( iii ) Astigmatism In this defect , a person cannot focus on horizontal and vertical lines at the same distance at the same time .
This defect can be removed by using suitable cylindrical lenses .
( iv ) Colour Blindness In this defect , distinguish between few colours . a person is unable to The reason of this defect is the absence few colours . of cone cells sensitive for
This defect cannot be removed .
( v ) Cataract In this defect . an opaque white membrane is developed on cornea due to which person lost power of vision partially on completely .
This defect can be removed by removing this membrane through surgery .
Camera
A photograph camera consists of a light proof box , at one end of which a converging lens system is fitted . A light sensitive film is fixed at the other end of the box , opposite to the lens system . A real inverted image of the object is formed on the film by the lens system .
f-Number for a Camera : The f-number represent the size of the aperture . f-number = Focal length of the lens ( F ) / Diameter of the lens ( d ) Generally 2 , 2.8 , 4 , 5.6 , 8 , 11 , 22 , 32 are f-numbers .
The amount of light ( L ) entering the camera is directly proportional to the area ( A ) of the aperture , i . e .,
L ∝A∝ d 2
Brightness of Image ∝ ( d2 / f2 )
where , d = dia meter of the lens and F = focal length of the lens . Exposure time is the time for which light is incident of photographic film .