In this paper, we report experimental results on laser beam guiding by a self-tightening lattice photo induced in a strontium barium niobate (SBN) photorefractive crystal. In such setting, optical lattice (or periodical array of strongly-coupled waveguides) gradually transforms into array of weakly coupled ones. Experimental data are supported by computer simulation, which revealed crucial role of induced modulation instability in the strong laser beam guiding.