We present a tight-binding study of the strong quantum confined Stark effect (QCSE) involving direct transitions in quantum wells. Our aim is to provide a theoretical and numerical description of the experimental results by Kuo [Nature 437, 1334 (2005)] by means of a tight-binding model. In the presence and in the absence of external electric fields, we are able to assign of the states involved in the observed transitions. Oscillator strengths for normal and parallel incident radiation are evaluated. In particular, the genuine direct transitions in the Ge region, at the point, and the direct transitions coming from states along the lines folded at , are discriminated; their energy shift as a function of a superimposed field is evaluated, and their role in the QCSE is evidenced. Excitonic effects below and above the interband threshold are also included in our calculations; they contribute to a close reproduction of the experimental absorption spectra for different superimposed uniform electric fields.