Photar, thanks for the reply. There are three other prisms that I know of: The original "PD Prism" finder which had manual centre-weighted Silicon Photo-Diode cell which went from 8s to 1/500th and the "PD Prsim Finder S", which goes up from 8s to 1/1000th. There's an Aperture-priority only Prism as well, but I think one has a cds cell.