The aperture diaphragm is not opened wide enough The condenser is too low The objective does not belong to this microscope The front lens of the objective is dirty An immersion object has been used without immersion oil The immersion oil contains air bubbles The condenser is not correctly centred The recommended immersion oil has not been used Dirt / dust on the objective Dirt /dust on the front lens of the condenser Dirt / dust on the eyepieces Dirt / dust on the front lens of the condenser Dirt / dust on the object The stage was not correctly fitted The objective is not positioned correctly on the beam path The nosepiece is not correctly fitted The upper side of the object is facing down The nosepiece is not correctly fitted The objective is not positioned correctly on the beam path The condenser has not been correctly centred The rotational resistance brake is too tight The table is blocked by a solid body The rotational resistance brake is not tight enough The stage was not correctly fitted