Adjusting image density
In certain lighting situations, it is nearly impossible to achieve an optimal balance of density in an image. When that happens, it can be difficult to adjust the brightness and contrast in Photoshop without losing information in the midtones and highlights.
For example, in a photo of a landscape at sunset, the sky might be properly exposed, but the rest of the image underexposed. A curve for the whole image would open the dark area, but we would lose the information in the sky.
Below is an image taken in the late afternoon. It´s somewhat underexposed, and the water and the walkway are quite dark. If we open up the whole image by adjusting the curve, we will lose information in the sky area and possibly burnout the reflections on the skyscrapers.
To properly adjust the image, we should select the dark area in the foreground with the polygonal lasso . Don´t worry too much about making a perfect selection since we will be “feathering” the edges of the selection.
Once we have selected the area to be adjusted, we should use the feather tool to blur the edges of the selection, so that our adjustment will not stand out. The feather tool (found at Select / Modify / Feather) has a default value of 0 and a maximum value of 250 pixels.
You can experiment with different values. Since this is a large area, we will use the maximum value of 250 pixels.
Once we have the selection made and “feathered”, we can apply a curves adjustment to the selected area. This enables us to open up the foreground shadows without losing density in the rest of the image.
Once we´ve done that, we can invert the selection (Select / Inverse) so that the selected area is the sky and buildings. If we adjust the curves a bit, we will gain luminance, and the result is an image with a full dynamic range.