The authors present the results on a new sample of faint, optically selected quasar candidates. Two different search techniques (multicolour and grism) have been applied to the same field of 0.69 square degrees. The cross-check of the two lists of candidates allows to estimate the level of completeness of each technique. The authors find about 50 quasars per square degree with J < 21.0. In the magnitude range J = 21 - 22 the results are in very good agreement with those of other existing surveys.