Hidden Lake and Barehat Mountain, Glacier National Park

This is the third of a series of posts on digital darkroom techniques describing digital darkroom techniques that “combine” groups of images towards various ends. Part two covered focus blending, this post will discuss stitching.

Stitching is the easiest of these techniques to understand, we simply take a handful of overlapping images and “stick them together” to create a higher-resolution image that shows a broader part of the scene. Image stitching is often used to create panoramic images such as the simple example above, which was stitched from five vertical images.

As easy as it is to understand conceptually, getting a good result from image stitching can be quite a challenge. The stitching software has to decide, for example, how to handle the perspective of the resulting image. For horizontal panoramic images such as this one, it’s essential to make sure the component images are centered across the horizon, the effects of not doing that well are reflected in this example in the strange wide-angle sort of distortion most visible on the left and right edges. This means that it’s critical for the tripod to be absolutely level before shooting. For best results, particularly when there are nearby objects in the scene, even the parallax that comes from the camera being rotated around the camera rather than the “nodal point” of the lens introduces problems that can throw stitching software. And moving objects (including clouds and water ripples) can cause problems for any of the image combining techniques we’re discussing.

You’ll also want to avoid using a polarizing filter when shooting your compoent images, and be sure to end up with a consistent exposure and white balance. I often use manual exposure when shooting panoramics, but find that I often have to reshoot the whole series when it turns out my exposure settings work fine on the first few component images but, for whatever reason, blow out highlights in one of the later ones.

Creating an image from those component images is pretty straightforward, when thing go smoothly. Start by selecting File > Automate > Photomerge in Photoshop, then select “Auto” and choose the source files you’d like to merge. You’ll want to check the boxes to remove geometric distortion and vignettes, as well as “Blend Images Together.” Then press OK. Photoshop will chew for a good bit of time, but with luck you’ll end up with the images still as separate layers but overlapping each other.

Now take a good look at the result. Do the images line up with eath other correctly? Doe the sky colors and other image tones show edges where the images were stitched together? If the results look good, go ahead and merge the layers together and crop the result, which will otherwise have a ragged edge.

There are a large number of third-party image stitching applications, ImageAssembler from PanaVue, PTgui and PTMac (both front ends to the Panorama Tools programs) and Autopano, but good results are possible in many cases from CS4’s built-in tools, and if you’re interested in dipping your toe into the world of image stitching, and already own Photoshop, that’s a great place to start.

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