First find out if you have enough detail in the dark areas to make it worthwhile. I would not use the brightness control to determine this. I would use levels.
Go to Image/Adjust/Levels, pull the white and black point arrows to the beginning of the data at each end of the histogram, and then slide the middle arrow towards the left until the dark areas are where you want them. This will blow out the highlights in the lighter area but this is ok since this is just a test to see how much detail in available in the darker areas. If the data looks good then there are several ways to go about fixing it. BTW, click 'Cancel' when done with the test.
If the differential is not too great between the dark and light areas you can use curves to correct a given range, however in doing to you will also change/loose any shadow detail in the lighter areas if this shadow detail is as dark as any of the darker areas you are correcting for. BTW, anyone else out there using a resolution of 1600x1200 or higher that thinks the curves dialog box should be much larger?
If the skyline separating the dark from the light areas is smooth (ie, it doesn't have lots of trees protruding in to the lighter sky or mountains), you might be able to use the lasso tool to select the darker area, use the Select/Feather tool at about 5-10 pixels to smooth the selection, and then use the Image/Adjust/Levels control to adjust as needed. The problem with this approach is that if your selection is sloppy, then you will have to use more feathering, and this can cause a halo effect around the selection point if too much level adjustment is done.
The third way method (my preference when dealing with such problems) is to duplicate the layer, 'extract' the area that needs adjustment, and then flatten the layers when done.
Before Image example: http://www.drgrease.com/test/Extract_before.jpg
From the layers panel, right click on the 'background' layer and select 'Duplicate layer'. This will be the only one to select unless you have created other layers. It doesn't matter what the name is so just hit 'ok' when the name dialog box comes up. It will be automatically placed on top and will be selected (blue) after you click 'ok'.
Select 'Image/Extract' and wait for the extract tool dialog box to come up.
When it comes up, use the top tool on the left (Edge Highlight) to select the edge between the light and dark areas across the image. This tool works great when there a lot of contrast at the selection point which is why I use it in such situations. There is no need to enclose and area as long as the selection goes from one edge of the image to another (does not have to be opposing edges and you can select multiple areas).
Now select the Fill tool (bucket) and click in the dark area or any area you wish to extract for adjustment. This will fill the entire area up to and including areas within the selected area until it hits what it determines to be an edge.
Selected and Filled Image example: http://www.drgrease.com/test/Extract_work.jpg
Preview of extracted layer example: http://www.drgrease.com/test/Extracted.jpg
Click on the Preview button and then click Extract. the image will not look like it has been changed but now you can use the Image/Adjust/Levels control on just this extracted layer. This will allow more adjustment without affecting any detail of highlights in the non extracted areas. When you are done select Layer/Flatten Layers to create a single image that can then be saved however you wish.
