Projection optical system

A projection optical system comprising, from an enlargement side to a reduction side:

• a first lens unit having a negative optical power and including only a negative lens element, the first lens unit having at least one aspherical surface;
• a second lens unit having a positive optical power and including at least one positive lens element;
• a third lens unit having a positive optical power and including a lens element;
• and
• an aperture diaphragm disposed at a front focal point of the third lens unit,
• wherein the lens element of the third lens unit fulfills the following conditions:
• 0.015<Θ−(0.644−0.00168·νd)<0.06
• 65<νd<100
• where,
• Θ=(ng−nF)/(nF−nC)
• νd=(nd−1)/(nF−nC)
• ng represents refractive index for g-line (wavelength is 435.84 nm);
• nF represents refractive index for F-line (wavelength is 486.13 nm);
• nd represents refractive index for d-line (wavelength is 587.56 nm);
• and
• nC represents refractive index for C-line (wavelength is 656.28 nm).