One of my favorite techniques of photography is "long exposure", night photos that take minutes or more to complete. The usual problem with these is picking the exposure.
Built-in luxmeters of most cameras will give you a flat zero or a noise around it, and even if they don't, that gets seriously biased by point light sources that will remain similar bright dots whether I set exposure to 30 seconds or to 10 minutes. It's the dark background that matters and that I want to be visible.
Currently, I'm working by trial and error, which is problematic considering 3-4 tries to get the exposure just right, each try taking 15 minutes or so.
I wonder though, are there any fault-proof (and not budget-destroying) methods of sure-fire adjustment of shutter time for night photos that don't take hours? For example cameras / firmware hacks, where you could obtain "exposure progress" to show? Like, I watch the screen, where the image gets gradually brighter following the light gathered by the lens, then I decide "it's enough", press the remote button and the camera finalizes making the photo?