In the first part of this series, I wrote about training the eye to ‘see’ in greyscale tones by converting colour into shades of grey. In this part of the series, we’ll break that down a little further.
In that first part of the series, we looked at how colours can translate into the same or similar shades of grey. We also talked about the use of colour contrast filters with black & white film to block or pass certain wavelengths (colours) of light to expose the film differently and create tonal contrast. We also looked at how this can be mimiced in the digital darkroom with the available tools.
In this third part we’re going to break colour down a little further and see what impact each separate component has on the shades of grey.
What we think of as ‘colour’ or ‘color’ for my friends in the U.S. is actually made up of three parts – Hue, Saturation and Luminance (or Brightness).
Hue is the shade of a colour. Blue is a hue, red is a hue, mauve is a hue. Saturation is the depth or intensity of the colour. It’s a measure of how much Hue a colour has. More hue means more saturation and a deeper or more intense version of the colour. Less hue is a less saturated colour. Think of it this way. If you start with a tin of white paint which has zero hue and zero saturation and begin adding drops of pure green, the more pure green you add, the more intense the colour of the white paint becomes. Adding those extra drops of hue is increasing saturation. The last component of a colour is its luminance or brightness. Luminance determines how light or dark a colour appears.
Each of Saturation and Luminance have a different impact on shades of grey so let’s see what that effect is.
Below is a set of green colour patches. The Hue is the same in all three at 128. Brightness is the same in all three at 100. Saturation is different starting at 100 and dropping to 25.
We get the expected result, the intensity of the green in the blocks is reduced as we reduce the saturation or the number of drops of green paint in the white bucket of paint. If we set Saturation at 0, we’d have a block of white.
Now let’s look at what happens when we convert this to grey tones. To do this, I’ve used the default Channel Mixer settings of Red-40, Green-40 and Blue-20.
This result should really be no surprise. As we reduce the drops of green paint, the level of grey in a conversion also is reduced and we have a lighter shade of grey. Keep in mind that Brightness is maintained as a constant in this example.
What happens when we leave Saturation as a constant and adjust Brightness? Let’s have a look.
The set of blocks below show the same Hue level as the previous example at 128. Saturation is maintained at 100 and Brightness ranges from 100 down to 25.
The left most block is the same as in the saturation example. The middle and right most blocks are quite a bit different – darker. Those two blocks are darker because even though we’ve maintained saturation at 100, we’ve reduced Brightness. We need light in order to see colour. If you’re in a room with no light, you won’t be able to tell what colour the walls are. As the light level comes up in the room, we begin to be able to discern the colour of the walls.
How does this translate to shades of grey? The image below shows us.
The same Channel Mixer defaults were used to make this conversion. The block of grey on the left is the same as in the Saturation example. This is not surprising since the numbers are the same. As we reduce the amount of light in the room, the ability to discern colour falls and the corresponding shade of grey is darker.
What does this all mean for our photography? Well, a few things. In very simplistic terms, we can think of it as Saturation affecting more the brighter end of the grey tone scale and Luminance or Brightness affecting the darker end of the grey tone scale. Colours we see in the real world aren’t as we know, simply made of up just Hue and one of either Saturation or Brightness so the real world isn’t quite as simple. But if we begin to look at our colours in terms of their combined saturation and brightness, we can begin to translate those into grey tones. So let’s do that. Let’s compare combinations of Saturation and Luminance and see what we get.
The image below shows a mixture of Saturation and Luminance values with Hue remaining at 128.
What we see here is Saturation and Luminance working in combination. It’s not as simple as Saturation affecting the lighter shades of grey and Brightness affecting the darker because the two work in combination.
The combination of Saturation and Luminance makes it a little more complex but not tremendously. For any level of Brightness, lower Saturation will produce lighter shades of grey. For any level of Brightness, higher Saturation will produce darker shades of grey. For any level of Saturation, higher Brightness will produce lighter shades of grey. And for any level of Saturation, lower Brightness will produce darker shades of grey.
You can test this out for yourself by creating blocks of colour similar to the ones I’ve used here to see the impact. This can also be helpful for training your eyes to ‘see’ in shades of grey. Try different Hue settings as well. Look at the the Red, Green & Blue values as you change the Hue. This will give some additional insight into how the various sliders in the Channel Mixer or the B&W tool or in ACR/Lightroom will impact the shade of grey you end up with.