(Mental Health Warning - the following is quite advanced (even geeky) and I must assume the reader's full knowledge of channels, levels, curves, blending modes, tools etc as well as how the main color models work (RGB, Lab, HSB).)
Much has been written about converting colour to Black and White: we all know that there are many different ways to do this, some more effective than others. Differences between the various methods are usually explained in terms of visual appeal or the ability to blend the various colour channels to emulate traditional B+W filters. What has not been mentioned is exactly why different greyscale conversions give different results, and more importantly, the fact that this principle can be used to make more accurate adjustments to colour images.
This last point seems quite contradictory, what has greyscale conversion got to do with colour adjustments? Well, quite a lot really when you remember that all colour images are comprised of 3 different channels of greyscale information and that the relationship between them give us 'colour'. If you alter a colour image by, say, dodging or burning, you are actually altering 3 greyscale channels at the same time and unless the relationship between those 3 channels stays the same, there might be a shift of hue or saturation which is of course hardly the aim of the adjustment.
How we see Luminosity.
Photoshop very rarely uses the term Luminosity. It is not Brightness in the Hue, Saturation, Brightness (HSB) colour model. It is not the Lightness channel in Lab mode and it is not the K value when you use the colour picker in Greyscale mode.
Luminosity is the perceived brightness of a colour, not it's numerical value under the above colour models. Look at this image - 3 patches of full strength RGB. Each has a 100% Brightness and a 100% Saturation, all that differs between them is the Hue. However, I think all would agree that the green is perceptually lighter, or more luminous, than the red which is in turn lighter than the blue. So, whilst the numbers show a 100% brightness, you see quite different tones.
Here is the same image converted to B+W using Image > Adjust > Desaturate. All the separation disappears - which is why this is a very poor way to make decent B+W images.
Here is the image converted another simple way : Image > Mode > Greyscale. It's better, but the Blue now reads as 100% Black now - surely a bright blue should be visually lighter than Black?
Even the Lightness channel in Lab, shown below, does not really depict the relative tones as we see them. Apart from the Blue still converting to 100% Black, it is actually a bit closer to the perceived luminance and this will become useful later in this article.
Colour scientists have assigned differing perceptual 'weights' to different colours - in fact 60% Green, 30% Red and 10% Blue. (Remember the Bayer sensor array in digital cameras? 2 Green sensors for every Red and Blue one.) This is a pretty good approximation to the way we see different colour tonalities. This last picture is more like we see the relationship between the colour tones and represents the true Luminosity of the image as we perceive it. The Blue is now 94% Black which is much more perceptually correct. This was done using Image > Adjust > Channel Mixer set to 30%R:60%G:10%B
Luminosity in Colour Images
Photoshop is a highly developed but imperfect tool. Many of the operations in Levels and Curves oversimplify the relationshps between the RGB values and cause colours to do odd things if too extreme an adjustment is made. The Burn or Dodge Tools are good examples of this - have you ever noticed how, if you burn in too much, the colours seem to intensify and alter slightly even as they darken? This is because you are applying a blunt instrument to all 3 colour channels at once and it can only do so much before making hue and saturation shifts as well as the required 'brightness' changes.
None of the native Photoshop tools use a true Luminosity value to make changes. You can however use 'Curves' to measure Luminosity by setting the readout to Luminosity rather than levels. Do this by clicking on the little double arrow in the centre of the lower greyscale bar. The readout is now in genuine Luminosity and by Alt-Clicking on the image you can see where the true Luminosity lies on the curve.
Whilst this is all very interesting it does not solve the problem of making tonal adjustments to an image without affecting the colour in any way. Photoshop does not have an HSL model, only RGB and HSB neither of which include Luminosity as a channel. Lab is the nearest, even though, while the Lightness channel is not a genuine Luminosity channel, it is similar in principle if a bit too 'light' perceptually.
Many advanced users will be familiar with the "USM / Fade 100% with Luminosity blending mode" technique which avoids colour casts in the USM fringes and is considered best practice for sharpening. Why not take this a step further? Can we use the Luminosity blending mode for other tonal adjustments like Dodging and Burning? Unfortunately no, there is no blending option with these tools. All is not lost, all we have to do is convert to Lab, use the tools on the Lightness channel only and convert back to RGB. Whilst this is not quite the same as using the Luminosity information, it is very similar in principle and it is the technique I want to demonstrate in the following example.
Orpheus Island
These two images were taken at the AIPP workshop held on Orpheus Island last year. I did a simple twin exposure to keep detail in the sky.
Once opened in Photoshop dragging one image onto the other whilst holding down the Shift key adds the image precisely over the other image, to pixel accuracy. I added a Layer Mask and used a Black brush to paint over the sky on the top (lighter) layer to reveal the darker layer below and thus bring the sky into a reasonable tonal range.
The result, after flattening but before any tonal work, appears thus:
Next I changed the Mode to Lab - Image / Mode / Lab. (Notice how the image now appears lighter - this is what I meant when I described the Lightness channel in Lab as being perceptually too light.)
I now select the Lightness channel only and use the Burn Tool (set to Midtones) to burn in the surrounding beach and the Dodge Tool (set to Highlight) to lighten the tree. I end up with a better image with the tree well separated from the rest of the image.
Click back on the composite Lab channel to add back the colours and you will get this result.
It might not look so different from doing the same tonal adjustments in RGB but if you looked closely into the new shadows you will find that the Hue and Saturation is exactly the same, just darker. If you did that in RGB the Hue and Sat would have changed somewhat. I like this technique because I know that I will not be altering any colours, only tones.
This all boils down to one simple principle - for the most accurate image adjustments make tonal changes only to Luminosity and make colour changes only to Hue and Saturation. The best option in Photoshop for tonal changes is therefore the Lightness channel in Lab mode.
Making accurate Colour changes may be the subject for another article as it is quite a bit more complex, but I just want to take a moment to introduce you to a new friend of mine - the Lobster.
Lobster.
If the above has you hooked on the idea of full and accurate control over your tones, but the idea of entering the arcane 'Land of LAB' is too scary, you need to know about Lobster.
Lobster is a script written for Photoshop by Australian Michael Cutter. What it does - and this is far cleverer than it seems - is to take an RGB file of the usual 3 colour channels and convert it into an RGB file, still with 3 channels, but broken up into layers. One layer is the original, the next is a true Luminosity layer, followed by a Layer Group of three Chromacity layers, one each for Red, Blue and Green.
Here is a Lobster file with the layers displayed and here is the same file with only the Luminosity layer active.
See how there is no change in the perceived overall 'brightness' of the image, just a lack of colour. This is quite unlike the Lab conversion mentioned above where the Lightness channel is appreciably 'lighter' than the overall colour composite.
These screenshots show the Luminosity of a Lobster file next to a Lightness channel of Lab - from the same colour file. See how the Lab image on the left is lighter and flatter.
Lobster allows you to make tonal changes to the true Luminosity of an image without affecting the colours in any way. It is the perfect place for dodging, burning, sharpening etc. Not only that but each of the colours can be adjusted without affecting the other colours, or even the Luminosity. Imagine being able to add contrast to only the greens in a forest scene without affecting the other colours at all, and without affecting the Luminosity of the image. Take it from me, you cannot do this properly in RGB - sure, you can run a curve on the Green channel but it will affect the perceived brightness at the same time as it makes the image more or less green. Using a Lobster file you can alter colours without affecting the Luminosity one bit.
If you are interested there is a Lobster demo available, and the full version only costs US$50. Check out www.freegamma.com for more details. There is a manual, in fact there are 3, and whilst they are seriously complex they give an amazing insight into how Photoshop works, where it doesn't work and what to do about it. Highly Recommended.
Nick Rains 2006
