There’s always something to learn in Photoshop. One could spend a lifetime, and still not learn all there is to know about this wonderful program. Which is one thing I like about Photoshop!
Stacking is a technique that is useful for combining exposures created over time. One common application is to create composite images of star trails, for example 15 four minutes exposures stacked together rather than a single 60 minute exposures. There are a number of advantages to this technique, most significantly it results in less noise.
To stack in either the regular version of Photoshop or the Extended version, load the separate images in Photoshop as layers. One way to do this is to select the images in Adobe Bridge, then choose Tools > Photoshop > Load Files into Photoshop Layers.
Once in Photoshop, combine the layers into a Smart Object by first selecting them (Select > All Layers). Next, choose Layer > Smart Objects > Convert to Smart Object.
With the layers encapsulated in a Smart Object, you can then apply one of the Stack Modes from the menu found at Smart Objects > Stack Mode. Note, for star trails usually setting the Stack Mode to Maximum shows the best trails, provided the sky is dark.
To get to the same place I have been using the Statistics action, available in the Extended Edition of Photoshop CS5. It turns out that the Statistics action only encapsulates the procedure I have just described, and it is good to know that you don’t need the Extended version to perform statistically-based stacking.
Note correction: The stack modes are only available in the Extended edition of CS5. If you have the standard edition these menu items will appear grayed out. I was misinformed, and I am sorry if you spent time trying to follow my original instructions with the standard edition.
This semi-abstract star trail image is a view north from Point Reyes. I’ve licensed it for use as a Trader Joe’s greeting card. The image was created using 12 exposures, each at 4 minutes, f/4, and ISO 100, for a total total exposure time of about 48 minutes, with the Stack mode set to Maximum.