Imaging optical system and optical apparatus including the same

An imaging optical system configured to form a conjugate relation between a conjugate point on an enlargement side having a long distance and a conjugate point on a reduction side having a short distance, the imaging optical system comprising:

• in order from the enlargement side,
• a first lens unit having a negative refractive power;
• a second lens unit having a positive refractive power;
• a third lens unit having a negative refractive power;
• and
• a fourth lens unit having a positive refractive power, wherein
• the first lens unit includes a first negative lens positioned closest to the enlargement side and a first cemented lens positioned closest to the reduction side,
• the second lens unit includes, in order from the enlargement side, a second cemented lens, one or more positive lens components, and a third cemented lens,
• the lens components form a lens block in which only an enlargement-side surface and a reduction-side surface are in contact with air in an optical path,
• the first cemented lens includes a negative lens closest to the reduction side and has a surface closest to the reduction side having a concave surface facing toward the reduction side,
• the second cemented lens has a positive refractive power and has a surface closest to the enlargement side having a concave surface facing toward the enlargement side, and
• the third cemented lens has a positive refractive power and includes a negative lens on the reduction side.