>>3547391>>3547388A digital camera does it all for you, you adjust the minutes of exposure until the needle or menu says the exposure will turn out correctly.
For film I personally use two apps, an app called Light Meter (it has a little icon with f16 on it) and another called Exposure Assistant. I use the Light Meter app to take a few spot meters around the scene and average those out in my head. Then I plug the suggested exposure time into Exposure Assistant. That works out how much longer you have to wait due to failure reciprocity. Basically each film has an amount of light it can take before the effect needs to be extended, a failure rate. Typically lower ISO film has a better reciprocity failure rate which means less waiting around.
In terms of settings, on both digital and film I choose low ISO. Basically as low as I can go without the exposure time skyrocketing. Then I use as small an aperture as possible, also keeping in mind how long I want to wait. So most long exposures I do are like ISO 100 f16 8 minutes. I keep it that long because I'm often standing on a street corner the whole time and frankly don't want to be fielding questions or battling drunks as I wait. If you have the opportunity to wait longer you'll have better images, especially if you add an ND filter on top to extend it further.
Colour film usually turns out better at night because the contrast between light and shadowed areas really stands out. I used black and white because it's what I like but I know these photos would look "better" in colour.
