X-ray vs. optical redshift for the 35 sources with spectroscopic or photometric redshift (three sources do not appear since they have z_X > 1.2). Red symbols are for clusters whose redshift is found in the literature after inspecting the NED database (circles and triangles are for photometric and spectroscopic redshifts, respectively). Magenta squares are for clusters whose photometric redshift has been computed by us with a dedicated TNG program or with VLT archive images (one case). Error bars on z_X and z_opt correspond to 1σ.