There are two types of image stacking methods that are popular today. "Focus stacking" is used to extend the depth of field. Whereas "exposure stacking" can be used to extend the apparent exposure time. This post is intended to be a quick introduction to the latter.
Note: Some would say there is a third type called "action sequencing" but we consider that to be somewhat distinct from focus and exposure stacking. At least for our purposes.
When to use exposure stacking
On occasion, regardless of neutral density filters or external conditions, you may run into subject matter where you can not capture a long enough exposure to obtain the desired result. In these instances exposure stacking may be the answer. The concept is simple. Capture multiple images over time and put them together to create a single "longer" exposure.
How to create the images
To create the multiple images we recommend the use of an intervalometer. Your camera may already have one built-in or you can pickup a simple one from Amazon, B&H Photo, and most photography suppliers from around $12 and up. The intervalometer is most often associated with time-lapse photography. And, that's exactly what you will be doing here, e.g. capturing a series of images over time. The difference is instead of composing a video with your time-lapse images (although you could do that too), the goal here is to composite the images into a single stacked image to simulate a longer exposure.
Tip: Depending on your desired workflow you may prefer to process all of the images in Camera Raw or Lightroom before creating your stack. For example, if you are working on a star trails image now would be the time to edit out aircraft and other distractions from the individual images. If you do, just be sure to do your camera raw work on a master image and copy those settings to all of the other images so that they will be consistent.
How to stack the images
There are commercial software products dedicated to stacking images. However, for the purposes of this post we will use the Photoshop method.
Step 1: File > Scripts > Load images into stack...
Browse to multi-select all of the images you wish to stack (they should already be in the same folder). The selected images will load into the tool. Note: If you use Lightroom, start in Grid mode, select all of the images you wish to stack, then select Edit In → Edit in Adobe Photoshop...
In most cases, but not always, you will want to check the box to "Attempt to Automatically Align Source Images".
Always check the second box: "Create Smart Object after Loading Layers"
Step 2: Click the "OK" button on the dialog to begin the stacking and blending processes. This may take some time.
Step 3: Navigate to Layer → Smart Objects → Stack Mode → Mean
In most cases, the "Mean" mode, will work fine.
We find that some subjects such as star trails work better with the Maximum mode. Experimentation is recommended.
Step 4: At this point you no longer need the Smart Object. So, right-click on the Layer and select "Rasterize layer".
Step 5: Now you can process the resulting image as you ordinary would in Photoshop.
Tip: Be judicious about sharpening at this point. Sharpening your purposefully moving elements (clouds, water, etc.) will usually make them look grainy... unless of course that is your intention.
Next steps
There are many ways to compose and produce images for exposure stacking. And many imaging techniques to produce your stacked images. Far too many to cover in this short introduction. We are planning to offer both online and live workshops in the near future on this and other techniques. We encourage you to subscribe to our mailing list to stay informed (see the bottom of the page to subscribe) .