  Unavoidable variations in size and position of the building blocks of
photonic crystals cause light scattering and extinction of coherent beams. We
present a new model for both 2 and 3-dimensional photonic crystals that relates
the extinction length to the magnitude of the variations. The predicted lengths
agree well with our new experiments on high-quality opals and inverse opals,
and with literature data analyzed by us. As a result, control over photons is
limited to distances up to 50 lattice parameters ($\sim 15 \mu$m) in
state-of-the-art structures, thereby impeding large-scale applications such as
integrated circuits. Conversely, scattering in photonic crystals may lead to
novel physics such as Anderson localization and non-classical diffusion.
