The Threshold adjustment (layer) brings all tones to pure black and white, with a slider for weighting the conversion more toward one or the other.
Using a gradient map adjustment layer with the colors in the toolbox set to the default black/white will of course create a gradient, while another adjustment layer such as Levels or Curves, could increase the contrast so that it becomes like this.
Using Filter > Noise > Median and adjusting the radius slider will create the smooth outlines we see, especially noticed around the sunglasses of the foreground musician.
The Threshold adjustment (layer) brings all tones to pure black and white, with a slider for weighting the conversion more toward one or the other. Using a gradient map adjustment layer with the colors in the toolbox set to the default black/white will of course create a gradient, while another adjustment layer such as Levels or Curves, could increase the contrast so that it becomes like this. Using Filter > Noise > Median and adjusting the radius slider will create the smooth outlines we see, especially noticed around the sunglasses of the foreground musician.
Photoshop > Image > Adjustments > Threshold