Photoshop is a massive and nuanced program, for which there are many ways to accomplish any one task. However, one of the most useful techniques to know is luminosity masks, which are powerful and versatile additions to your editing arsenal. This helpful video will get you started with them.
Coming to you from Matt Kloskowski, this great video will introduce you to luminosity masks, both how they work and how to use them. Luminosity masks are tremendously useful, as they provide a way to create layer masks based on the luminance values; in other words, they allow you to mask effects into the highlights and gradually blend them into the midtones, simply bring up the shadows, and much more. You can also generate them very quickly as opposed to the time it would take to make an intricate selection and then fade the mask to blend it. For example, in the video, notice how when Kloskowski adjusts the exposure of the sky, it also adjusts the reflection in the water, keeping the effect realistic, whereas his quick selection mask misses this. But luminosity masks aren't just for exposure adjustments; you can use them to mask in things like color toning or whatever you want. Check out the video above for more!