Photography

Photography is one of the strongest modes of visual communication ad expression, with an aesthetic sense. It is a technology in which images are produced through the action of visible light, ultraviolet, infra-red, x-ray, radiation from radio-active materials etc. The word Photography is derived from the Greek roots photos (light) and graphos (to draw) .During the 16th 17th century, it become known that a number of substances change color when exposed to light. This provided the first hint that perhaps an image of any object could be captured permanently. The possibility of quickly producing duplicates also seemed bright. The first practical process of photography was invented by Louis J.M.Daguerre while negative-positive methods were introduced by William H. Fox Talbot of England in 1851
     The ancient Greek Philosopher Aristotle observed that light passing through a small hole in the wall of a room formed an inverted image of the object. The first crude camera was made in 15558 A.D. in Italy and was called Camera Obseura (dark Chamber) by Giovanni Battista Porta. Depending on the size and application, traditional cameras are divided into two distinct classes-small portable ones and large immovable types. Large cameras with horizontal and vertical viewing systems are still used in large scale print publication. The introduction of digital cameras has however changed the scenario completely
    In its simplest from; the camera is a light-tight box with a small aperture (opening) at one end and a photographic film at the other end. A device called shutter opens the aperture while taking photography. When light falls on any object, parts of its get reflected from it. This reflected light passes through a lens system which concentrates (focuses) them on a film which is placed at distance behind the lens. This distance (Focal Length) depends on the distance between the object and the kens. Most cameras have a focusing mechanism that moves the lens forward and backward and other are with fixed lens .The amount of light i.e. the exposure can be controlled in most of the cameras. This is done by shutter speed and aperture size. Shutter speed is the time for which the shutter remains open to let light expose the film. A slow shutter speed lets in a large amount of light and a fast speed admits little amount of light. Aperture size is charged by a device called a diaphragm, which consists of a circle of overlapping metal leaves. The diaphragm expands to make the aperture larger and admit more light. Changes in the size of the aperture affect the overall, the area of sharpness of the picture. As the aperture become smaller, the are of sharpness in front and behind the subject becomes larger. The area or sharpness is called depth of field. As the aperture opens, the area become shallower and background and foreground of object become blurred.
   Based on the viewing system, cameras can be divided into three groups –range finder, single lens reflex (SLR) and twin lens reflex. In range finder cameras, the viewing system is separate from lens. An angled mirror behind the lens reflects a second image in the viewfinder. In single lens reflex camera, a subject is viewed directly through lens. A mirror mechanism between the lens and the film reflects the image into a viewing above the picture taking reflex cameras; the viewing klens is situated directly above the picture taking lens. The image is them screen is much larger and cleaner.
  Photographic films are flexible polyester strips coated with light sensitive emulsion which change its characteristics with incidence of light. There are three main kinds of photographic films, based on the type of pictures produced. Black and white prints are made black and white negative film, color films for color films and color reversal films, In Black and White negative film, the emulsion contains silver halides (element from halogen group-chlorine, bromine, iodine)- often silver bromide (AgBr), which changes into metallic silver when light falls on it. As light from lighter areas of the object reflects back and reaches the film, silver halides on those areas change to metallic silver and in other areas, they remain uncharged .A latent image, which is not visible at this stage is formed on the permanent. After exposure, the film is taken to a dark room or a photographic laboratory, where it is developed. The developer converts the exposed silver salts on the film into metallic silver. The fixer hypo (sodium thiosulphate) dissolves the unexposed silver salts and they are washed away. The fixer also contains a special hardening agent that makes the emulsion resistant to scratches. The image formed on the film records the lighter parts as dark areas and we get a reverse image of the film records the lighter parts as dark areas and we get a reverse image of the object that is way it is known as negatives. Making a positive print is similar to exposing and developing film, Like film, printing paper is coated with a light-sensitive emulsion. Light passes through the negative and exposes the paper, forming a latent image. After development and chemical treatment, the image on the paper is visible and permanent. During exposure, the dark areas of the negative hold back much light. These areas show up as light areas on the print. The light and clear areas let large amount of light pass through to the paper and appear as dark areas o the print .Thus, it reproduces the tones of the object photographed
For color photography, every thing remains the same except the construction of the film, which becomes multilayered. The majority of color films use the integral tripack.In color films the bottom most layer is sensitive to red parts of the reflected light and not to others. The middle one is sensitive to green (and to some extent to blue light but not to red) and the topmost is sensitive to blue. The topmost layer is dyed to yellow gelatin and it restricts blue light from reaching down layers. During the development, dyes are deposited in direct proportion to the amount of exposure received by each layer. From the negative, the positive is obtained by exposing it to photosensitive paper. Chemical compounds called couplers-which cause the development of a particular color, are also incorporated.
  We now get color photographs in a minute with the invention of Polaroid cameras. Immediately after exposure and within the camera the film is developed in a solution containing combined developer-fixer agents. The emulsion is in contact with a special white paper. The negative image is formed on the original film. Whereas a positive image appears on the receiving sheet.
   Digital photography is faster and easier. It captures images with a digital camera that gives image in digital from. Once captured, it is stored in a digital format which can be used in a computer, can be viewed on television or attached in e-mail or used on the internet. It captures image, using a solid state image sensor instead of traditional film. These tiny silicon chips contain millions of photosensitive diodes called photosites. Each photosite records the intensity or brightness of the light that falls on it and accumulates an electrical charge. The more the light-the higher the charge. It is then stored as a set of number that can be used to set color and brightness of dots on the screen of on the printed page to reconstruct the image. The screen or printer creates images with dots called picture element or pixels. The color of each is calculated with the pixels surrounding it.
        Though the technology is drastically different from traditional photography, the digital cameras are almost same in principle like film cameras. The biggest replacement is the film with a solid state image sensor either the charge coupled diodes (CCD) or CMOS image sensors: with the facility of a preview screen, one can choose a picture before print. Depending on the number of pixels or dots per squared inch (ppi or dpi), the resolution of the captured image can be changed. Digital imaging has found applications I different fields due to its immediacy and flexibility. The image are easy to insert into documents or web pages , in journalism , multimedia application, composite imaging, photos for ID cards , obtaining photographs from spare telescope microscopy etc.