Selecting the Correct Lens, Aperture range

Selecting the Correct Lens

Aperture range

The aperture is the clear opening in the centre of the lens that allows light to pass to the pick-up device.

The aperture size is described in relation to the focal length (F). Thus the aperture is specified by an F number. If the lens has a focal length of 50mm and if the diameter of the clear opening of the lens is 25mm, then the lens is said to have an aperture of F:2 (50mm divided by 25mm = 2). A 50mm lens with an aperture of F:2 has the same light gathering ability as a 150mm F:2 lens but the glass components will be more complex in order to achieve the telephoto effect.

Most CCTV lenses have a built-in mechanism called an iris. This allows the aperture to be changed to accommodate varying light levels. The largest aperture setting is often used to define the maximum light gathering capability of the lens, the lens speed. A lens with a maximum aperture of F:1.6 is referred to as an F:1.6 lens and is said to be ‘faster’ than, say, an F:4.2 or F:8 lens.

The iris setting of the lens is numbered from the largest F stop down to the

smallest F stop. CCTV systems are rarely installed in situations where light levels are constant, so lenses with automatic iris adjustment are important components of most systems.

Focus set to 20m.

Lens f = 1.4

Depth of Field

Lens f = 5.6

Depth of Field

The Sony Guide to CCTV . Issue 3