Most modern black and white prints are gelatin silver prints - where silver halide salt crystals are suspended in a thin gelatin layer both film and paper. These crystals are the reason why some images can appear grainy larger crystals, higher film ISO , more light sensitivity. Daguerreotypes don't have this issue because the surface of the plate is pure and light exposure is decided by each individual molecule as opposed to a whole grouping of molecules.
After the daguerreotype, countless new methods for capturing and preserving images came to light: salted paper prints, Wet plates, dry plates, cyanotypes, albumen prints and gum bichromate prints just to name a few.
The gum bichromate process also known as the gum dichromate process is actually pretty simple to perform. The importance of these imperfections is quite astounding as they are the key to the image forming process on film. Let us see how the latent image is formed in the following series of illustrations.
The admitted theory reckons 3 steps in the forming process of this very distinctive latent image. The first step of the latent image forming process is the exposure. In doing so it kicks out the supplementary electron of the large Bromide ion. During the second phase of the latent image forming process, the kicked out electron, which has too much energy to be captured by the Silver ion, goes on to roam around the entire crystal.
And so until being trapped by an imperfection, a local defect in the crystal structure, or by an impurity in the form of a foreign atom or molecule. Then after several photons have stricken the crystal and kicked out electrons of their Bromide ion, an accumulation of those electrons is created around this local imperfection or impurity. In the third step, this local accumulation of electrons generate a negative electric field that can attract towards itself the positive Silver ions not well set in the crystal structure.
Each positive silver ion combining with an electron gives birth to a metallic Silver atom. We rapidly end-up with a small gathering of metallic Silver atoms which correspond to a development seed. All crystals in the emulsion containing a development seed, form together what is called the latent image. At macroscopic scale this image is not sufficiently contrasted to be usable as is.
After all there is very little difference between an exposed crystal that contains a development seed and an unexposed crystal that does not contain such a seed i. This is where the developer agent function comes handy: it amplifies the difference between exposed crystals and non exposed ones by transforming the entire exposed crystal into metallic Silver just from the seed presence. The development seed acts as catalyst of the transformation.
The thought that the developer agent acts only on the Silver Halide crystal containing a development seed is only true during a certain time. In reality if we leave the emulsion in contact with the developer agent long enough, it will transform all crystals in the emulsion.
The developer acts more on the difference in the reduction speed of Silver contained in the exposed crystals versus Silver contained in the unexposed ones. The still wet paper was then exposed in a camera until a dark silver image appeared in the light-struck regions, and the remaining silver chloride was removed by washing with a concentrated salt solution or a sodium thiosulfate solution.
By waxing or oiling the negative sheet, Talbot made the paper transparent, and then by making an exposure of diffuse light through the negative onto another sensitized sheet, he produced a positive image. An unlimited number of copies of a photograph could thus be made from any one negative. Both Daguerre's and Talbot's methods were inconvenient because they required long exposures in the camera—sometimes as long as 60 minutes.
In Talbot greatly improved his process. The latent image was then "developed" into a visible image by treatment with a solution of gallic acid and silver nitrate. The main difference between Talbot's process and modern photographic practice is that now the silver halide, in the form of approximately micron-sized crystals or "grains," is suspended in gelatin. The gelatin emulsion is coated as a thin film on glass plates or flexible sheets of plastic or paper.
When a photon is absorbed by a silver halide grain, an electron is ejected from a halide ion and temporarily held at some site in the crystal. A silver ion can migrate to the site and combine with the electron to form a silver atom. The atom is not stable; it can decompose back into a silver ion and a free electron. However, during its lifetime, the atom can trap a second electron if one becomes available.
If this second electron remains trapped until the arrival of a second silver ion, a two-atom cluster forms. This buildup of a silver cluster can continue as long as photoelectrons are available. The smallest cluster corresponding to a stable latent image speck is believed to consist of three or four silver atoms. Specks of this size or greater on the crystal surface can catalyze the subsequent action of a developer. Exposure of the sensitive material, usually a gelatin emulsion of silver halides on a cellulose acetate film, in the camera.
Development in the darkroom by treating the film with a solution of organic reducing agents such as hydroquinone and N-methyl paraaminophenol. The reagents reduce to metallic silver those silver halide crystals that acquired latent-image silver clusters.
The film has four layers: The first layer is a protective coating which protects the emulsion layer that has the gelatin and the silver halide crystals in it. Higher speeds of film have multiple layers of emulsion. The third layer is the film base.
The film base is a polymer that is chemically stable and flexible. The fourth layer is the anti-halation backing which prevents reflections off the back of the film.
There is a lot of science in photography, but it is also a form of expression and art. Photography fills books and many pages of the internet. There are many techniques and forms for photography. Photos can show motion and can also create many emotions in those who look at them. There is a lot of chemistry in photography.
Like Chemistry, such as, chemical reactions and reactions with light. There are many parts to a film camera. The most important part is the lens. The lens draws the light into the camera and focuses the light onto the film which is directly behind it.
The shutter is in between the lens and the film and opens and closes to control the exposure of light on the film.
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