Small problem with the lens
The phenomenon shows up on Canon 5D Mk III and 1Dx cameras only. T4i and 7D bodies with firmware upgrade do NOT do this, nor do any other older cameras we could test.
The Sigma 180 OS and Sigma 150 OS macro lenses both show the effect and its identical. No other Sigma lenses that we stock showed the effect, nor did any Tamron or Tokina lenses. Obviously I cant test what we dont carry.
If you turn off Illumination Correction in the menu the effect goes away. To repeat, though, Illumination Correction in the 7D and T4i, on or off, doesnt cause the effect.