JEE Main & Advanced Physics Ray Optics Lens Camera

(1) In lens camera a converging lens of adjustable aperture is used.

(2) Distance of film from lens is also adjustable.

(3) In photographing an object, the image is first focused on the film by adjusting the distance between lens and film. It is called focusing. After focusing, aperture is set to a specific value and then film is exposed to light for a given time through shutter.

(4) f-number : The ratio of focal length to the aperture of lens is called f-number of the camera.

2, 2.8, 4, 5.6, 8, 11, 22, 32 are the f-numbers marked on aperture.

f-number \[=\frac{\text{Focal length}}{\text{Aperture}}\] \[\Rightarrow \]Aperture \[\propto \] \[\frac{1}{f\text{-number}}\]

(5) Time of exposure : It is the time for which the shutter opens and light enters the camera to expose film.

(i) If intensity of light is kept fixed then for proper exposure

Time of exposure (t) \[\propto \frac{1}{{{\text{(Aperture)}}^{\text{2}}}}\]

(ii) If aperture is kept fixed then for proper exposure

Time of exposure (t) \[\propto \frac{1}{{{\text{ }\!\![\!\!\text{ Intensity}\,(I)]}^{2}}}\]

\[\Rightarrow \] \[I\,t=\,\text{constant}\]\[\Rightarrow \]\[{{I}_{1}}{{t}_{1}}={{I}_{2}}{{t}_{2}}\]

(iii) Smaller the f-number larger will be the aperture and lesser will be the time of exposure and faster will be the camera.

(6) Depth of focus : It refers to the range of distance over which the object may lie so as to form a good quality image. Large f-number increase the depth of focus.