|With some high contrast images levels and curves adjustments do not yield satisfactory results or require complex masking to apply different degrees of adjustment to different areas of the image.
Using a contrast mask is one possible approach to dealing with contrast. Briefly, contrast masking involves overlaying a colour image with a inverted greyscale image which has been blurred to make a soft mask; for a full description of the process see http://www.luminous-landscape.com/tutorials/contrast_masking.shtml
With practice contrast masking is an excellent approach to taming the contrast range within an image, and with experience can give good results. However one drawback of contrast masking is that the degree of Gaussian blur used to create the mask can make a dramatic difference to the final image. Too little blurring gives an image which looks flat or has unnatural looking edges; too much blurring can lead to contours or haloes of different density at tonal transitions in the image. Since Gaussian blur cannot be applied as an adjustment layer it is difficult to adjust the contrast masking.
After some trial an error I've come up with an alternative method which allows the contrast of an image to be controlled using an adjustment layer and hence allows the degree of contrast control to be adjusted even after the image has been saved.