing’. Often people tend to use the term “Developing” as synonymous with ‘PROCESSING’. But, the truth is, developing is just a part of the process as ‘developing’ alone cannot render a stable image on tha film/ paper. Basic B&W film processing includes three essential steps                      1] developing 2 ] neutralizing [stop bath]and 3] fixing.

Each step renders the film fit for the next stage in film processing.

PRINCIPLE: A photo image  is created by “play of light’ on a film /paper. The image so formed arises from the light reflected off an object. Such reflected light rays are trapped by light sensitive silverhalides distributed as fine coat over a base transparent [film] or opaque [ paper].  These ‘halides’ are quite sensitive to light and they trap the light reflected on to them from the object. The trapped light is ‘arrested’ on the sensitive surface as LATENT IMAGE[ NOT VISIBLE TO THE EYE]. It is ‘film processing’ that confers visibility on to the latent matter. In other words , the invisible image turns visible when the light affected silver halides are converted to metallic silver through ‘developing’ step of the film processing. Chemists call this conversion as  ‘reduction’. SILVER HALIDES TURN METALLIC SILVER UPON REDUCTION and so the developer chemical is a reducing agent. Since every reduction involves oxidation , the chemical gets oxidized after each use   and so has a limited life.

THE DEVELOPER [THE DEVELOPING AGENT]

Developing should be carried to a limit so that the metallic silver formed is proportional to the extent of ‘exposure’ received on the film or paper. This means a degree of control must be kept over the speed of the reaction by a developer. Such control is provided by Sodium bromide as it binds the reducing agent and in effect the developer acts on light affected silver halides proportionate to the extent of ‘exposure’ received by such silver halides.

That the developing agent gets oxidized as it reduces the light affected halides to metallic silver, places a demand on the developing agent to stand protected against oxygen [aerial oxidation].So an agent to rapidly quench oxygen in the medium is used [Sodium sulphite]. But then the activity of developer requires alkaline pH; Borax[ Sodium borate [mild] Sodium carbonate [more alkaline]orSodium hydroxide / Potassium hydroxide [strong alkali] is employed

With these adjuvants,  the film developer is actually a team in which the function of converting halide to metal is done by a twin action provided by METOL [Mono-methyl-p-aminophenol sulphate] an QUINOL  [Hydroquinone] Sometimes in place of Metol , PHENIDONE [1-Phenyl, 3-pyrazolidinone] is used . Accordingly, developers are either MQ or PQ by constitution.

Thus , a developer has a] Developing agents, b] Preservative  [Anti-oxidizer], c] Alkali, d] Restrainer as a functional team in a liquid [Distilled Water].

To stop developer actionsoon after the time limit,  a mild acidic bath 2-3%|Acetic acid ] is used as STOP BATH and ultimately to render the  image permanent  FIXER [Sodium thiosulphate [Hypo]] isused and is repeatedly used for several batches .

More to follow

K.Raman