Focus Stacking, Part 1: Potential and Pitfalls
April 29, 2014
Landscape photographers have always been frustrated by the difficulty of keeping everything in focus from the closest foreground flower to the most distant mountain. One solution, in some cases, is to use a tilt-shift lens. As I discuss in my blog post, these are superb lenses, but they’re also heavy, bulky, and expensive. Digital imaging offers an alternative: focus stacking. In brief, the idea is to make a series of frames of the same subject that differ only in focus point, then combine the sharp parts of each image in software. The result, with the right technique and the right subject, can be extensive depth of field that cannot be duplicated by stopping the lens down.
Here’s the procedure in more detail. Mount the camera on a solid tripod, compose the shot, focus on the closest part of the subject, and shoot a frame. Focus a little bit farther away, and shoot a second frame. Continue until you have focused on the most distant part of the subject and captured a frame. Be sure to use manual focus, manual exposure, manual ISO, and manual white balance. Shoot in order from near to far rather than focusing near, focusing far, then focusing somewhere in between.
Download all the frames to Lightroom, select them, then choose Photo>Edit In>Open as Layers in Photoshop. If you don’t use Lightroom, you can select the images in Bridge, then choose Tools>Photoshop>Load Files Into Photoshop Layers. Click on the top layer, then shift-click on the bottom layer to select all the layers. Choose Edit>Auto-Align Layers. I often choose Auto for Projection, but in some cases Perspective will produce a more pleasing result, particularly with subjects containing lots of straight lines. I usually check Vignette Removal and Geometric Distortion as well.
Once Photoshop has finished aligning all the images, and while all the layers are still selected, choose Edit>Auto-Blend Layers. Under Blend Images choose Stack rather than Panorama and be sure Seamless Tones and Colors is checked. Hit okay and prepare to be amazed.
And then, sometimes, frustrated. At first glance, the image will probably look perfect. Before declaring victory, however, you need to zoom in to 100 percent and inspect carefully. Look particularly at the edges of foreground objects placed against distant backgrounds (as opposed to foreground elements whose background is just a short distance away). You may find out-of-focus haloes around those foreground elements. The problem is not in your technique, or in the software. The problem is that in some cases there simply is no frame where that thin strip of image is sharp. When you focus on the foreground, all of the background is fuzzy. When you focus on the background, all of it is sharp except for those areas obscured by the fuzzy foreground elements. Not only are those foreground elements soft, but they have also “bloomed.” The area occupied by the out-of-focus foreground object is larger than the area occupied by the same object when sharp.
Problems are also common along the edges. Remember my advice to use Auto-Align Layers before using Auto-Blend Layers? You might think, why bother? My camera is on a good tripod. The images are already aligned. But look closely at your focus-stacked set. You’ll see that image size changes as you change focus distance. The subject size is larger when you are focused on the closest part of the scene than it is when you are focused at infinity. Photoshop has to do a lot of stretching and warping to make everything fit together. It doesn’t always do so perfectly, and the errors are most commonly seen along the edges. The easiest solution by far is to compose generously, including some subject matter along the edges that you are willing to crop away.
Focus stacking works best when most of the subject elements lie in one plane. An expansive field of flowers is a good example. It fails most often when you have a single subject directly in front of a distant background and nothing but air in between the two picture elements. The technique also works best with wide-angle lenses rather than telephotos. Macro work is another field where focus stacking excels. Despite its limitations, focus stacking is a powerful tool that will allow you achieve otherwise impossible images.
Next month: Helicon Focus, a specialized focus-stacking program.
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