  The theoretically observable limit of electron density distribution by
single-crystal X-ray diffraction is discussed. When F_{orb} and {\delta}F are
defined as, respectively, the partial structure factor for an orbital and the
deviation of the observed F from the true F, the accuracy of electron density
attributable to F_{orb} is chiefly determined by the number of reflections
satisfying the condition F_{orb}/F > {\delta}F/F. Since F_{orb}/F, which is
generally small for crystals with large F(0,0,0), is constant under a given set
of experimental conditions, {\delta}F/F must be reduced to increase the number
of reflections satisfying F_{orb}/F > {\delta}F/F. The present paper
demonstrates how to reduce {\delta}F mathematically and experimentally, and the
following topics are covered: the Poisson statistics, accumulation of errors in
the data collection and reduction procedure, multiple diffraction, conversion
error from F^2 to F in refinement programs, which is unavoidable when the input
quantities have different dimension from F, weighting of reflections, and tips.
For demonstration, observation of the electron density of the Ti-3d^1 orbital
in YTiO_3 by synchrotron single-crystal X-ray diffraction is presented.
