  The reaction gamma + p -> K+ + Sigma + pi was used to determine the invariant
mass distributions or "line shapes" of the Sigma+ pi-, Sigma- pi+ and Sigma0
pi0 final states, from threshold at 1328 MeV/c^2 through the mass range of the
Lambda(1405) and the Lambda(1520). The measurements were made with the CLAS
system at Jefferson Lab using tagged real photons, for center-of-mass energies
1.95 < W < 2.85 GeV. The three mass distributions differ strongly in the
vicinity of the I=0 \Lambda(1405), indicating the presence of substantial I=1
strength in the reaction. Background contributions to the data from the
Sigma0(1385) and from K^* Sigma production were studied and shown to have
negligible influence. To separate the isospin amplitudes, Breit-Wigner model
fits were made that included channel-coupling distortions due to the NKbar
threshold. A best fit to all the data was obtained after including a
phenomenological I=1, J^P = 1/2^- amplitude with a centroid at 1394\pm20
MeV/c^2 and a second I=1 amplitude at 1413\pm10 MeV/c^2. The centroid of the
I=0 Lambda(1405) strength was found at the Sigma pi threshold, with the
observed shape determined largely by channel-coupling, leading to an apparent
overall peak near 1405 MeV/c^2.
