Projection optical system, projection apparatus, and projection system

A projection optical system, comprising:

• a reflective optical system on a magnification side along an optical path of the projection optical system, the reflective optical system including at least one reflective optical element having a power;
• and
• a refractive optical system on a reduction side along the optical path, the refractive optical system including:
• a front group on the magnification side;
• and
• a rear group on the reduction side having a positive refractive power,
• wherein the front group includes a plurality of focusing lens groups that moves to at least one of the magnification side and the reduction side in a change in focus from a long distance to a short distance,
• the plurality of focusing lens groups include a first focusing lens group closest to the magnification side, a second focusing lens group second closest to the magnification side in the focusing lens group, and a third focusing lens group third closest to the magnification side,
• a distance between the first focusing lens group and the third focusing lens varies in the change in focus from the long distance to the short distance, and
• the rear group of the refractive optical system is fixed during the change in focus.