I made some samples. For all images I made the standard Groundprop all black (so no indirect light from this one).

One Spotlight, 300%, no HDRI

Same Spot, HDRI intensity 0.05

Same Spot, HDRI intensity 0.10

HDRI Background

The "hsv2 value" is the intensity (mentioned above). Intensity depends on the image used. Saturation is reduced to zero, so the image has no colour information (b/w). For special effects one may let the colour information alone. Also worth checking out: False colours.

Remember that the ground prop should be all black. The background shouldn't bounce light (unless one wants to achieve special effects).

With this method you get fast renders with just one key light.

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