How to do Exposure Stacking with Manual Alignment in Photoshop

Astrophotography can yield some amazing results even when working with a simple point-and-shoot camera. It helps, however, to know how to reduce the noise that inevitably comes with night photography by stacking multiple exposures. Photoshop’s Auto-Align Layers feature generally does a great job, but when it doesn’t work you’ll need to do it manually. Sound daunting? It’s not as bad as it sounds. Check out this tutorial from astrophotographer Ian Norman:

Image stacking is sort of magical for getting a whole lot more detail in the final image with a lot less noise. Basically, the more exposures you take, the more information is available and the greater your ability to reduce noise (Norman suggests 8 exposures). Photoshop’s auto-align will normally line up your exposures for you with next to no hassle, but occasionally you’ll end up with a blurry result like the one below:

When Auto Align Fails

This can happen when there is more than one element competing for Photoshop’s attention or when you’re using a lens that has a fair amount of distortion. In the example Norman is using, there are clouds moving in one direction while the stars and the earth’s rotation are moving the other way. This makes it hard for Photoshop to choose. As a result, Norman has come up with a fairly pain free way to stack exposures by hand using the smart object median stack mode. (This feature is only available in PS6 extended and CC, so if you’re using a previous version you’ll need to average them as layers instead.)

The technique? Use the “difference” blending mode to highlight the differences between the images and use free transform to line the larger discrepancies up and warp to line up the smaller ones.

Using the Difference Blending mode

Using the Difference Blending mode to see where the stars are not aligned.

Norman starts by checking the center of the image (above) before zooming in to check each of the corners (below).

Skewing with Free Transform

Afterwards he converts the exposures into a smart object and stacks them with the median function. This, combined with a separate median stack for the foreground gives him a beautifully lined up image. Return to Lightroom and reduce the noise, and voilá, an infinitely more detailed image with much less noise. Ingenious!

A fully aligned Image

And even better, this shot was taken on a Sony RX100III point and shoot! How cool is that?

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