STH-DCSG
U
SER
’
S
M
ANUAL
©
2005
V
IDERE
D
ESIGN
15
6 Lenses
The STH-DCSG/-C uses standard C or CS-mount lenses. 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.
6.1 Locking
Lenses
Some lenses come with thumbscrew locks for holding the focus and iris
settings. We recommend these locking lenses for all applications in which
the device is moving, or in which people or objects may touch the lens. For
the STH-DCSG, locking lenses are standard (2.8, 4.0, and 8.0 mm).
6.2 C and CS-Mount Lenses
The STH-DCSG will accommodate either C or CS-mount lenses. CS-
mount lenses have a mounting distance of 12.5 mm from the imager. C-
mount lenses have a mounting distance of 17.5 mm. Normally, 1/3 format
lenses are of the CS-mount variety. To use C-mount lenses, add a 5mm
spacer behind each lens. Spacers are available from Videre.
6.3 Changing
Lenses
Standard C/CS-mount lenses have a 1” diameter, 28 threads-per-inch screw
on their back end. The screw mates with the lens holder opening. To insert
a lens, place its 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 lens holder or lens
threads.
Once the threads are engaged, continue screwing it on until it seats firmly.
You can snug it down, but do not tighten it excessively, since this can
damage the lens and the lens holder threads.
Removing the lens is the reverse process: unscrew the lens counter-
clockwise. There will be some initial resistance, and then it should unscrew
smoothly.
Normal care should be used in taking care of the lenses, as with lenses for
any good-quality camera.
6.4 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.
6.5 Imager
Size
The
imager size
is the largest size of imager that can be covered by the lens.
For the STH-DCSG, the lens must be 1/3” or greater.
6.6 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-DCSG 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. In any case, it is useful to have a manual iris for high
light situations. While the imagers can have electronic exposure and gain
control to automatically compensate for different light conditions, the
acceptable illumination range can be extended by mechanical adjustment of
the lens opening.
