raw - Can 30 1-second exposures be combined to a form the equivalent of a single 30s exposure?

I need to take 30 1s exposure shots in raw at night. The image becomes ALMOST black... Is it possible to take this tiny amount of light/information and sum them all toghether and get as a result a single image that equals a longer expoaure? I am not talking about brightening an image, I am talking about summing the single light information of the pictures together. Would that be possible in photoshop? Would the result be any good?

Answer

Fundamentally you're capturing the same amount of light in either case so the results should be the same.

Practically, there are 2 differences between stacking 30 one second exposures and shooting one 30 second exposure. The first is the light lost between each one second exposure after the shutter closes before it reopens for the next exposure. This can cause problems with light trails of fast moving objects (such as cars), but is otherwise unlikely to cause significant problems.

The second is that doing 30 one second exposures requires 30 reads from the sensor, so you will get 30x the read noise (errors that are picked up as the signal is read off the sensor before it is converted to a digital value).

The fact that you state each exposure is "almost black" suggests that this could be significant problem, if each individual exposure isn't bright enough to rise above the read noise then your image stack will just contain noise, whereas a 30 second exposure would be brighter than the read noise.

There are some things you can do in this situation, push the ISO up as high as it will go (disregard what you have been told about increasing ISO causing noise, it doesn't apply in this case). Read noise occurs after ISO amplification, so amplifying the signal raises it above the read noise.

You could switch to a camera with lower read noise, the latest sensors from Sony (which have found their way into many camera brands) are the leaders in ultra-low read noise.

Finally you could increase the brightness of each exposure by using a wider aperture lens, or switching to say, 15 two-second exposures, or 10 three-second exposures.