The key here is to understand that in all conver-sions, a particular colour (Hue) will be translated to
a specific grey value. A blue sky, by default, will be
converted to a medium to dark grey, skin tones to a
pale grey, green grass to a medium grey, yellow f lowers
to a light grey etc.
The question is: precisely which grey values do spe-
cific colours convert to? Controlling this is one of the
secrets of good B+W imagery, at least from a tonal
point of view - it’s still got to be an interesting photo!
To understand the question properly, you need tounderstand a little bit about how we see.
Computers work with numbers in a very literal
sense; we, on the other hand, subjectively ‘experi-
ence’ colours more than we objectively ‘measure’
them. This makes it difficult for computers to
emulate colour vision and there are a large variety
of what are known as colour appearance models
(CAMs) that cater for this mathematically. For our
purposes RGB is the most familiar since that’s how
digital cameras ‘see’ and it’s also similar to our own
colour vision - the ‘tristimulus model’.
There is another CAM that is more useful for
B+W conversions – the HSL model. That’s Hue,
Saturation and Lightness (a.k.a. the HSB model).
The power of this model lies in its capacity to sepa-
rate Hue from Lightness and allow adjustments of
them independently, something hard to do using
the RGB model.
For B+W conversions this means that we can
choose what greyscale value any particular Hue is
translated to by varying it’s brightness. For example,
a blue sky will convert to a value of grey, let’s say 50%.
If we change the Lightness of the blue Hues we can
translate the blue sky to different greyscale values so
that we could, for example, emphasise the tonal dif-
ference between white clouds and blue sky, adding
impact and drama.
This HSL model is what’s being using when you
make B+W conversions in Lightroom - open up the
HSL /B+W pane in the Develop pane and you’ll see a
whole load of sliders controlling the Hues, Saturations
and Lightnesses of eight different colour (hue) ranges.
With B+W selected in the Basic pane, these change
to sliders controlling only the Lightness of those same
eight different Hues such that you can adjust the light-
ness or darkness of those colours as they are converted
to greyscale values. Lower the Blue slider and your
blue skies will darken but the other colours remain
unaltered. This is very powerful and allows some
very subtle control of which colours convert to which
greyscale values.