Sony SSC-C108P Посібник - Сторінка 17

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Selecting the

Correct Lens

The quality of the lens is vital in determining the quality of the final image.

The basic optical characteristics of a lens are the focal length (in zoom lenses

the focal length range) or angle of coverage, and the aperture range. We'll

look at these two characteristics together with a third important

characteristic, the format.

Focal Length (F)

The focal length represents the distance from the optical centre of the lens to the

point at which the image is in sharpest focus and where the pick-up device in the

CCTV camera is located.

The focal length determines the angle of coverage of the lens. The longer the

focal length, the narrower the angle of coverage and the shorter the focal length,

the wider the angle of coverage.

One particular angle of coverage produces a picture very similar to that seen by

the human eye. A lens giving this angle is called a 'normal' lens. The focal length of

a normal lens is approximately equal to the diagonal of the picture area. For

example, for a 2/3 inch camera, the normal lens would have a focal length of

16mm. A 1/2 inch camera would have a normal lens of 12.5mm and a 1/3 inch

camera, an 8mm normal lens.

A focal length shorter than a normal

focal length produces a wide angle

view. A longer focal length

produces a telephoto image.

Some lenses have variable

focal lengths and are called

zoom lenses.

Basic Lens

= Focal Length

= Lens Diameter or Aperture

= Lens Stop Expressed as a ratio

of Focal Length to Aperture

For more information contact the Sony Business Information Centre: 01932 816340