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1. A strict technical definition of focal length is
difficult without providing a lot of background
in lens theory, so we will use a simplification.
You can think of focal length as the distance
between the imaging plane (e.g. the image
chip in a digital camera) and a point where all
light rays intersect inside the lens (the ‘optical
center’). So a focal length of 20mm means that
the distance from the optical center to the
imaging plane is 20mm long (about ¾ of an
inch).
What is Camera Focal Length?

2. • The focal length number tells us how much of
the scene is captured in the picture. The lower
the number the wider the view, and the more
we can see. The higher the number, the
narrower the view, and the less we can
see. This is illustrated below – where the
camera is stationary and the focal length (in
white numerals) changes:
What does the focal length number mean

4. • A camera typically has focal length in a range
of 10mm to 500mm. Different types of
camera can have different ranges and
speciality lenses can extend outside this range
as well. A 10mm focal length would be a very
wide lens (capturing a lot of the scene), and
500mm would be a very narrow lens
(capturing only a small part of the scene –
giving a large magnification like binoculars or a
telescope).