Since the ISO value directly determines how noisy an image is and apertures are fixed on smartphones, leaving the shutter open longer is your only option for getting cleaner images at a given exposure.
The longer the shutter speed, however, the more movement will be blurred, so this technique is best for static subjects – unless you want motion blur for artistic effect.
Theoretically, a long enough shutter shutter speed could result in nighttime photos as noise-free as those taken during the day. In fact, extending the shutter period is exactly how optical image stabilization works, which brings us to the next point…
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