You may have heard the term ‘stacking’ in relation to image editing before, perhaps most commonly in reference to astrophotography, where several long exposures are merged together to reduce noise and composite star trails for a more exciting result.
There are many uses for stacking besides photographing the skies, so let’s jump in and demystify this term.
What does stacking do?
Stacking takes a series of images (sometimes aligned beforehand) and averages their contents, looking for changes between the images. How it performs the averaging is determined by a mathematical operator—most commonly, this would be Mean or Median (remember those from Maths at school!?).
That’s how it works technically—its practical use still remains as clear as mud, so let’s put stacking into practice:
Sometimes when shooting a scene, there are people in your composition and there’s nothing you can do about it. You could come back another time, of course, but that may be impractical and the lighting may have changed. However, taking several shots of the same composition allows you to remove everything that changes between shots; in most cases, the people.
This technique does rely on your obstructions moving between shots. If you have static subjects, like people sitting or loitering, they likely won’t move enough between each shot to remove them effectively. This method of stacking is best applied to busy scenes where there is a constant hustle and bustle of pedestrians and traffic.
Object removal stacking is a very similar principal to aligning your images and selectively masking areas to remove unwanted elements. Performing a Median operation on the images will achieve a similar result whilst cutting out the manual work involved.
Digital sensor noise is sometimes unavoidable and mostly undesirable. Low light photography, unless shot with a tripod, requires high ISO values and large apertures to achieve suitable exposures, and that means noisier images.
Noise performance benefits as sensor technology improves over the years. Despite that, noise can prove tricky to remove. Noise removal algorithms do a very respectable job at cleaning it up, but there is still a loss of detail, noticeable in the ‘waxy’, smooth appearance of subjects that should have fine detail.
Sensor noise is random—it’s never the same in each frame. Instead of taking one shot, we can take a burst of shots and stack them. When we average those shots, we are identifying and averaging anything that changes from shot to shot, which includes the noise. By doing this, we are increasing the signal-to-noise ratio, and can therefore more accurately reconstruct fine detail in the image.
Exposure merging is not HDR, nor is it a substitute. However, by stacking together different exposures of the same composition, averaging them can give the appearance of more dynamic range in the image. This provides a great starting point to do some further work on the image with more tonal range to play with.
Long exposure simulation
An unexpected but surprisingly effective use of stacking is to simulate long exposure photography. Producing good long exposure photography requires certain equipment; notably, a form of stabilisation (such as a tripod) and neutral density filters to reduce the amount of light entering the lens, thus allowing slower shutter speeds.
In the event of factors such as forgetting to bring these pieces of equipment, not being able to budget for them, or simply not wanting to carry them around, we can take several shots of our intended long exposure subject (typically water) and average between them. The effect, while not as striking, is not too dissimilar to that of a genuine long exposure.
Maximum and Minimum operators average the greatest and smallest RGB values respectively, and they work best on images that either have a dark background and bright subject (Maximum), or a light background and dark subject (Minimum). In the above example, several shots of a bonfire shot against a dark background are averaged using the Maximum operator, which results in the brightest areas of each image being combined to produce a more powerful-looking image.
Stacking in Affinity Photo
Affinity Photo offers an intuitive stacking feature (via File>New Stack) that automatically aligns a series of images and creates a stacked group with them. Changing the stacking operator for different results is a simple case of changing the method from a dropdown on the Layers Panel.
We have a selection of video tutorials that cover stacking: