Proxima ASA STH-MD1/-C - Manual (Page 8)

STH-MD1 U SER ’ S M ANUAL
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2001 V IDERE D ESIGN
8
4 Lenses
The STH-MD1/-C uses standard C-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.
4.1 Changing Lenses
Standard C-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 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 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.
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-MD1, the lens must be 2/3” or 1”. 2/3” is an acceptable
size, but there will be a little vignetting (darkening) at the edge of the
image. A 1” lens will give much better illumination, but is typically
available only in longer focal lengths (12 mm or greater).
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-MD1 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.
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