Lens Selection

The PR-MINI recorders utilize an externally mounted lens on the recorder case. This provides for increased flexibility in offering the ideal field of view (telephoto size) for the video capture and recording.

Depending on your room setup you may wish to capture the podium area only, or just the chalkboard or whiteboard or simply a wide-angle shot of the entire room including the audience.

Two varifocal lens styles are available. The focal length of varifocal lenses is adjustable allowing for wide-angle or telephoto fields of view. A varifocal lens is like a traditional zoom lens except the focus must be re-adjusted each time the focal length changes. A zoom lens will automatically adjust for focal length changes, but they are considerably more expensive to purchase.

All lenses offer one or more adjustments: iris, focus and focal length. The iris controls the amount of light entering the sensor and can be stopped up or down depending on the light available in the room and the depth of field desired. The smaller the iris the greater depth of field. The focal length is the “zoom” of the lens and determines the image magnification and field of view. Finally, the focus brings the image into sharp detail.

To adjust the zoom lens, first stop down the iris to the point where the image begins to darken and then back off a bit. This will maximize the depth of field and prevent the lens sensor from being over-exposed. Next, adjust focal length to provide for the angle of view desired. Finally, adjust the focus to maximize image detail.

The adjustment knobs on the lenses will move freely if they are rotated to the left and will lock in place if rotated to the right. Lock all the adjustments firmly once the desired image is achieved.

Be sure to mount the lens onto the recorder case in a dust-free area and keep the protective caps on the recorder and on the lens when they are not mounted. Only expose the sensor for the minimum time needed to mount the lens. A sensor that shows dust particles may be cleaned with a photographic air duster (a rubber bulb with a nozzle that pumps air onto the surface of the sensor or surface of the lens). Do not blow on the sensor or use air cans that might leak chemicals onto the sensor surface.

The required focal length of the lens is determined by the field of view (target width) to be captured and the camera distance to the target. The underlying formula is stated as

Focal-Length = Sensor-Size x Working-Distance / Field-of-View

It’s easiest to keep all dimensions in millimeters (mm.) when making this calculation, and you need to know the dimensions of the sensor in use, which in this case is 3.76 mm. (width) x 2.74 mm. (height).

As a specific example, if we have a conference room with an 8 foot whiteboard (2438.4 mm.) and the camera lens is 35 feet (10668 mm.) distant, then the focal length required is 3.76 x 10668 / 2438.4 or 16.45 mm. So a lens similar to the 15mm-50mm model offered would be required in this example.

One final comment concerns the type of lens used. The case supports an industry standard “CS-Mount” lens which is widely used in the CCTV, Security and Machine-vision industries. Many of these lenses, however, are only for “Standard Definition” cameras (for use with the older NTSC TV’s). Such lenses are not optimal for recording the high-definition images captured by these recorders. A “Megapixel” lens is required, where the capture resolution is 2 Megapixels (2MP) or greater.