I have been shooting in color for years and converting to black and white as an afterthought. Click the B&W button in Lightroom, maybe drag the contrast slider, done. That is the wrong way to do it and my photos looked flat because of it.
Black and white conversion is not a filter. It is a choice about what tones map to what brightness levels, and if you let the software decide for you, you get muddy midtones and no separation.
Start With the Color Channels
Every color in your photo maps to a specific shade of gray when you convert. Red does not become the same gray as blue. This is the whole game.
In Lightroom, the B&W panel gives you eight sliders: Red, Orange, Yellow, Green, Aqua, Blue, Purple, Magenta. Each one controls how bright or dark that color becomes in the final grayscale image.
I used to ignore these. Now they are the first thing I touch.
Skin Tones Are Orange and Red
If you shoot people, the Orange slider is your best friend. Drag it right and skin gets brighter. Drag it left and skin goes dark and moody.
For portraits I usually push Orange up by 10 to 15 and pull Red up slightly. This separates skin from the background without making it look washed out.
If the background is green ( trees, grass, whatever ), drag Green down to darken it. Now your subject pops without any local adjustment brush work.
The Sky Is Blue ( Usually )
Blue slider down for a darker, more dramatic sky. Blue slider up for a softer, high-key look. I go down almost every time, maybe -20 to -40 depending on how much sky is in the frame.
If you shot on a cloudy day with no real blue in the sky, this slider does nothing. That is fine. Move on.
Contrast Comes After, Not Before
I used to crank contrast first, then convert. Wrong order. Channel mixing changes the tonal relationships. If you add contrast first, you clip highlights and crush shadows before the conversion even happens.
The order I use now:
1. Convert to B&W
2. Set color channel sliders
3. Adjust exposure and white balance ( yes, white balance still matters in B&W )
4. Add contrast
5. Clarity and texture for local punch
White balance in B&W sounds stupid but it is not. Shifting temperature warmer brightens warm colors in the mix. Shifting cooler brightens blues. It is another way to control tonal separation.
Clarity Is Not a Substitute for Good Tones
Clarity adds midtone contrast. It makes things look punchy and gritty. It also makes everything look like an Instagram filter from 2014 if you overdo it.
I use +10 to +15 clarity on most B&W shots. Sometimes +25 for architecture or street where I want hard edges. Never more than that.
Texture is the more subtle version. I use +10 texture on almost everything now. It adds micro-contrast without the halo effect clarity gives you.
darktable Does It Differently
If you use darktable instead of Lightroom, the module you want is called Monochrome. It has a color contrast section with red/cyan and blue/yellow sliders instead of eight individual channels.
It takes getting used to but I actually prefer it for quick edits. Two sliders instead of eight. Less fiddling.
The trick in darktable is to combine Monochrome with the Color Balance RGB module. You can shift individual color zones there before they hit the monochrome conversion, which gives you the same control as Lightroom's eight sliders but with more precision.
When to Actually Shoot B&W in Camera
Never. Shoot RAW in color always. You lose the color channels if you shoot B&W JPEG, and then you cannot do any of what I just described.
The only reason to switch your camera to B&W mode is to preview composition on the EVF or rear screen. Some cameras let you shoot RAW + JPEG where the JPEG is B&W and the RAW stays color. That is the only setting worth using.
Conclusion
Stop desaturating and calling it black and white. Spend two minutes on the color channel sliders and your B&W work will look like it was done on purpose for once.
:)