Long exposures are as old as photography itself and used to properly expose a dark scene, using very slow shutter speeds. Image-stacking, on the other hand, is a technique used in digital photography and has the very technical scope of improving the digital signal-to-noise ratio.

With image-stacking, we refer to the idea of averaging a photographic sequence. Differently than shooting for doing HDR or focus-stacking, with image-stacking, the camera settings and the focus stay the same for the entire image sequence.

The result is a cleaner and more detailed image

The result is a cleaner and more detailed image, as details that were “diluted” in the background noise in the single images now stand above it in the stacked image.

The more images you stack, the cleaner your final results are, but the improvement of the signal-to-noise ratio is not linear with the number of images. Instead, it goes as the square root of the number of images you stack.

For night-time photography and astrophotography in particular, image-stacking is an absolute must to reduce noise while revealing better details in your images. Unfortunately, the benefits of image-stacking are not something you can appreciate directly in the field, as the magic is done during post-processing.