  The Berkovits formulation of open superstring field theory is based on the
large Hilbert space of the superconformal ghost sector. We discuss its relation
to the Witten formulation based on the small Hilbert space. We introduce a
one-parameter family of conditions for partial gauge fixing of the Berkovits
formulation such that the cubic interaction of the theory under the partial
gauge fixing reduces to that of the Witten formulation in a singular limit. The
local picture-changing operator at the open-string midpoint in the Witten
formulation is regularized in our approach, and the divergence in on-shell
four-point amplitudes coming from collision of picture-changing operators is
resolved. The quartic interaction inherited from the Berkovits formulation
plays a role of adjusting different behaviors of the picture-changing operators
in the $s$ channel and in the $t$ channel of Feynman diagrams with two cubic
vertices, and correct amplitudes in the world-sheet theory are reproduced.
While gauge invariance at the second order in the coupling constant is obscured
in the Witten formulation by collision of picture-changing operators, it is
well defined in our approach and is recovered by including the quartic
interaction inherited from the Berkovits formulation.
