I've learnt a lot from all the posts in this thread. Thanks to everyone for contributing. One other thing: the histogram in PP software such as ACR or Lightroom is not quite the same as the camera's (especially if you shoot RAW), but can be used to correct overall exposure and tone based on the same principles. Any good book on e.g. Lightroom will provide a thorough explanation. The colour channels are, however, more useful than implied elsewhere. You can use them to check your HSL settings in addition to getting the exposure right.