Sociologists use a number of different types of observation in their research. They can be participant or non-participant and can be covert or overt. Overt observation is where those being observed are aware of the fact. The researcher may still participate in the activity being observed (overt participant observation) or might play no part and simply observe (overt non-participant observation).
This is the most ethical form of observation, as it requires no deception and participants are able to give their informed consent.
However, this form of observation is the method most at risk of the Hawthorne Effect. When people know that they are being observed they behave differently (consciously or unconsciously). However the researcher can still impact what is being observed if it is covert, if they themselves are participating.