An optical system includes a front lens unit, a first focusing unit having a negative refractive power, and a second focusing unit having a negative refractive power in this order from an object side to an image side. The optical system is a single focus optical system. When a focus is shifted from an object at infinity to an object at a short distance, the first focusing unit and the second focusing unit move to change a distance between the first focusing unit and the second focusing unit on an optical axis.

A projection optical system that projects, while enlarging, an image displayed on an image display surface includes in order from an enlargement side: a first negative refractive power lens group; a second negative refractive power lens group; at least one positive or negative refractive power lens group; a stop; and a first positive refractive power lens group. The projection optical system causes magnification variation by varying distances between lens groups. The at least one positive or negative refractive power lens groups disposed between the second negative refractive power lens group and the stop is movable during the magnification variation. The first positive refractive power lens group is an only lens group that is positioned closer to a reduction side than the stop and is also movable during the magnification variation.

An optical system includes a front lens unit, a first focusing unit having a negative refractive power, and a second focusing unit having a negative refractive power in this order from an object side to an image side. The optical system is a single focus optical system. When a focus is shifted from an object at infinity to an object at a short distance, the first focusing unit and the second focusing unit move to change a distance between the first focusing unit and the second focusing unit on an optical axis.

A projection optical system that projects, while enlarging, an image displayed on an image display surface includes in order from an enlargement side: a first negative refractive power lens group; a second negative refractive power lens group; at least one positive or negative refractive power lens group; a stop; and a first positive refractive power lens group. The projection optical system causes magnification variation by varying distances between lens groups. The at least one positive or negative refractive power lens groups disposed between the second negative refractive power lens group and the stop is movable during the magnification variation. The first positive refractive power lens group is an only lens group that is positioned closer to a reduction side than the stop and is also movable during the magnification variation.