Photographic camera
A photographic camera is essentially a converging lens system used for producing a real image on the photographic film.
Construction: A photographic camera consists of following parts:
(а) A light proof box: This box is blackened inside to absorb only stray radiations and to control internal reflections.
(b) Objective: Photographic objective is a converging lens system fitted in front of camera and the lens system is corrected for spherical and chromatic aberrations. It forms real, inverted and diminished image of the object on the photographic film.
(c) Shutter: It is placed between the lens and the film. It opens and closes quickly when photograph of an object is taken.
In a camera following adjustments and controls are provided.
(i) Exposure time: In order to obtain a clear image for a given aperture, the film has to be exposed for a certain time. It determines the total amount of light falling on the film. If the photographer is taken in bright sun light the exposure time should be small and it should be large (i) shade condition or for indoor photography. The normal, exposure times used in camera are:
\(\frac{1}{500}s, \frac{1}{200}s, \frac{1}{125}s, \frac{1}{60}s, \frac{1}{30s} \ etc.\)
(ii) Aperture of the lens (f-numbers).
It refers to diameter of circular opening through which light passes into the camera. It is generally expressed in f-numbers, f-number indicates the light generating power, hence is referred to as the speed of the lens.
If D is diameter of the circle of lens and f is focal length, then
f-number = \(\frac{\text{focal length of the lens }}{\text{diameter of the aperture }}\)
= \(\frac{f}{D}\)
So, amount of light per unit area of image is proportional to \(\frac{D^2}{f^2}\)
It means, smaller the f-number, higher the speed of the lens of the camera.
In almost all cameras, the aperture used are
\(\frac{f}{2}, \frac{f}{2.8}(\frac{f}{2\sqrt2}), \frac{f}{4}, \frac{f}{5.6}, \frac{f}{4\sqrt2}, \frac{f}{8}, \frac{f}{11.2}, (\frac{f}{8\sqrt2}), \frac{f}{16}\)
f-number 16 means that the focal length is 16-times the diameter.
(iii) Film speed: Film speed is a measure of how quickly the film be exposed when in use. A fast film requires comparatively slow time exposure while a slow film requires relatively longer time. Thus fast films are used in poor light conditions and very slow films for all still object photography.
(iv) Exposure meter: Modern cameras have built in exposure meters. It has light sensitive surface. When light falls over it, a current, proportional to the quantity of light falling, flows in the meter and photographer can adjust a suitable combination of aperture and exposure times for correct exposure.
(v) Depth of focus: It indicates the range of object distances over which the focussing is reasonably good. This factor becomes important when the aperture has to be made large due to poor light conditions. Under such condtions, the image is not focussed in a single plane but gets spread out due to aberrations.
