There are many times you may want to combine multiple images together to produce a single final photo. The source images may be a bracketed set, each taken at a different exposure, or you may opt to process a single image in multiple ways and then combine those various ideas together, taking only the best pieces of each photo. In either case, you’re going to need layer masks in Photoshop (or the freeware Gimp alternative).
In the case of manually blending multiple images together to produce a natural ‘HDR’ image, you’ll often want to effect only the darkest or the lightest regions of the image, and this is where the concept of luminosity masks comes into play. Simply creating a mask based on the darkness (or lightness) of the image. While there are a multitude of ways to creating luminosity masks, I’ve found that the easiest way to create a simple luminosity mask in Photoshop is to use the ‘Apply Image’ command in the Image menu. First start with a pure white mask. Ensure that the image you want to use as the basis of your luminosity mask is visible, then select the mask before clicking Image -> Apply Image. Leave the default options (Blend Mode Multiply, 100% opacity) in place and you’ll get a black and white rendition of your image applied to the mask.
This standard mask is a good starting point, but rarely am I able to use it exactly as it comes out of the Apply Image mask. I have also found it useful if you do not try to create the mask from the same image you are applying it to. Doing often creates overly flat and muddy looking images when doing a blend of multiple exposures. Instead, use the ‘middle’ frame as your Apply Image reference image.
Invert & Opacity
Note that there are many ways to modify the way the mask is created. Either by changing the color channel used, changing the blend mode, changing the opacity of the effect, or simply hitting that ‘Invert’ checkbox next to the Channel dropdown. The two that I most often utilize is Opacity and Invert. Opacity does exactly what you would think, at 100% opacity any region of the source image that was pure black will create a pure black mask. At 50% opacity that same region of the mask will only be 50% gray, and of curse everything else is scaled as well. Invert, again, is a straight forward term you are probably familiar with. Useful when you want the dark areas of your photo to create a white region in the mask instead of black.
Nearly anything you can do with an image layer in Photoshop can also be done with a layer mask. You can brush blacks/grays/white onto it. You can apply filters to it. You can draw gradients onto it. You can apply adjustments to it. You can select parts of it. And quite likely much more I haven’t figured out yet. Those however are some of my more frequently used modifications to masks either on their own, or more often in conjunction with Apply Image to fine tune a mask to just the right effect.
Starting with one of the more simple adjustments to an existing mask, is the Curves (Cntrl+M on PC), or Levels (Cntrl+L on PC), or Invert (Cntrl+I on PC). Curves and Invert are my two go-to adjustments. Curves is used either increase the contrast (or decrease the black and/or white points, etc) of a mask. Compare the mask in the photo above to the original mask shown further up. Shown to the right is the curves adjustment used to modify the mask. Invert is useful if the mask you are trying to create is easier to create in the ‘negative’ space for one reason or another, then invert the mask to create the desired mask.
Say you want to blend in a darker sky from one image, but the darker foreground, even for the brightest parts of your ‘middle’ image don’t need to be effected. Either start with a white mask and draw in a black gradient (or gray if you want it just a little modified), and then use the apply image as above. You’ll get a mask similar to the above. The sky has white, but near the horizon the gradient fades the mask to pure black. You can, of course, do the operation in the opposite order, first creating the mask and then adding black (or white, or grays) to fill in more or less effect.
The sky is the limit to how complex you get with your masks. The above mask was created for the ‘dark’ frame in a 3-image blend I was recently working on. I wanted the sky to be pulled from the dark frame to prevent it from being blown out. But then I decided I wanted a little of the dark frame to influence the shadows of some of the foreground. Brushing in white on the foreground along side the boardwalk I darkened those parts I wanted to be deeper in shadow. However, I only wanted the darkest parts of those regions to be influenced by the dark frame. I using the rectangle selection tool I selected only the lower part of the mask, then applied image ‘Inverted’ so the brighter areas of the image removed the mask effect. One mask, one frame, two effects.
For further reading, try this link: Mastering the Apply Image Tool in Photoshop
Also the Phlearn Channel on YouTube has some excellent videos on masking that goes into a lot more depth and tricks than this article could ever dream to. Aaron Nace is truly a Photoshop master well worth your time to watch in action.