Document 800-05662V1 Rev A 97
04/11
C
Understanding Automatic Exposure and
NightShot
This appendix contains the following sections:
• About the Lens Iris or Aperture and the F-Number, page 97
• About Video Gain, Shutter Speed, and Spectral Response, page 98
• About ACUIX PTZ Dome Manual Exposure Control, page 100
• Operating NightShot (TDN) on ACUIX PTZ Camera Blocks, page 101
This appendix provides additional information about automatic exposure (AE) to
assist you in configuring the ACUIX High Speed PTZ dome to meet your specific
requirements.
About the Lens Iris or Aperture and the F-Number
The lens iris, or aperture, determines how much light is transmitted through the
lens to the sensor. This is measured as an f-number, which is the focal length of
the lens divided by the equivalent diameter of the front lens element. Therefore,
a large f-number transmits less light to the sensor. The standard f-numbers are
integer powers of the square root of 2 (for example, 1, 1.4, 2, 2.8, 4, 5.6, 8, 11,
16, 22, 32, 45, 64, 90, 128, and so on). Each successive f-number transmits one
half the amount of light as the previous f-number.
The f-number is the first factor changed in an auto exposure system. That is to
say, the other factors are not changed until the lens aperture reaches its open or
closed limit. The lens aperture also controls the depth of field of the lens. The
depth of field is a measure of the distance that an image is in focus to an
acceptable degree. The larger the f-number (a smaller aperture) the greater the
depth of field.
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