What are Luminosity Masks?
Firstly, what are luminosity masks? Well, a luminosity mask allows us to make advanced selections of an image in Photoshop by selecting certain luminance (or brightness) values. Using these advanced selections, we can blend parts of one image with another. In order to use luminosity masks you need to shoot on a tripod and you need to have at least two different exposures. So the thinking behind it is to replace the blown out highlights of an image with correctly exposed areas OR to replace the underexposed areas with correctly exposed areas, depending on how you choose to look at it.
Step 1 – Adjust RAW images and bring into Photoshop
If you are not shooting RAW you can skip this step, but I would highly advise shooting in RAW. RAW images retain more information that will be needed in the merging process and it gives you more room to adjust the properties of the final image.