SOYO STH-DCAM user manual Lenses

STH-DCAM USER’S MANUAL 2001 VIDERE DESIGN

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.

4.1Changing 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.

4.2Cleaning 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

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small amount of methyl alcohol or similar lens-cleaning solvent, and wipe the imager glass surface gently. Dry with a similar tissue.

4.3Imager 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.

4.4F 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.

4.5Focal 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.