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Physics for Secondary Schools
shutter and a mounting base. Figure 6.12 an object that is far away. You have
shows some parts of a lens camera. probably seen a photograph where an
object is in focus but the background
Shutter release button is blurred; this is often done with a
telephoto lens.
(d) Interchangeable lens which offers
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the photographer the opportunity to
select a focal length that is best for
a given situation. In recent years,
variable focal length or “zoom”
Film lenses have become very popular.
The diaphragm determines the amount
Lens Focusing knob of light that passes through the lens by
changing the size of the aperture. The
Figure 6.12: Parts of a lens camera aperture is an opening whose diameter
is adjustable. The size of the aperture
The lens is the image-forming device is measured in f-number; the larger the
in a camera. The focal length of a lens number the smaller the aperture. Most
determines the size of the image that will cameras use an iris-type diaphragm, which
be formed on the film. Therefore, four consists of a number of very thin metal
basic categories of lenses exist. blades. They are mounted so that the size
of the lens opening can be changed by a
These are: rotating ring or a moving lever.
(a) Normal lens which allows the viewing
field of approximately 50 degrees, The shutter is a mechanical device that
giving a picture that is normal in acts as a gate, controlling the duration of
size relative to the background and time that light is allowed to pass through
which looks natural to the viewer. the lens and fall on the film.
(b) Wide angle lens which enables A film is a light-sensitive surface of the
a view of 90 degrees and is used camera. It is normally rolled to the back
when smaller objects are to be of the camera. In most cameras, the film is
photographed.
wound up on a spool with an interleaving
(c) Telephoto lens or long-focus lens light-tight backing paper. However, in
which has a wider field of view than a digital camera, an electronic detector
the normal lens. It shows enlarged called a charge-coupled device (CCD)
detail of the image over the same array is used instead of a film. The digital
film area. The telephoto lens has a information is stored on a memory chip
long reach, allowing you to capture for play back on the screen of the camera.
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Student’s Book Form Two
Physics Form 2 Final.indd 226 25/10/2025 10:28
