The chemical composition of the photo

The correctly exposed film is different from the unexposed film only in atomic state, that is, the former has undergone slight chemical changes, forming an invisible image or "latent image". In the subsequent development process, the developed drug must act on the film in the dark to enhance the latent image into a more realistic image that persists under ordinary light. These chemicals are all used in liquid form, and each cleaning solution has a special effect on the membrane suitable for it. For example, for most black-and-white films, the first chemical liquid (developer) will develop the illuminated area into black silver particles. The second liquid medicine (fixing solution) used later can dissolve (fix) the silver halide in the unexposed part, leaving a transparent film. Therefore, chemical by-products are washed away, and the washing result obtained after drying is a black-and-white negative.

The process of developing color film into color negative is similar to the above, but the chemical composition is more complicated. During the development of color film, the developer forms dyes in three emulsion layers of yellow, magenta and cyan, including black silver. Then the silver is removed and fixed, leaving a color picture composed of only three layers of dyes, which is complementary to the color of the subject. Color slides need more developing steps. First, a black-and-white negative developer is used, and then the film (usually fixed) is developed in color to produce a positive image of silver and dye. After all the silver is removed and fixed, what remains is a color slide with a positive dye image.

print

The next production step is printing, or more generally enlarging. The film of the existing picture is installed in a vertical projector called a magnifying glass, and the lens of the magnifier can be imaged on photographic paper, and its size can be almost arbitrarily selected as needed. During exposure, the amount of light shining on photographic paper through transparent areas on the film is more than that through dense areas. The latent image recorded on the photographic paper should also be washed away by the subsequent chemical liquid.

Generally speaking, these development steps are similar to those required when developing films. For example, black-and-white photographic paper is exposed on a black-and-white negative, then developed, fixed and washed, and the "negative of the negative" will appear. This is the positive image, which is the familiar black-and-white photo. After color paper exposure, a series of developing steps, such as color development, bleaching and fixing, can form color negative and positive films on color negative. In addition, color photos can also be obtained from color slides and some other photosensitive materials and the development process.

The important feature of printing (besides changing the image size and printing multiple copies) is that you can correct or control the picture from beginning to end. Unnecessary parts near the edge can be cut off, and the tone of the selected area can be lightened or deepened. For color printing, you can use a magnifying glass filter with a wide working range to fine-tune the color balance of photos or produce some effects. In addition, according to experience, some parts of several films can be combined into a photo to form a picture with some positive films and some negative films, and so on. Flowing instead of sodium sulfate and sodium bicarbonate