Unfortunately this will only work in the full version of Photoshop or Corel phot paint so I apologise in advance for excluding elements, psp, picasa and photo impact users.

First of all a word or two about colour spaces.

The colour space which is used by all cameras, desktop printers and web browsers is RGB (red, green, blue) and the most commonly used variant is sRGB though others such as Adobe RGB(1998), colormatch RGB and ProPhoto RGB can be chosen as options in photoshop. The main difference between these different varieties of RGB is the gamut or range of colours that can be captured or reproduced. sRGB has a comparatively narrow gamut, ProPhoto RGB has a comparatively wide gamut but they all work in the same way by combining different amounts of red, green and blue light to produce all of the available colours and shades.

Lab on the other hand separates colour from luminosity (brightness) and has an unbelievably wide gamut, in that it includes so called impossible colours (colours that cannot be reproduced by any output device) Like RGB it has 3 channels but unlike RGB it has one for luminosity and two for colour. L, surprisingly, is the luminosity channel and handles all brightness information. 'a' is best thought of as a constant gradient between green and magenta so colours in the 'a' channel are described as being less green, more magenta or vice versa. 'b' is the same but the gradient is from blue to yellow. For the purposes of this recipe that is all that you need to know

Though this is by definition an advanced technique, it is a very easy recipe and takes about 30 seconds. You can also make an action in photoshop to do it in one click. It doesn't work with all types of images but is most successful with images like landscapes such as this one that are dominated by a fairly narrow range of hues, in this case browns and greens and a restricted contrast range.

Before you use this recipe make sure that you have removed any colour casts because if the image has a cast then that wll be intensified and you will not like

the result.The starting image is a jpeg converted straight from a raw file, the only adjustment was to neutralise the white balance. I could have improved the colour and contrast in camera raw prior to converting to Lab but I wanted to show how this technique can work very well on jpegs with very little image degradation. The original full size images from this article are available to download from here: http://www.pbase.com/bottesini/labvhuesat to compare or work along if you choose but please don't share the images or my puppy will be sad;) The images you will find there are the original full size (6MP) file (choose original under the image to view and download the full size images), one prepared with the recipe, one prepared using the recipe to extremes and one using hue/saturation in RGB. I have also included copies of the final files sharpened and resized for web viewing.

As you can see from the starting image which was taken on a dull day from across a lake (Rydal water), the tonal range is restricted and the green and brown hues are very muddy and subdued. There are a number of different ways to bring out the colours in this image without moving out of RGB, all of which will give an acceptable result but which will also degrade the image because of the way that RGB colour space uses colour and contrast together. Because Lab separates Luminance from colour there is no change in brightness or contrast when the colours are altered and no colour shift when the contrast is altered so it is more efficient when used for this purpose.

The recipe:

1. Open your starting image in photoshop (yours or IMGP2287SOOC.jpg)

2. Go to IMAGE-MODE-LAB COLOR (Nothing happens but go to the channels pallette and look at the channels which are now called Lightness, a and b)

3. Click the layers tab then add a curves adjustment layer by clicking the half black/half white circle at the bottom of the layers pallette and choosing CURVES from the list.

4. This opens the curves dialogue box which opens with a 45 degree line from bottom left to top right showing the lightness channel.

Note:As you can see from my screen grabs I have my curves dialogue set with the gradients on the left and bottom starting with white and going to black at bottom right and top left. To display this in CS3/4 click the arrow next to curve display options and select the pigment ink radio button. In earlier versions click the double headed arrow on the bottom gradient and it will change the orientation.

5. Go to the channels drop down box and choose the a channel

6. Click the white arrow on the gradient at the bottom left of the grid box and move it in to the right until the input box just beneath it reads 25. The output box will stay at 0. You can do this by grabbing the point at bottom left of the grid and moving that but I find it's easier to use the arrow.

7. Click the black arrow at the right end of the gradient and move it to the left until the input box says 75, the output will stay at 100. This can also be done by moving the point at top right of the grid to the left. You will notice as you move the points that the colours change quite drastically but don't worry about that.

8. Go to the channels drop down and choose the b channel and do exactly the same moves that you did with the a channe. lThe colours are much more saturated now but the hue is the same. What this does is bring out all of the hidden colours in the original file and separates similar colours to improve local contrast but we are not finished, the next step is just as important.

9. Go to the channels drop down and choose the lightness channel

10. Mouse over the image with the left button pressed and you will see a circle moving up and down the line in the grid box of the curves dialogue. As you do
this make a mental note of the highest and lowest points in the circles travel then go to the grid and click on the line around the lowest and highest points.

11. Move the top point to the left and the bottom point to the right to create an 'S' curve. This has the effect of increasing the contrast in the important mid tone area of the photo. It will look something like this and the result will look like the image called IMGP2287Lab2575.

Compare this to the original and you will see that the colour is greatly improved and the image appears to have more depth.

12. If you are happy with the result click OK then convert back to RGB using IMAGE-MODE-RGB Color. If you feel that the colour is either too intense or not intense enough you can adjust the a and b curves

As a comparison I tried to replicate the result with hue/saturation and as you can see the result is not satisfactory because, in order to avoid a shift in hue the overall colour is not as saturated and it's softer due to colours bleeding into each other.

You can try this for yourself using my original file but you will find that in order to exactly match one colour all of the others will suffer and the colour balance will change quite drastically. My settings are in the file name (0 hue, 48 saturation and -1 lightness). I chose not to alter the individual colours because that would make it a lot harder to match the colours without creating inconsistency. It can be improved by adjusting levels or curves but the changes required would degrade the image even more. As an experiment I created some extreme lab curves which on their own look gross but you will notice that the colours, though a bit Sellafield are still clear and not smudged. If you try this just reduce the opacity of the curves layer until it looks OK.

I hope that some of you have found this useful. Even though it's a lot to read and seems complex, it is quite a simple recipe which, as I have said, takes about 30 seconds or can be made into an action. If you choose to do this though, stop recording after you have made the adjustments to the a and b channels so that you can alter the luminosity individually for each image and it leaves the a and b channels still editable.

A word of warning if you choose to try this yourself and need to alter the curves in the a and b channels. The line MUST pass through the centre point of the grid to keep the colours neutral and for the sake of this recipe you should move the a and b so that they match exactly.

This is the most basic move in Lab colour but there is a lot more to it than that. If anyone is interested in knowing more then I can tell you about sharpening without halos and noise reduction without softening as well as using the a and b curves to create some weird and wonderful colour shifts that are impossible in other colour spaces

Unfortunately this site appears to soften all pictures so it is best to go and look at the originals by following the link above