Aperture Range

• The most obvious difference is that the 1.2L II has an aperture range of f/1.2-16. The use of a larger aperture results in a blurry background. The capability to go to f/1.2 is an extremely rare feature in lenses. This large aperture allows a photographer to use a slower shutter speed and also create a blurred background, which draws focus to the subject of the image. The 1.8 lens has a range of f/1.8-22. The maximum aperture of f/1.8 is still considerably large compared to most lenses.
  
  

Build Quality

• The 1.2 lens is an L-series lens, the "L" standing for luxury. This means a much tougher build quality as well as the use of higher quality glass. The L-series is the best than Canon offers and is preferred by professionals.
  
  

Chromatic Aberrations

• The 1.2L II features special glass coatings to reduce chromatic aberrations. The 1.8, due to poorer build quality, suffers from more noticeable chromatic aberrations compared to the 1.2L II.