Black and white conversion
A black and white conversion removes the color from your photo, but lets you control how each color translates to a grey tone. You can make a red sweater light or dark grey, separate from the sky or grass. Good black and white is about that tonal control, not just setting saturation to zero.
Use the black and white panel (or the B&W mixer) to steer brightness per color: darken a blue sky for drama, or lighten skin for a soft portrait. Setting saturation to zero often gives a flat result, so really work with the color sliders. Then strengthen with contrast, an S-curve and maybe grain for a filmic look. A light split toning can give your black and white that warm or cool edge.
Read also the in-depth explanation: Black and white conversion explained.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.