Desert Sunset Composite

As I’ve said in the past, sometimes Sky Lights is just about pretty pictures. This is one of them. On March 2nd I saw what was one of the most beautiful desert sunsets since I moved to Arizona in 1998. I don’t have the luxury enjoyed by some professional photographers — two cameras, one with a standard lens and another with a wide angle lens, both always at the ready. I have a single Canon EOS 20D, an early model DSLR. It takes 10 minutes to change the lens since this must be done in a dust-free environment. That means sometimes I miss a great photo op or have to improvise. For this shot I took two photos and digitally merged them. The thumbnails below are the two original photos.

When you need to merge two images you have several options (none as good as a single shot through a wide-angle lens). I’ve had to do this often enough that I thought I’d share what I’ve learned about that process. So if you find yourself stuck with a standard lens, typically an 18–55 mm zoom lens, but need a wide-angle shot, just follow the steps below.

I should mention that there are many apps that claim to join two (or more) images “automatically” into a panorama. Some are better than others, some are free, and higher-end graphics editors often have the tool as a standard feature or 3rd-party plugin. But all require some manual touch-up after joining and can benefit from these preparatory guidelines.

Most importantly: If you have a tripod or other device for holding your camera, and have time to set it up, make it so. Ensure it’s level and attach your camera securely. Take however many shots are required to capture your entire field of interest, overlapping each shot with the others by 10-20%. All this will make the digital joinery easier. If the camera must be hand-held, take care to keep the horizontal elevation of the camera constant for all shots. When hand-holding the camera, I take multiple images of each shot to improve the odds of getting a good horizontal match. And it’s best to start with large image sizes — you don’t need “raw” but you’ll want the next largest.

We’ll assume you have the images at this point. The rest of the process is purely digital and will require a graphics editing app like Photoshop, Photo-Paint, or the open source GIMP. Sorry, but the stock image editors that come with your OS (like Windows Paint) are just not up to the task.

  • First try to get a close match on the overall brightness. Play around with the gamma, brightness, and contrast settings. The problem you’re trying to overcome is a result of the sky brightness always being slightly different in different directions, and the camera trying to compensate. If your graphics app allows gradient effects, this would be a good time to learn how to use them. Note: Variations in brightness can be minimized by shooting at full manual instead of auto. But you need to know how to use full manual and be able to change settings quickly.
  • When you have the images as close as possible in brightness you’re ready to merge them. Increase the “paper size” enough to fit all images side-by-side. Make the first image 50% transparent and slide the next image “under” it until the horizons (or other visual cues) line up. If either image needs to be rotated slightly this is where you’ll notice. Perform any needed rotations until the images overlap seamlessly. Repeat for additional images.
  • Now you can “flatten” the image and preform the actual merge. You can crop as needed at this point, but don’t reduce the resolution (resample) just yet.
  • Here’s the tedious part … with magnification you’ll see discontinuities where the two images meet regardless of how careful you’ve been. This is the time to use the editing tools usually called “smear” and “smudge” to blend along the boundaries of the images. I find that setting these tools at 50% transparency allows a finer touch. You’ll never get it perfect, but do the best you can.
  • Finally, reduce the resolution to what is needed. This will often make changes from the previous step invisible. I use 700 dpi for the width of my main image in Sky Lights. Starting with the 3504×2336 original from my Canon DSLR this will shrink any joinery errors by about 80% (your mileage may vary).

A final tweak of the contrast, sharpness, color balance, and saturation should get your photo into a usable form. My photo above was created using this process. Can you tell where the two images merge?

Next Week in Sky Lights ⇒ Another Strange Cloud

Jupiter and Comet 41P
Q&A: Another Strange Cloud