Axis 2400+ Manual do Utilizador descarregar pdf (Página 27)

Iris

The role of the iris is to adjust the amount of light

passing through the lens. There are three different types

of irises on lenses:

• Manual Iris. Turn a ring to adjust the iris, or use a

ﬁ xed iris lens. A selection of iris apertures is available

e.g. F1.4, F 2.0, etc

• DC Auto Iris. Connected to the output of a camera, the

iris is steered by the camera’s Digital Signal Processor

(DSP)

• Video Auto Iris. The iris is steered by video signal

Auto iris lenses are recommended for outdoor

applications. The iris automatically adjusts the amount

of light reaching the camera and gives best results, as well

as protecting the image sensor from too strong light.

A small iris diameter reduces the amount of light, giv ing

a better depth of ﬁ eld (focus over a greater distance). A

large iris diameter, on the other hand, gives better images

in low light. The iris is deﬁ ned by the F-number.

F-number = Focal length / Iris diameter

A lower F value gives better pictures under low light

conditions.

With an auto iris lens, always set the focus in low

light. If you make the adjustment in sunlight it is very

easy to focus: but then at night the iris diameter increases

and the image is no longer in focus. You can also get

special dark ﬁ lters which reduce the light e.g. 10 times.

These ﬁ lters are helpful when you install cameras.

This is an example of the depth of ﬁ eld for a telephoto lens. A large F-number gives focus over a greater distance. For typical 3.5-8 mm lenses you can focus from

10 ft/3 m to 50 ft/15 m.

C AMERA BASICS

F5.6

F16

0 1.5m 3m 4.5m 6m

7.5m

15'5'0 10' 20' 25'

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