Modulation instability and pattern formation by spatially incoherent light is investigated experimentally in a nonlinear ring cavity using a photorefractive strontium barium niobate crystal as the nonlinear medium. A step-like threshold for the onset of pattern formation is observed experimentally for the case of high optical feedback. When compared to the case without feedback, this threshold is shifted towards smaller nonlinearities and a significant increase of the modulation degree of the obtained patterns is obtained. Our measurements also show that, above threshold, the dominating spatial frequency of the patterns decreases monotonically with both increasing nonlinearity and increasing feedback.