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Since film development is becoming a fashion again, I decided to write some posts on this topic. But I warn you beforehand: this is a quite complex topic, and it can take many (intense) hours from your time...

 

In this first post I will try to describe what is a film development process without using technical words or strange words. This will remain for the second post, only for the case that you found this interesting...

The film

Photographic film consists of three main parts: a passive support (transparent plastic ribbon), a gelatin layer and the chemical compounds (salts) that will react to light. In case of colour film (either negative or dia), there may be several salts layers, each one sensitive to a specific colour.

 

The gelatin (also transparent) holds the salts inside, in suspension, uniformly distributed. Those salts have two main characteristics: the chemical composition (responsible of their behaviour) and the size of each salt "grain". Typically, the total amoount of salts in the different films is quite similar.

Capturing

Salts in the film are sensitive to light; this means that when light photns strike the salt grains, those become "active", changing their state slightly. The more light is cast over the film, the more salt grains will be activated.

 

An important point here, salt grains react to the very impact of photons. This impact is random, quite easy at the beginning, since all grains are available. But, as time passes, it becomes more difficult for a photon to hit a "fresh" grain, so film response to light changes and it becomes less sensitive.

 

Technically, the film response is called "logarithmic", and it is the reason why you need to allow for double the light to obtain just one mre step illumination in your photography...

Processing

Once the film has been exposed to light (with active salt areas), it goes through a chemical bath that reacts with those active salts, making them grow and become opaque, visible. At the same time, the chemical process may remove some of the inactive salt contents.

 

The result is the processed film, already. But it remains sensitive to light. At this point, it may not be exposed to (ambient) light again. To stop this sensitivity, a second chemical bath is applied, called "stop bath". This bath blocks the light sensitivity of the salts, both active and inactive.

 

Finally, there is a third step, a wash with water, to remove all rests of chemical products (developer and stop bath), in order to allow for the film to remain unaltered for years. Now you have the final negative, you just need to let it dry...

Positive

The transfer of the negative to photo paper is similar to the film processing. With a special equipment, a focused image of the negative is projected onto the paper, which is also light-sensitive.

 

Paper is developed in a similar process (development, fix, wash), and it must also dry to get the final picture.

 

In later posts to this thread, I will work further on the above topics, step by step, introducing technical terms and more complexity little by little...

 

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Tag(s) : #Photography