Correct The Exposure: Dynamic Range

The most common fix used in digital photo post-processing is the exposure correction. Any camera sensor can record a limited amount of detail between highlights and shadows in an image. This is called the dynamic range. For most of the scenes you will want that your photo have recorded pixels in the entire range. This is the case for those photos which was correctly exposed. If most of the pixels are in the dark side, most probably your photo is underexposed. The opposite is overexposed. Sometimes, depending on the subject and/or the photographer's artistic intentions, these situations are desirable, but for most of the cases indicates an anomaly in the exposure. By correcting a highly underexposed image usually you will face lot of noise. On the other side, a highly overexposed photo rarely can be fixed since the detail in the highlight zone is missing. For non-extreme situations you can obtain quite good improvements by correcting the exposure to use the entire dynamic range.

For most of your photos this fix will be enough. For most of the others, this fix represents the fist step to go. For this tutorial I used GIMP 2.6.1. You may face some small differences in the GIMP's interface if you are using a different version.

Step 1: Check The Dynamic Range

Bellow is the photography that we want to fix in this tutorial (click on it to enlarge):

Well... there is something wrong with this photo. Can you guess what? Which aberration is most obvious to you, the dynamic range or the semaphore lights? This funny (and real) shot was taken in London (on the left is The Big Ben). As if it were not enough that you have to drive on the left side of the road, you have to deal with such semaphore also.

But I'm sure that you were more concerned about the dynamic range, not the red/green traffic lights. So let's take a look over the histogram in the levels tool:

In a well balanced photo, the histogram is expanded over the hole horizontal axis. Dynamic range in photography describes the ratio between the maximum and minimum measurable light intensities (white and black, respectively). In a standard 8 bit RGB digital image, for each pixel, in each channel we can store a value between 0 an 255. As you can see from the histogram, our photo is missing quite an impressive number of values in the highlights zone (right) and also some in the shadows zone (left). This means that we do not benefit from all the values range we can use. Our lightest pixel is around 160, far away from a white point (255). On the opposite side we miss less range, but enough to reduce the quality our photo.