Apparently luminosity is a term that is used incorrectly by Adobe Photoshop and photographers in general. I won’t go into the correct astronomy definition of the term. However, the inaccurate concept of luminosity as it pertains to photography is still particularly useful. In photography, luminosity is considered to be the measure of the density of luminous intensity coming from or through a surface. In other words it is an indicator of how bright a surface will appear. Because of the limited ability of film and digital sensors to capture as much tonal range of light as the human eye can, photographic images are often challenged by areas that appear to have too much luminosity (over exposure) and areas that have too little luminosity (under exposure). In a scene where the human eye can easily see color and detail on a sunlit mountainside and in the shadow of a tree at the same time, the detail and saturation in these areas of a photograph of the scene would be compromised. Luminosity masking is a masking technique in Photoshop that allows one to accurately select and adjust the luminosity of different tonal values within an image. There are many ways of working with luminosity in Photoshop, including blending different exposures of the same image, dodging and burning, screening and multiplying and using curves or levels adjustments. While all are useful in a variety of situations, they also have their limitations because of the difficulty of confining the adjustments to specific areas of an image and easily blending the adjusted area with unadjusted areas so it looks natural.
Luminosity masking makes it possible to adjust the luminosity of varying degrees of dark, light and mid tone areas in an image in a way that is gradual and completely blended. This is done by converting luminosity channels into highly detailed masks that can then be used for targeted curves adjustments. It sounds complicated, which it is to some degree. However, the power, subtlety and precision the technique provides makes learning it a worthwhile effort for the serious photographer.
The following are a links to two photographer sites that have well written tutorials on luminosity masking. Tony Kuyper’s tutorial at www.goodlight.us is particularly excellent and has an accompanying set of actions that can be downloaded. Bob Johnson’s site www.earthboundlight.com has a tutorial on luminosity masking that takes a slightly different approach towards the same goal.