Specifications
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3.2.6 Depth of eld
A criterion that may be important to a video surveillance application is depth of eld. Depth of
eld refers to the distance in front of and beyond the point of focus where objects appear to be
sharp simultaneously. Depth of eld may be important, for instance, in monitoring a parking lot,
where there may be a need to identify license plates of cars at 20, 30 and 50 meters (60, 90 and
150 feet) away.
Depth of eld is aected by three factors: focal length, iris diameter and distance of the camera
to the subject. A long focal length, a large iris opening or a short distance between the camera
and the subject will limit the depth of eld.
Figure 3.2e Depth of eld: Imagine a line of people standing behind each other. If the focus is in the middle of the
line and it is possible to identify the faces of all in front and behind the mid-point more than 15 meters (45 feet) away,
the depth of eld is good.
Figure 3.2f Iris opening and depth of eld. The above illustration is an example of the depth of eld for dierent
f-numbers with a focal distance of 2 meters (7 feet). A large f-number (smaller iris opening) enables objects to be in
focus over a longer range. (Depending on the pixel size, very small iris openings may blur an image due to dirac-
tion.)
CAMERA ELEMENTS - CHAPTER 3