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This effect creates darker somber appearance through intentional underexposure, and emphasizes the highlight. Color hue can be adjusted.
Here are some examples
Even the same route home looks like a different place when I press shutter.
I found a swaying sparkling heart while out for a walk and captured it with Low Key filter to pick up the light beautifully.
The black silhouette with the sky in the back gives a deeper impact with the Low Key filter.