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My microscope images always have a circular pattern on them. What am I doing wrong? 

Is the LM digital adapter faulty?

The phenomenon you are describing often occurs when using digital cameras with fixed, inbuilt zoom lenses. The reason for this has nothing to do with either the microscope itself or the LM digital adapter that is being used. It is far more likely to result from small artefacts on the lens of the camera, which are caused when the lenses are being produced. In general photography and macrophotography, these flaws are not visible; they can only be seen in parallel light paths in optical instruments.

Often, this phenomenon only appears after a certain amount of time. Sometimes plastic lenses are used in compact cameras, which are aged by UV light and other environmental influences. Since it is unfortunately not possible with compact cameras to exchange the lens, there is also no possibility to improve the image quality. The only thing that can be tried is to see whether a shorter focal length of the camera lens lessens the visibility of this circular pattern.

For professional work in microscopy we therefore recommend using digital single lens reflex cameras with Live View function.

A detailed discussion of this subject can be found at:
http://www.microscopy-uk.org.uk/mag/imgjan04/coolpix-artefact-report.pdf [ Artefacts in Photomicrographs caused by Nikon Digital Cameras, Models Coolpix 990, 995 and 4500]