The technique of recording the image of an object on a photosensitive film with the help of a camera is called photography. In the beginning, metal or glass plates were used to records the images. Plastic roll films were developed in 1889. In the middle of the nineteenth century, several other chemical methods were invented for developing images in addition to Daguerretutype and collotype negative-positive methods. The negative-positive method is in the even today.
All the cameras have a lens by which an inverted image of the scene is formed on the photo film. A photosensitive layer of silver bromide is coated on photo film. This is called exposing. After exposing, the film is developed in chemicals and then fixed. In this way, a permanent negative of a scene is made on the film. From this negative, prints of required size can be made on bromide paper using an enlarger.
In colored photography, two types of films are used. The first one is color reversal film, on which color positive, slide or transparencies are made. The other one is color negative film, which is used of making color prints. Both of these films are sensitive to the three primary colors i.e. blue, green and red. In one color film, we have three layers of photo sensitive emulsion. The first layer is sensitive to blue color, second for green and third for red. Each emulsion layer absorbs or subtracts a definite amount of light. This is called subtractive process. The colored films are developed in a special developer and prints of desired size are made from them.