When running an HDMI or DVI cable, you have the choice of using copper or fiber optics. A similar debate has raged for years when it comes to digital audio, but in digital video things are different:

However, full motion uncompressed digital video signal is a different story. Due to the nature of digital signals and impedance of copper, fiber optic conversion technology is the best connection medium for DVI and HDMI signal extension. … Typically, 1080p signal would mean 1920×1080 pixels refreshing over 40 times per second. At this high rate of speed and large bandwidth of data, impedance caused by copper begins to cause signal loss - which results in digital artifacts, pixilation and/or no picture (blue screen) - if the distance is longer than 15 feet. 720p and 1080i are better off with fiber optic connections if the distance is longer than 25 feet.

Link: Projector Central - HDMI/DVI - Copper or Fiber Optics.