The modification is to use levels to help fix the underexposure of the shot. Generally it is best to get the exposure right in camera by adjusting the exposure compensation using the histogram. But sometimes its not possible, especially with a fully automatic camera, then it is time to reach for the levels tool in Phootoshop.
Select Image>Adjustments>Levels and you will see something like this:
The important bit is the graph in the middle. What this is showing is the range of the tones in the image from pure black on the left to pure white on the right with the midtones marked by the pointer in the middle. But hang on a second! this is a colour image why am I talking about Black and White?
Well as far as the levels are concerned everything can be represented on a scale of Dark to Light. A very dark blue being a little to the right of black and a very pale blue appearing just to the left of white. Looking at the graph (histogram) we can see that the image has most of its tones gathered to the left (dark) end of the scale with very little happening in the light end of the image.
If we desaturate the image we can see that there is a lot of gray tones in the image but few pure whites..which is basically what the histigram is telling us:
So how do we improve the exposure?
Simply grab the pointers under the histogram and drag them in to the edge of where the graph starts. I have done this for both the white and black pointers. This repositions the definition of black & white to the extremes found in the image:
Clicking OK, should produce a much better exposed image:
A quick look at the levels dialogue again shows that the images are much more evenly distributed:
Just for fun I ran a Digital Velvia action to boost the colours:
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