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3.2.6 Depth of field
A criterion that may be important to a video surveillance application is depth of field. Depth of
field refers to the distance in front of and beyond the point of focus where objects appear to be
sharp simultaneously. Depth of field 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 field is affected 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 field.
Figure 3.2e Depth of field: 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 field is good.
Figure 3.2f Iris opening and depth of field. The above illustration is an example of the depth of field for different
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 diffrac-
tion.)
CAMERA ELEMENTS - CHAPTER 3
