Canon patent for EF 28-70mm F2.8 USM lens

Not sure why Canon filed that patent two years ago, since the old Canon EF 28-70mm f/2.8 L USM lens was replaced by the EF 24-70mm f/2.8L USM.

The details:

  • Patent Publication No. 2010-145831
    • published 2010/07/01
    • filled 2008/12/19
  • Embodiment 3
    • focus distance f=28.7-68.00mm
    • Fno=2.92
    • angle of view 74.02-35.30
    • image circle 43.28mm
    • lens length 177.91-168.98mm
    • back focus 46.71-75.33mm
  • Zooming, there are few changes of distortion
  • Retrofocus optical in wide angle
  • Tele-photo optical in tele-photo
    • short full length
  • Distortion is big in wide angle
    • astigmatism turns worse if it use the aspherical where the quantity is big
    • problem solving by a certain a lens was arranged in symmetry, and denying the distortion
  • Use aspherical
    • improve distortion
    • Because it is not necessary to make non-spherical surface quantity big, the astigmatism does not turn worse
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