Night scene multiple stacks: Photoshop alignment, Mean and Median workflow

When is it appropriate to stack multiple photos?

Photoshop Image Stacking combines a set of photos with similar reference angles but slightly different quality or content; Adobe lists uses such as reducing noise/distortion and removing incidental objects from a set of still photos. The content of the images needs to be similar enough so that the software can register the layers. Moving subjects, obvious perspective changes, or widely varying frames may make the composition less than ideal, and the results should be checked with a group photo first. [G-STACK-01, G-STACK-02]

Import photos and try to align them automatically

In Photoshop, select File > Scripts > Load Files into Stack to add the photos to be processed; check Attempt To Automatically Align Source Images if necessary. Adobe explains that image stacks containing at least two layers are more conducive to registration if the assets are of the same size and have similar content. Not every set of photos can be automatically aligned successfully. After processing, you should check whether the edges of the subject and the background are ghosted. [G-STACK-02, G-STACK-03]

Create Smart Object and select stack mode

After selecting all layers, go to Edit > Auto-Align Layers, then create a stack through Layer > Smart Objects > Convert to Smart Object; then select the mode from Layer > Smart Objects > Stack Mode. Adobe recommends Mean or Median for noise reduction; if you're trying to remove the occasional object in a still burst, try Median. Different photos may need to compare mode results. 〔G-STACK-03〕

Preserve adjustable original stack

Photoshop's image stack is saved as a Smart Object. It can be re-rendered when switching Stack Mode, and the original stack image is still retained; this mode application is non-destructive editing. Save the composition when you are satisfied; to retain room for subsequent adjustments, keep the file containing the original layers, not just the rasterized final image. 〔G-STACK-04〕

Review with end use instead of chasing fixed framerates

First observe whether there are ghosts in the aligned photos, and then compare the noise, details and motion areas output by Mean / Median; judge whether it meets the needs according to the final viewing size or printing size. Adobe's instructions don't give a fixed number of shots, ISO equivalents, or quality improvement ratios that will apply to all handheld night scenes, so test shots should be based on your current scene and stacking isn't considered a universal alternative to tripod long exposures.