We report some results of a new analysis (Trèvese et al. 1993, ApJ (submitted) (T93)) of the variability of the faint QSO sample of SA57 (Koo, Kron and Cudworth, 1986, PASP, 98, 285), concerning the intrinsic time scales of variability and the dependence of the amplitude of variability on absolute magnitude and redshift. Prime focus plates of SA57 have been obtained with the Mayall 4-m telescope at the Kitt Peak National Observatory since 1974. The present analysis is based on 14 BJ plates spanning 15 years at 11 independent epochs. The digitization, object selection, image classification and photometry are described in Koo (1986, ApJ, 311, 651) and Trèvese et al. (1989, AJ, 98, 108 (T89)). The magnitude limit of the sample is BJ = 22.5 and stellar objects are selected with a threshold in image size. This criteria give a sample of 694 objects in the field, 34 of which are QSOs whose spectra have been measured with the Mayall 4-m telescope. The r.m.s. error in the BJ band is 0.05 mag at BJ Ã 22 mag. All but one of these QSOs appear to be variable according to T93.