Imager and Lens Characteristics
Imager size - The imager on the MDCS3 cameras (either stereo or monocular) has square pixels, 5.2 um on a side. The standard active area (1280x960) has size 6.66 x 4.99 mm. This is just slightly larger than the nominal 1/2" imager size (6.4 x 4.8 mm). Most 1/2" format lenses will be adequate, and there should be no noticeable darkening of the corners of the image ("vignetting").
C vs. CS mount - The STOC-9cm, STH-DCSG-9cm, STH-DCSG-VAR/-X, STH-MDCS3/-VAR/-X, MDCS, and DCSG take either C or CS-mount lenses. Any C-mount lens can be converted to a CS-mount lens by the addition of a 5 mm adapter, which is included with the devices.
Field of View (FOV) - This is the most important parameter for most applications. The FOV of the image depends on the focal length of the lens. The table below lists FOV for some common focal lengths. The formula for FOV of an ideal (pinhole) lens is
HFOV = (180/pi) * 2 * tan-1(H/2f), H = 6.66 mm
VFOV = (180/pi) * 2 * tan-1(V/2f), V = 4.99 mm
where H is the horizontal size of the imager in mm, V is the vertical size (mm), and f is the focal length (mm). Note that the horizontal and vertical FOV are generally different, because the imager frame is rectangular. Also, the FOV for shorter focal lengths is generally greater than the pinhole model, because the image is compressed towards the edges. Finally, for stereo applications, there will be erosion of the output disparity image, because some amount of margin is needed on the left and the right of the image for running the stereo algorithm.
Focus and Iris - The lenses we supply for the MDCS3 devices come with a manual focus and iris adjustment. The depth-of-field for a lens varies with a number of factors, including the iris setting (aperture) and the focal length. Generally speaking, unless the application demands extreme close-ups, it is sufficient to set the focus of the lens so that objects are in focus from some point near the camera out to infinity. For example, with 6.0 mm lenses and a wide-open iris, a single focus setting suffices for about 0.5 m to infinity.
The manual iris controls the aperture of the lens, the amount of light that is passed. In most situations, the iris is left wide open to allow the maximum amount of light through. The MDCS3 cameras have electronic exposure control to compensate for lighter and darker conditions. The electronic exposure can be set under program control, or via an auto adjustment algorithm. For some situations, where there is very bright light, the iris can be closed down to prevent the imager from over-saturating. Also, if a very large depth-of-field is required, shutting down the aperture can help.
Because of electronic exposure control, a motorized iris is not necessary on the MDCS3 cameras.
Varifocal lenses - If the FOV for an application is known, then the correct lens can be chosen based on the FOV formulas above. For experimentation, it is possible to use varifocal lenses. These lenses have an additional setting to change the focal length of the lens. In stereo applications, changing the focal length requires a re-calibration of the stereo parameters. Since the calibration procedure is fairly simple, varifocal lenses provide an easy way to experiment with various FOV settings. Varifocal lenses must have a locking screw to preserve the focal length setting (see below).
Locking vs. non-locking lens - Lenses with manual controls (focus, iris, focal length) may have optional locking screws. These locking screws allow the desired settings to be rigidly kept, once the lenses are tuned for the application. Locking screws are not required (except on varifocal lenses, see above), since friction will generally hold the iris and focus settings. If the camera is moved around, subject to vibration, or can be touched, then locking screws are a good way to insure that the desired settings are kept. |
