Last Revision Date: 5/16/2014
A darkening of the corners of an image relative to its center is likely a result of vignetting. This article describes vignetting and potential remedies.
A darkening of the corners of an image relative to its center (see example below) is likely a result of vignetting, which is an expected effect with many lenses. There are different types of vignetting, including optical, natural, and mechanical vignetting.
One way to reduce or eliminate this effect is to buy a better lens, such as a C- or CS-mount lens. You can get these from companies such as Pentax, Fujinon, Computar and Edmund Optics. Another option is to buy a larger format lens. For example, if you are using a Dragonfly2, which uses a 1/3" CCD, a lens built for a 2/3" CCD might reduce this effect. However, be aware that focal lengths can differ significntly between different lens formats.
Some imaging products, such as the Dragonfly2, Firefly MV, and Chameleon allow for microlens and C/CS-mount lens holders that can be mounted to the bare printed circuit board (PCB). The imaging sensor is centered relative to the 2 mounting holes for these lens holders. These holes allow for some small adjustments in where the lens holder sits relative to the sensor. If you are seeing vignetting primarily on one side of the image, it may be possible to reposition the lens holder to reduce this effect.
The above products can also be enclosed in a plastic or aluminum case enclosure. Similar to what was described above, when assembling this case and the PCB there is a small amount of movement in the position of the PCB relative to the case. For example, if there is distinct vignetting only on the left side of the image, try unscrewing the PCB from the case, push the PCB farther to the opposite side, and screw the board back down.