Chemically prepared or sensitized material

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INTRODUCTION OF PHOTOGRAPHY

Photography is defined as the science and art of producing an exact impression of an object on chemically prepared or sensitized material by using light. It depends on the sensitiveness of silver compound to light. The photographs are usually taken by means of a photographic camera. Camera is an instrument consisting of a light-tight box with a place for the light sensitive film at one end and covering lens or a combination of lenses by which a real, inverted and diminished image can be obtained on the other hand.

This instrument called camera is so devised that lens may be moved forward and backward by screw arrangement, with a view to getting a distinct and clear image of an object, placed before the lens, on the film.

While it is easy to make comparisons between the pupil of the eye and the f-stop of a camera or between the retina of the eye and photographic film .Once we get past the basic similarities of the optics of the two systems comparisons begin to rapidly break down. The eye is not only much more complex than a camera and its film but the two imaging devices function by different chemical mechanisms. The photographer the automatic exposure system of the camera regulates the f-stop opening and time of exposure of her camera to match the sensitivity of film while the iris and retina sensitivity of the eye adjust to correspond to the light level of the scene.

This first installment on the chemistry of photography is intended to introduce in a simplified way the basic concepts of silver halide photography. It will not delve into the physics of optics the functioning of cameras and lenses, photographic techniques, non-silver processes, or the artistic aspects of photography.

Photography encompasses many things - there is a science to photography, but as a means of expression, it is ultimately an art form. There are many techniques and forms of photography which fill many books in the library and many pages on the Internet. Beyond this point, the actual taking of pictures, style, and good composition will not be mentioned again. Chemistry is an even larger subject area, encompassing all areas of life and invisible details that are often mind boggling. To couple photography and chemistry is narrower than either subject on its own, but still includes a lot of material. These pages will look at some basics, presenting only a small fraction of the information relevant to the chemistry of photography.

Photography encompasses many things - there is a science to photography, but as a means of expression, it is ultimately an art form. There are many techniques and forms of photography which fill many books in the library and many pages on the Internet. Beyond this point, the actual taking of pictures, style, and good composition will not be mentioned again. Chemistry is an even larger subject area, encompassing all areas of life and invisible details that are often mind boggling. To couple photography and chemistry is narrower than either subject on its own, but still includes a lot of material. These pages will look at some basics, presenting only a small fraction of the information relevant to the chemistry of photography.

ABBREVIATION LIST

  • NH4Br→Ammonium bromide.
  • AgNO3→Silver nitrate.
  • Ag Br→ Silver bromide.
  • NH4NO3→Ammonium nitrate.
  • 1, 4-dihydroxy benzene.
  • 2, 4-diaminophenol.
  • C6H6OH→PHENOL.

METHODOLOGY

  • Chemical reaction.
  • Process (propagation).
  • Techniques.
  • Material studied.
  • Images.

TYPES OF PHOTOGRAPHY:-

BLACK &WHITE PHOTOGRAPHY.

COLOUR PHOTOGRAPHY.

BLACK & WHITE PHOTOGRAPHY

Black and white photography is known for its unique charm, clarity and pureness. It strikes a fine balance between information and inspiration. These features of black and white photography have made the art most appealing though it has tough competition with digital color photography in the present era. Black and white photography is generally practiced by the monochrome photographers all over the world.

Monochrome photography consists of black and white photographs

A large part of monochrome photography consists of black and white photographs which retained their charm long after the trend of color photography had taken shape. The prime reasons behind the prevalence of black and white photography even in the present era are comparatively lower cost and classic look of black and white photographs.

It is interesting to know that all the monochrome photographs are not necessarily pure black and white. There are some black and white photographs which are produced by Cyanotype process and they display shades of blue and white. There was a process called "albumen process." It was in use some 150 years ago and the black and white photographs developed with the help of this process had brown shades.

(1) Preparation of a photographic film→

Photographic film is thin sheet of cellulose acetate or other polymeric support coated with the photographic emulsion. This emulsion is prepared by adding 20% solution of silver nitrate (AgNO3) to the solution of ammonium (NH4Br) containing gelatin.

NH4Br + AgNO3→AgBr + NH4NO3

After the mixing, the emulsion is ripened either by keeping it at high temperature for some time or by digesting it at a lower temperature with ammonia. It is then cooled till it sets to a jelly. The jelly, like mass is washed with water to remove ammonium nitrate. Then the emulsion is applied on a cellulose acetate film is then dried and carefully packed. To the fastness of film depends upon the size the grains of (Ag Br).The larger the grain the faster the film.

(2)Formation of latent image by exposure→

Photographic film is mounted in a camera and exposed few second to the image of a properly focused object. An invisible change occurs in parts of film on which light falls depending upon the shape, and intensity of the light reflected from different parts of the object. On the film, the inverted image of the object is formed. This image is called latent image and is not visible in presence of the sunlight.

2Ag Br→2Ag +Br

(3)Developing latent image and preparing the negative→

The exposed film is immersed in a solution of a weak reducing agent (developer) like ferrous oxalate or alkaline solution of organic reducing agent like hydroquinone (1, 4-dihydroxy benzene) and (2, 4-diaminophenol). This developer reduces the activated Ag Br grains to Ag at an appreciable greater rate than the unexposed Ag Br according to the equation.

This the (developer) act as much more rapidly upon these portions of the film where the illumination was most intense. Here the reduced silver is deposited in the form of black silver. Thus the film becomes darkest where the object was bright and vice-versa. It is called Negative.

2Ag Br+ (1, 4-dihydroxy benzene)+ 2(OH) -→2Ag+(2, 4-diaminophenol) +2H2O +2Br

(4)Fixing the image→

After developing the film is till sensitive to light due to the presence of unchanged the Ag Br. There for in order to get the permanent image, it is necessary to remove unchanged Ag Br. This is done by dipping the negative in the fixed bath. This process of the removal of the excess Ag Br is called fixing. Before the fixing and after developing the alkali is removed by dipping the negative in 0-5% acetic acid for short time.

Ag Br +2Na2S2O3→Na3[Ag(S2O3)2 +Na Br

(5)Printing→

In this process, the negative is placed over a sensitive paper printing paper that has been coated with photographic emulsion (silver bromide with gelatin).The paper is then exposed to light for a fraction of second so that the light passes through the negative. Darker spot on the negative allow less light and dark are reversed and we get a positive. The positive is than developed, fixed, and dried to give the black and white photography.

INTRODUCTION TO USING FILTER

The use of filters makes the digital black and white photography really interesting and effective. Through this feature, you can easily make out how a particular color appears in a black and white photograph. While using a digital camera for taking black and white photographs, you would require a filter holder so that the filter might be attached at the time of taking the shot. You cannot see the change of tones on the viewfinder. Once you complete taking the image, you can have a preview on the screen.

Filters are the addition to the ‘eyes of the camera' your lens. Additive, subtractive and special effect are the three basic categories of filters. So before we move on to some advanced types and uses of filters, get acquainted.

Filters can be glass or plastic:-Are used for enhancing a photo or adding special effects to the image. At the very least, any filter will act as a cover for your lens.

Plain glass or transparent filters:

These are primary filters that only serve as protection for your lens. They are not supposed to affect the image in any way. So it is a good idea to check this out before buying. No matter what the season or conditions you are shooting in, it is always recommended to have one of these on your lens. After all, you can't be too careful.

Color filters and black-and-white photography:

Color filters are best used in black and white photographs to alter or intensify contrasts. The basic filters include Red, Green and Blue along with Yellow. For instance, a yellow filter will make the outlines in an image and it background appear lighter in contrast. So if you are clicking your subject against a dark background then using a yellow filter will make the object seem lighter and brighter in contrast.

IMAGES OF BLACK & WHITE PHOTOGRAPHY

Common Silver Halide Developers

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