A photography defect (unless it is intentional: in which case, it is a photography effect, possibly by a pretentious photographer).
Underexposure happens when the film (negative or dia) receives less light than what it should.
How much light should film receive ? Light metering tells you that, but in general film should receive enough light to form a recognizable image, if possible exploiting most of its full contrast range (if you have a contrasty scene, of course).

How to recognize underexposure:

  • color slide: looks dark, with no shadow detail and no clear highlights. Notice that slides can tolerate underexposure better than overexposure.
  • color negative: looks "thin", with little saturation. When printed, grain becomes very visible (that's because grain is in the film base, and it is not influenced by exposure), and the resulting print has little contrast.
    Before this effects become apparent, you have to be off by a couple of f stops.
  • black and white film: the negative looks characteristically "thin", with a lack of black areas and a prevalence of clear film base.
    Inspection with a loupe reveals absence of detail in shadow areas.

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