Ghosts from the past
If you are new to photography, chances are that you have never heard of a strange effect called reciprocity failure.
If, instead, you are an old-timer from the film age, then you will be very pleased to learn that the annoying effect called reciprocity failure does not exist in digital photography.
Quite simply, digital sensors do not suffer from reciprocity failure.
Just to let everybody understand what I am writing about: reciprocity failure is a weird effect, resulting in underexposure and color shift, which affects (not only) film photography and is particularly apparent when shooting images with very long (or very short) exposures.
You can find more about reciprocity failure at wikipedia.org.
Good news
Yes, you read it right: no reciprocity failure in digital photography.
This is good news for those of us who like night shots as well as pinhole photography. We can now reliably calculate and set exposure duration even when it is much longer than one second or one minute: neither underexposure nor color shift is expected with digital long exposures.
Also, we can now shoot stunning photos of stars with much ease.
All that glitters is not gold
Still, digital sensors are not immune to problems arising when exposure duration is in the range of many second, or minutes, or hours. There is a broad range of technical issues with digital sensors which are not solved yet and which generate what is generally known as noise.
Photographs taken with long exposure times suffer from a peculiar issue known as dark current noise: this is a typical fixed-pattern noise and its effect is linear with time, so it can be subtracted out with a blank frame of the same exposure duration as the actual shot. Such a trick, which once required post-processing tools (say Gimp, Photoshop, or even some ad hoc software), is now performed in-camera by the vast majority of the most modern cameras, once you select the long exposure noise reduction feature.
If you like night photography you might well be interested in some of these books:
