So let’s say your Art Director is a tyrant and asshole…no…just follow me on this one.
“Yeah so I like the field and the color of the sky from this first shot but we need to have this lightning from this cityscape. I don’t want the sky from it, just the bolt…think you can get me a comp to look at by 2pm? Thanks.”
You could do this with layer masks and cloning, it will look sloppy and take you all day. Or you hope either Multiply, Screen or Overlay is gonna magically solve this problem. Instead we’re going to use a feature Photoshop has had for years and I bet you never even touched it. First dump the lightning photo into a layer above the field photo, don’t even worry about cropping anything.
Now double-click on the layer thumbnail of the lightning (or select Layer Options from the Layers Palette) to bring up the Layer Style menu.
Note the Blend If at the bottom, and drag the leftmost slider for This Layer to the right.
Notice how the shadows start to drop out of the layer and slowly start encroaching on the lightning. Here is what is happening, Photoshop is dropping the dark pixels of the layer to allow the underlying layer to come through. If you were to move the rightmost slider on This layer, the lightning would drop out.
Now it’s time to smooth the transition. Move the slider so the lightning still has a healthy amount of its original sky around it in all the channels, Grey, Red, Green and Blue, then hold down [ALT] to split the slider for each.
This makes the transition between lighter and darker pixels smoother. Some channels will have a more dramatic effect on dropping out the pixels you don’t need and keeping what the pixels you do. Play around with each channel to see what combination of all 4 works best. Also try playing with Underlying Layer to see how this affects your blend.