Can anyone explain what exactly happens with light waves as the pass through a maxed out variable ND filter that causes the X pattern to appear? Not that it matters much in practice but I'm curious.

  • Theory: The polarizing filters also act as diffraction gratings... – rackandboneman Jan 10 at 10:02
  • related: photo.stackexchange.com/questions/63940/… – jhamon Jan 10 at 10:38
  • Coating a plate glass with paint of some description, then scratching fine lines into the paint, makes an interesting though very dark sunstar filter... a polarizer might be similar in construction... – rackandboneman Jan 10 at 21:20
  • The two polarizers are not aligned perfectly, or the glass between them causes some rotation of the light, so that complete extinction is not possible. BTW, check the quality of photos taken with variable ND filters: the many layers of plastic polarizer, glass and air causes considerable degradation. – DrMoishe Pippik Jan 11 at 1:59

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