Why need that? I was thinking about setting aperture and using the bulb mode. In bulb pykake can control the exposure time for every image in the bracketing. So just input aperture and iso in pyKake and some algorithm will find out the exposure times and then using buld for every shot. pyKake would use the N900 light sensor for calculating the exposure time. There is already a light meter there for N900: http://maemo.org/packages/view/luxus/