Using LAB and impossible colors to fix blown images
In RGB, which is the color space that your monitor is also running in, the lightest any color channel can be is 255, when all 3 channels R G and B is at 255 we get white, and that’s as far as you can go.
In LAB however we have luminance (The L channel) split from the color channels (A and B) and LAB is device independant so you can have an image with an L value of 100 (Max) AND have positive values in the A and B channels which in RGB would mean values above 255. This is not possible to show on a RGB device such as your monitor, but photoshop tries anyway to map these impossible colors to something that looks alright, and that’s a great thing.
One thing you can use impossible colors in LAB to do, is to fix blown highlights.
First load an image with some moderate areas of blown highlights, like a portrait shot with blown highlights on the forehead or shoulders for example and switch the color mode to LAB.
Duplicate the background layer so we are not working on the original. Now select the duplicate layer and set the blend mode to Color, use the color picker to select a color near the blown highlight and simply paint over the blown areas, don’t worry if this looks horrid and don’t try to be to presize, just make sure you painted over the blown areas. What you are doing is telling photoshop to leave the luminance alone (It’s maxed at 100) and ADD color to the area only. Well that’s not possible since that means more than 255 on your monitor, but photoshop tries anyway.
Now add a curves layer clipped to the color blend mode layer and fiddle with the color to get it to match the surrounding area as best you can. If you don’t know, you clip an adjustment layer to the underlying layer by holding down ALT and clicking right between the layers. Clipping adjustment layers to the underlying layer means the adjustment layer will ONLY affect the layer below.
Now comes the reason I said don’t worry about being presize. Doubleclick beside the color mode layer to bring up the layer adjustments dialog. See the sliders labelled Blend If Make sure you are using the Lightness channel and select the Underlying layer slider and grab the white arrow and pull it to the middle. This allows us to affect ONLY the impossible color we created and blend it out. Just pulling the white slider creates harsh transitions but if you hold down the ALT key and grab either side of that little arrow you can split it in two and create feathered edges that blends much better.
If there’s colors you can’t blend out properly using the lightness channel you may need to select the A or B channel and play around until you find the correct color to blend the impossible color out of.
