TUTORIAL
Long Exposures
(excerpt from the book Stephen Johnson on Digital Photography unreleased revised electronic version)
Long exposures, at night for star trails, or simply because of low light, once meant simply the bulb setting (or T) on the shutter, and some calculations (guesstimations) of reciprocity failure exposure compensation, multiple tries and faith. Absolutely beautiful work could be done, but it took practice and patience–of course, most photography does.
Nowadays things are different, but challenges remain. Silicon builds up noise with long exposures, with heat being one of the problems. In astrophotography this is battled back by cooling circuits on the cameras, or even liquid nitrogen for the big guys.
Many of our cameras now have long exposure noise reduction modes which very cleverly takes a photograph subsequent to your exposure, of the same length, with the shutter closed, thus producing a noise map of the sensor. That so -called dark current image is then subtracted from the image. These features are well worth using.
Additionally, lower resolution dSLR cameras tend to be more sensitive as their pixel wells are larger, therefore have more silicon per pixel to gather photons. I've seen the difference between a 6 megapixel dSLR and a 22 megapixel camera, at the same ISO, aperture and time, reveal the Milky Way in the lower res photo, and barely see stars in the other.
Focusing can also be a real problem on low light photography. A green laser can sometimes help by providing a bright pinpoint of light to focus on when pointed at your subject.
Star Trails and Crosses (combined from 3 different exposures). Mission San Antonio. 2011
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