Definition of Focal Length: -

When many parallel light rays strike on a lens, they tend to converge at a particular point at the back of the lens. This point is known as the Focal Point. The distance from the middle of the lens to the focal point is known as the Focal Length. In the case of digital cameras, focal length is defined as the distance (in millimetres) between the optical centre of the lens and the image sensor.

Importance of Focal Length: -

Focal length is used to determine the magnification available through the camera. For example in case of a 35mm camera, a 50mm lens is required to get a natural view of the subject. The magnification of the objects increases in direct proportion with the focal length and as the focal length decreases, the objects appear farther away, however more area is covered. Focal Length is mentioned in terms of 35mm-lens length equivalent. This is because the traditional wide usage of film based cameras, where many  are acquainted with the focal length that is required to project images on a 35mm film.

Now let us compare the actual focal length of a 1.3 mega-pixel digital camera with that of a 35mm film based camera.

For Example

• Let focal length = 5.4mm; in a film based camera it is equivalent to 35mm. At this focal length, objects look small and far away. However it can be used to take wide coverage area shots like landscapes, groups of persons, etc.

• Let focal length = 7.7mm; in a film based camera it is equivalent to 50mm. At this focal length, objects look at about the same distance as they would appear to the naked eye. This is used to capture normal pictures of people.

• Let focal length = 16.2mm; in a film based camera it is equivalent to 105mm. At this focal length, objects look magnified. This is used to capture Telephoto shots, as well as close-up shots.

Types of Lens: -

• Wide-Angle Lens: – A lens with the focal length less than 50mm or 55mm is considered as a Wide-Angle type of Lens. They are best suited for taking wide area images.

• Normal Lens: – A lens with focal length of 50mm or 55mm is considered as a Normal type of Lens. They are suited for natural images.

• Telephoto Lens: – A lens with focal length greater than 50mm or 55mm is considered as a Telephoto type of Lens. These are suited for close-ups.

	Image using a Wide Angle Lens with focal length of 24mm		Image using a Wide Angle Lens with focal length of 35mm
	Image using a Normal Lens with focal length of 50mm		Image using a Telephoto Lens with focal length of 100mm
	Image using a Telephoto Lens with focal length of 200mm		Image using a Telephoto Lens with focal length of 400mm

As the sensor sizes of compact cameras are of different sizes, the focal length of a camera is often defined in terms of its magnification power. This is directly related to the focal length, e.g; for a 3x zoom camera, the focal length works out between 58.mm to 17.4mm. Thus a shorter focal length gives more area coverage, while a longer focal length provides with more detailed images.

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