Principle of recording an Image

A definition of photography is the process of forming stable or permanent images directly, by the action of light, onto a light sensitive surface.

One of the first documented accounts of an image being permanently captured was in 1826 when chemist Nicephore Niepce captured the view of his window onto a copper plate. He achieved this by coating the copper plate with a silver solution. Photochemistry and photography evolved but limited the photographer as the copper/glass plates used were bulky and the cameras large and heavy to carry. It was not until 1889 when George Eastman developed emulsions that could be coated on a cellulose nitrate film base. These were flexible and could be rolled which allowed the camera’s that were available to the photographer, to made smaller and be more transportable.
Light is the essence of photography as the recording of an object is based on the relationship between light and the photo-sensitive chemicals on the films. Light that is visible to the human eye is made up of radiation. Specifically it is the wavelengths that we can see but there is a vast spectrum of wavelengths that we cannot see, such as x rays and Gamma rays. The small band of radiation, that is visible to the human eye, is made up with a variety of different wavelengths. The eye registers these different variances as colour. Our eyes perceive the shortest visible wavelengths as violet, and the longest as red. In between are the familiar colours of the spectrum, of which the eye can normally see seven, the colours of the rainbow. The key principle of photography is capturing that light onto the film.

LAYERS OF PHOTOGRAPHIC FILM


Film is made up from three layers, the first is the anti abrasion layer which protects the film from being marked. The second layer is the mixture of emulsion and suspended silver halides and is where the image is captured. The final layer is an anti- halation layer, which stops the light being projected back onto the film and causing “halos” of light on the film.
When the radiation reacts with the silver-halides they change to form a “latent image” and this is an negative replica of the original image, that is area of shadow in the original image are recorded as light areas in the negative and areas of highlight are recorded as areas of darkness. Photographic film react to different wavelengths, early film was mono-chrome and only reacted to shorter or blue wavelengths, further development in film added coloured dyes which could react to wider wavelengths, thus rendering colour. Film is classified by its speed, which is universally measured as an I.S.O (International Standards Organisation) speed. The higher the speed of the film the more sensitive the film is to light. This is because the film is coated with more silver-halides than a slower speed film. As there are more silver halides the final images is deemed as “grainy” as the final print is often mottled. Slower film react slower to light as they are covered in less silver-halide, therefore the final image is often described as “fine grain” as the final print is cleaner.

To record the image the camera is used to project the light from the object onto the film surface, this is achieved by the use of a lens which allows the image to be focused on to the film surface. The lens has the ability to reproduce the image in a variety of magnifications; this is known as “focal length”. The focal length of the lens is the distance from the rear of the lens element to the film plane, when the lens is focused on infinity. There are two main effects of varying focal lengths of a lens, magnification and angle of view. The more magnification of an image on the film, the greater the focal length of the lens which means that the further away the lens is to the film, the less magnification required the closer the lens is to the film plane. There is also variation in angles of view and perspective. The more magnified decreases the angle available to view of the image; it also compresses the image so objects appear closer together and so alters the perspective of the image.

To record an image on film is to be able alter the amount of light was reaches the film surface. This is controlled by the shutter and the iris (aperture). The aperture is found at the back of a lens. It is a diaphragm that can be wide or small, the wider the more light is let thru, the smaller the less light exposed. The aperture also records how much of the image is in focus. This is measured on the camera as an f stop, the greater the number e.g. f16 the smaller the aperture is. A wider aperture will only show the detail of the area of the image that is in focus, the smaller the aperture gives a greater level of sharpness in the image. This is known as the “depth of filed” and is the distance in front of and beyond the subject that appears to be in focus.

The shutter sits in front of the film and when activated exposes the film to the light. The faster it moves across the film the less light is exposed to it and a slower shutter speeds means that the films is exposed more to the light. This “shutter speed” is measured in fractions of a second. A fast shutter speed is often shown as 1/1000 sec and a slow shutter speed as 1/15 sec. This fraction denotes the amount of time the light is exposed to the light sensitive film.