camera    A  photographic  camera consists of (1) a light proof box,  (2)  a  converging camera lens or lens system on the front, which  forms  a real image of a distant or near object on a light sensitive  film,  (3)  a shutter that opens for short duration (10^-3 to a few seconds), and (4) a diaphragm that controls the aperture of the lens.

Illumination  of  the  image  is  one  of the most important factor  that  determines  the quality of final photograph. The illumination  of the image is given by the product of brightness, B  of the object and the solid angle w formed by the aperture of the  lens  at the  image  point.  w should be small for bright objects  and  vice  versa,  because the illumination of the image should be constant.

w is given by

w = (area of the entrance pupil)/(focal length)² = (p (1/4) diameter² ) /(focal length)²

The ratio of focal length to the diameter of the lens is called the f-number.

A  lens with focal length of 10 cm and an aperture of 2 cm has f-number  =  5,  which  is written as f /5. Reducing f-number means increasing  aperture.  For  poorly  lit objects (B small), large aperture  is required which means that f-number should be small. For brightly lit objects large f-number gives better result.

On  a  camera  the values of f-number which are indicated on the diaphragm adjustment are the following :

f - number 2 2.8 4 5.6 8 11 16

¬ (dim light) (bright light) ®

The  ratio  of  any  two successive f-numbers is 1.4, square of which  is  2. This shows that when brightness doubles we have to choose the next value of f-number on the camera.

Camera lenses: In a fixed focus or box camera the camera lens is  a  single  meniscus lens  with  a stop of fixed diameter, to reduce  aberrations  to  minimum.  Its aperture can not be large than  f /11.  Many  improved  lenses  are available with aperture f/6.3 or better.

A  telephoto  camera  lens  consists of two lenses, a convex lens  of  short  focal  length and a concave lens. The distance between the lenses is less than the focal length of the convex lens. The  use  of  telephoto  lens  avoids  the necessity of very long camera and helps to get highly magnified image, even with a short camera. A zoom lens is a system whose focal length can be varied without  moving  the  image out of focus. A zoom lens therefore provides  variable  magnification. Many zoom lenses are triplets consisting  of two convex lenses and a concave lens. The concave lens  is  in  between the  convex lenses, which are mechanically linked.    Moving  the two convex lenses forward makes the image smaller and moving them backwards makes the image larger.

Depth  of  focus  :  When a photograph is taken by a camera, the objects  are  at different  distances  at the same time. If the camera  is focused at an object at a distance d, then objects d ± D ,  will  also  appear  focused, but objects that are not within  this range  appear  blurred. D is called the depth of field.

The  maximum  allowable distance by which a film in a camera may  move  forward  or backward  along  the axis without causing blurring of the image is called depth of focus.

The  depth  of  focus and consequently depth of field may be increased by decreasing the aperture of the lens.