Artefact or object on microscope image

I have many microscope image of water in oil emulsion, most of them have a lot of halo like objects have don't have a firm black boundary. The ones with firm boundary are water drops dispersed in continuous oil medium. Any thoughts on what those halos might be? Is it an artefact? I use Zeiss Axioscope light transmission microscope with Axiocam Icc camera(1.4 MegaPixel) for image capture.