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. For a good summary of vignetting go to http://www.vanwalree.com/optics/vignetting.html#optical.
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 PL-B952, 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.
The most common causes of vignetting in microphotography and macrophotography:
- Optical axis of camera and microscope not in line
- Camera lens limits the light path
- No optical adjustment between the microscope and the camera
- An unsuitable projection lens is already integrated in the microscope’s photo tube or C-mount connection
- The optical system is off-centre