Lens Camera

 The lens is the image-forming device on a camera.

Structure of a lens camera

The important parts of the camera are:

(i) The converging lens that forms a real image of the object to be

photographed. This image is diminished and inverted. The lens can be moved

back and forward with the help of the focusing ring so that objects at different

distances can be brought to focus. A focused image is located on the film or

plate

(ii)The shutter controls whether light is allowed to reach the film or not. The

speed of the opening and closing of the shutter is used to control the exposure

time when taking a photograph.

(iii) The diaphragm which controls the amount of light falling on to the film per

second by using more or less of the aperture of the lens. The aperture of the

camera lens is usually described by the f-number or relative aperture. The

larger the number the smaller the aperture. Aperture is used to controls the

amount of light that will reach the camera's image sensor

(iv) Film which is a light – sensitive surface of the camera. It is normally rolled to

the back of the camera

Basic types of Lenses

• Normal (standard lens)

• wide angle lens

• telephoto (long-focus lens)

• Interchangeable lens

Normal (standard) Lens

• The viewing is much wider-about 50 degrees.

• The objects appear normal in size and shape, relative to the picture

background

Wide Angle Lens

• The viewing is much wider-about 90 degrees.

• Used to make smaller objects look larger or to photograph large objects from

close up.

Telephoto Lens

• It has wider fields of view than normal lenses.

• They show an enlarged detail of the image over the same film area.

Interchangeable lens

 It helps to offer the photography opportunity to select a focal length that is best

for a given situation. In recent years variable focal lengths or “zoom” lenses

have become very popular.

Mode of Action

• The image of the object must be sharply focused on the film by adjusting the

distance of the lens from the film

• After focusing and correctly setting the aperture size and shutter time, the click

button is pressed

• The shutter opens to allow light to enter and expose the film to form an image of

the object being photographed. The film is then developed to produce a

photograph of the object

Magnification

Since magnification is given by

′

,

From;

then

, then substitute

into the lens formula;

, it follows that

(reciprocate the two sides and make m

the subject)

풇

풉풆풏풄풆; 풎 =

풖 − 풇

therefore; the magnification produced by the lens camera depends on the focal

length f of the convex lens used and the object distance u

Worked Examples

1. A lens camera of focal length 15 cm is used to take a picture of a man of height

1.8 m. If the man is standing 10 m ahead of the camera. Determine the:

(a) Magnification of the image

(b) size of the image

SOLN:

Given: f = 15 cm = 0.15 m, H_O= 1.8 m, u = 10 m, m =?, H_I= ?

(a) From:

(b)Also;

= 0.027cm

2. A pin-hole camera is used to take a photograph of a student who is 1.8 m tall. The

student stands 2.7 m in front of the camera. Film is placed 10 cm behind the pin-

hole. Determine the height of the image produced.

ANS; Consider a free body diagram below;

From;

67m

Therefore, the height of image produced is 6.7 cm

Uses of Lens Camera

• The sine or video camera is used to take motion pictures.

• High-speed cameras used to record movement of particles.

• Closed-circuit television cameras are used for surveillance in high-security

• Digital cameras are used to capture images