Contrast masking is a technique used to correct photos that because of the high dynamic range some of it’s areas appear underexposed while the rest of the image appears overexposed. With this technique we are able to reveal details both from the underexposed areas and the overexposed areas. This technique isn’t something new, traditional photographers used to apply the equivalent of this technique in the dark room for quite sometime now while the digital version of this technique is also used by a lot of photographers for years. However, I have found that most digital photographers ignore this extremely useful technique so I decided to write this tutorial.
Lets start by opening our image. I am going to use this photo taken at Stockholm with a small compact digital camera.
You can see that the left side of the photo is underexposed, while the building is slightly overexposed.
Now we are going to duplicate the photo layer. Go to Layer->Duplicate Layer (Ctrl+J)
The next step is to desaturate the new layer we just created. Go to Image->Adjustments->Desaturate (Ctrl+Shift+U). Now you should have something like this.
Inverting the duplicate layer is our next step. Go to Image->Adjustments->Invert (Ctrl+I)
Set the blending mode of the layer to Overlay and you should get something like this.
As you can see a lot of details that weren’t previously visible are now visible. However our image looks strange, so we are going to do one final thing.
Go to Filter -> Blur -> Gaussian Blur and apply a gaussian blur. For this step there is no golden rule about the value of the radius of the gaussian blur that works best, you should experiment with the settings. For different images I have seen values from 10 up to more than 100 work. For this image I used a value of 60. You can also play with the opacity of the duplicate layer to achieve your desired result.
And here is my final result.