User`s manual

STOC USER’S MANUAL ©2005 VIDERE DESIGN
6 Lenses
The STOC/-C uses either 12mm diameter miniature lenses, or CS-mount
locking lenses. Miniature lenses are screwed into integral lens-holders
milled into the aluminum chassis. The lenses are focused at the factory, and
firmly glued into place. There are no adjustments to be performed on the
lenses. It is possible to change both types of lenses, although changing the
miniature lenses is not recommended; see Section
6.7 and 6.8 below.
Lenses are characterized optically by imager size, F number, and focal
length. Following subsections discuss the choice of these values.
6.1 Cleaning the Imagers
It should not be necessary to clean the imagers, since they are sealed off by
an IR filter inside the lens mount.
6.2 Imager Size
The imager size is the largest size of imager that can be covered by the lens.
The STOC imager is 1/3” size (about 6mm in diameter).
6.3 F Number
The F number is a measure of the light-gathering ability of a lens. The
lower the F number, the better it is at pulling in light, and the better the
STOC will see in low-illumination settings. For indoor work, an F number
of 1.8 is acceptable, and 1.4 is even better. For outdoors, higher F numbers
are fine. Miniature lenses do not have any iris control; instead, they rely on
electronic exposure and gain control to automatically compensate for
different light conditions.
6.4 Focal Length
The focal length is the distance from the lens virtual viewpoint to the
imager. It defines how large an angle the imager views through the lens.
The focal length is a primary determinant of the performance of a stereo
system. It affects two important aspects of the stereo system: how wide a
field of view the system can see, and how good the range resolution of the
stereo is. Unfortunately there’s a tradeoff here. A wide-angle lens (short
focal length) gives a great field of view, but causes a drop in range
resolution. A telephoto lens (long focal length) can only see a small field of
view, but gives better range resolution. So the choice of lens focal length
usually involves a compromise. In typical situations, one usually chooses
the focal length based on the narrowest field of view acceptable for an
application, and then takes whatever range resolution comes with it.
6.5 Field of View
The field of view is completely determined by the focal length. The
formulas for the FOV in horizontal and vertical directions are:
)/92.1arctan(2 fHFOV =
)/44.1arctan(2 fVFOV =
where f is in millimeters. For example, a 2.8 mm lens yields a horizontal
FOV of 87 degrees.
Table 4 shows the FOV for some standard focal lengths.
Lens focal length Horizontal FOV Vertical FOV
2.1 mm 85 deg 69 deg
3.6 56 44
5.7 37 27
8.0 27 20
Table 4 Horizontal and vertical field of view for different
lens focal lengths.
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