In this post I'll explain the basics of one of the fundamental techniques in modern digital photography - shooting two different exposures (different length shutters, or different focsu positions) and blending them together.
There are many scenarios in which a photographer might want to employ this technique. Firstly and perhaps the easiest to grasp is the case of a sunset when you don't have or want to use graduated ND filters. In this case, it's easy to imagine that the clouds above the horizon might be several stops brighter than the landscape in front of your camera, making it impossible to properly expose both the colorful clouds and the ground in the same exposure.
What do you do? Simple - take two different length exposures that each correctly exposes for the sky (exposure 1) and the ground at your feet (exposure 2). You must take EXTRA CARE to not move the camera AT ALL, not even the slightest amount, between exposures.
Another example might be the photo at the top of this post, where one image was exposed for the light painting, while the class was standing there in the scene, and then after they walked away, I made another, longer exposure at a higher ISO to suck out some of the starlight on the landscape. I could have done the same with a very, very long exposure if I had wanted the shy to be star trails.
How do we bring these together into one image? With Lightroom + Photoshop, its never been easier.
Step 1: Shoot two (or more) different exposures, one properly exposed for each part of your scene / frame
Step 2: Develop in Lightroom to taste, taking care to synchronize any cropping or lens corrections
Step 3: Bounce both frames into Photoshop as layers, with the "Edit In / Open as Layers In Photoshop" command. This should present you with both frames atop one another in the layers panel in Photoshop.
Step 4: Add a layer mask to the top layer (HINT: Use the little mask button at bottom of the layers panel)
Step 5: Use the paint Brush (HINT: "B" key activates the brush, square brackets on the keyboard change the size bigger/smaller) to add black paint to the mask to cover up areas of the image with the mask. REMEMBER: "Black conceals, white reveals." You really should say that out loud now at least three times!
Step 6: Carefully, methodically, slowly use different hardness and opacity values, as well as brush sizes, to ad and remove layers of black and white paint to adjust the mask so that it's difficult to tell where the parts of the two different images are fitting together. This is the "meat and potatoes" of this technique, and where skills really comes into play as to it's successful application. Use selection tools like the magnetic lasso, in conjunction with the "Refine Mask" panel to help you save time.
That's it! Simple right? Go ahead and try it on your next shoot. Even if you aren't quite ready to dive into this full-on, I recommend to my beginning students to bracket for multiple exposures anyway, since storage is cheap and why not have the frames on your hard drive ready to go for you whenever you do decide to try it.