Proxima-asa STH-MD1/-C Manuel d'utilisateur télécharger pdf (Page 8)

STH-MD1 USER’S MANUAL ?2001 VIDERE DESIGN

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

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Proxima-asa STH-MD1/-C Manuel d'utilisateur Page 8

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