Electron multiplying CCD cameras can respond to single photons with > 90% quantum efficiency [1] -- that's better than rhodopsin. (I use one on a semi-daily basis.) To take advantage of that sensitivity you need to cool the sensor to -80°C so that thermal noise doesn't swamp your signal, but obviously you can't do that with your eyes.

[1] http://www.andor.com/learning-academy/ccd,-emccd-and-iccd-co...