mikeber,
This is an area of photography that I did years ago with film, attempted recently with my Sigma SD14 dSLR with varying success, and one of the motivations behind returning to film and getting into large format.
No scene will ever contain total darkness. Whether reflected from lights in the area or even from the back ground glow of a light polluted night sky, given a sufficiently long exposure, your camera will record light from every point in the scene.
Long exposures which give a sense of a day lit scene will always carry a surreal look of night simply because of the lights from windows and street lighting.
A 30 minute exposure precludes digital technology, at least when performing the exposure with a single long exposure. I do not know of many cameras where the electronics allows for an exposure lasting more than a few minutes.
Alternatively, you can achieve long digital exposure in software by the technique of image stacking. This is commonly done in both professional and amateur astronomical photography. The process averages out the noise and enhances the imaged light by mathematically combining multiple digital exposures. Whether you go with digital or film, you may want to research these software packages to help you decide:
RegiStax
DeepSkyStacker
AIP4Win
I have all three and my best results have been with DeepSkyStacker. As long as it can identify "stars", small point sources of light surrounded by darkness, it will align and stack anywhere from a few to hundreds of exposures. ( Some amateur astronomers use web cam video cameras connected to a telescope and combine the stack of video frames. )
I recommend that you start out with film. The noise that you can be plagued with in low light digital photography can be really discouraging.
Film also has an advantage over digital in that you can perform a pre-exposure, exposing the whole sheet to a uniform low intensity light to move the starting exposure out from the initial nearly horizontal region of the exposure versus density curve. Done carefully will effectively increase your film's sensitivity.
Then don't. An easy way to eliminate focusing in low light is to set up your camera a couple hours before it gets so dark that you could no longer focus. This will allow you all the freedoms afforded by a large format camera. Another alternative will take some math. Calculate your lenses hyperfocal distance. When you are setting up your camera, set your lens to focus to the hyperfocal distance and then everything from 1/2 the hyperfocal distance through to infinity will be focused.
I hope this helps you pick a direction.
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