The method of summing up several pictures of the same scene is conceptually simple. Just take as many photographs as necessary so that when summing them up, the scene will be lighted correctly. The light of each photo will add to that of the others. By adding an appropriate number of such pictures, the photographer can get a final image properly exposed.
We can distinguish between two macro-cases. In the first one, we are taking a picture exploiting the already present natural ambient light. In the second case, we provide the main lighting. In the first case, the proposed method offers several advantages, but, in the second case, the benefits are even more.
Just Ambient Light
The first real macro advantage is that, due to thermal noise in CCDs, still digital cameras simply cannot offer arbitrarily long exposure time. That is why the authentic “B” pose does not exist in digital cameras. Therefore, the proposed method overcomes the problem of very long exposure, impossible with digital cameras.
Another advantage in using the proposed method is noise reduction. Indeed, at least a part of noise is stochastic, which means that it can add to or subtract from the signal. If we sum many noisy images, we make a sort of average, so reducing noise.
A third advantage is we can dismiss faulty images. In the sequence of images taken, some may be blurred by micro-movement of the camera (e.g. due to the wind) or imperfect in other ways. For instance, an airplane or a car passing by may leave an unwanted path of light. There could be zillions of reasons why an exposure could be faulty. If we take just one long exposure, we will end up with a useless picture. However, if we take a lot of them, keeping all of them short, we can dismiss some of the shots at the editing phase. This is even more so if the exposure requires some photographer’s actions, like firing sequences of flashes.
A fourth benefit arises from the powerful capabilities of image editing software. As an example, we can mask unwanted parts in some of the shots. This could be useful, for instance, in order to avoid saturation of the brightest parts of the image. This way, the lighting will be more uniform. Alternatively, we could do this creatively by dimming some portions of the final image purposely. Control over the brightness of the single parts of the image is much stronger.
We could even extend this technique to normally lighted daytime shots, taking advantage of the same benefits. In that case, each single shot would be extremely short (1/500s or faster).
We Provide the Main Lighting
If the photographer is providing the lighting of the scene being photographed, all the previous advantages still apply. Others can be listed, however.
Lighting equipment can be drastically reduced, thus reducing cost and ease of transport. It is not necessary having many lights lighting the scene simultaneously. This time we can shoot each picture with just one light at a time. They will be added during the editing phase. So, we need just one lamp for each type of lighting equipment being used (e.g. just one soft box instead of two). Moreover, the lights being used do not need to be very powerful, because, again, summing their effects via software we can achieve any brightness we desire.
In case of artistic “painting with light”, during the retouching session the photographer has a very wider range of freedom. He can easily try different compositions, perfectly matching the different lights as desired. This will enhance his creativity. For instance, if using two colored lights, the number of possible combinations is countless. In this same example, precision reached through an image retouching tool, summing the different images each with its own light, is unattainable simply dosing the light during the exposure.
Of course, this method has disadvantages, too. It is obviously the extra post-processing work with the editing software. In particular, the registration phase, i.e. the alignment of all the pictures to be added, is cumbersome. However, there are specific tools doing that automatically. Moreover, although it is true that we can try different combinations by dismissing some photographs or others or masking them in different ways, this may take a lot of time, too. On the other hand, this is a price for greater freedom.