STH-DCAM U
SER
’
S
M
ANUAL
2001 V
IDERE
D
ESIGN
4 Lenses
The STH-DCAM uses standard miniature lenses (12 x 0.5 mm). Good-
quality, fixed-focus lenses with low distortion and high light-gathering
capability are best.
Lenses are characterized optically by imager size, F number, and focal
length. Following subsections discuss the choice of these values.
4.1 Changing
Lenses
Standard miniature lenses have a 12 mm diameter, 0.5 mm pitch screw on
their back end. The screw mates with the lens holder opening. To insert a
lens, place it back end on the lens holder opening as straight as possible,
and gently turn it clockwise (looking down at the lens) until it engages the
threads of the lens holder. If you encounter a lot of resistance, you may be
cross-threading the lens. Forcing it on will damage the plastic lens holder
threads.
Once the threads are engaged, continue screwing it on until most of the
thread is in the holder. Turn on the device, and check the focus, adjusting it
until there is a clear image. The depth of focus of most miniature lenses is
very large, from several inches to infinity.
Removing the lens is the reverse process: unscrew the lens counter-
clockwise.
Normal care should be used in taking care of the lenses, as with lenses for
any good-quality camera.
4.2 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.
If dirt and dust are present on the IR filter surface, they can be cleaned in
the same manner as a lens. Wet a non-abrasive optic cleaning tissue with a
small amount of methyl alcohol or similar lens-cleaning solvent, and wipe
the imager glass surface gently. Dry with a similar tissue.
4.3 Imager
Size
The
imager size
is the largest size of imager that can be covered by the lens.
For the STH-DCAM, the lens must be 1/4” or larger.
4.4 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
STH-DCAM 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 have no mechanical iris for exposure
adjustment. Instead, they have electronic exposure and gain control to
automatically compensate for different light conditions.
4.5 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.
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