Depth of Field…it can be the bane of the photographer. It frequently seems as though we’re constantly dealing with more of it than we want or not enough of it. As a landscape photographer, I much more frequently have to deal with the latter than the former.
For our purposes, depth of field can be defined as representing the part of a scene that is rendered acceptably sharp. (In that sense, it’s admittedly subjective.) When it comes to landscapes, I frequently find myself unable to obtain sufficient depth of field without stopping the lens down (i.e. reduce the size of the aperture—increase the f-stop number) to levels best avoided. (Very small apertures can introduce image softening as a result of diffraction.) Sometimes it’s impossible to obtain the desired depth of field regardless of the aperture used.
All things being equal, shorter focal lengths and narrower apertures produce greater depth of field, but sometimes the desired shot requires greater depth of field than you can achieve in a single frame. This is where the focus stacking technique comes into play.
What Is Focus Stacking?
In general terms, focus stacking refers to the technique of taking two or more shots of the same scene, altering the focus point of each shot, and combining these images in post processing, either manually or via an automated, software-driven process. Regardless of the specific technique, the key to success is meeting the following criteria:
1) Keep the camera in a fixed position (i.e. use a tripod).
2) Adjust focus in a manner that covers the entire range of the scene that you want to be rendered sharp. In other words, every bit of the scene that is intended to be sharp must be properly focused in at least one frame. I generally work from front to back (i.e. beginning with the point of critical sharpness closest to the camera and ending with the point of critical sharpness farthest away from the camera) but there’s no inherent reason why you couldn’t do it the other way around.
3) Be sure that each frame overlaps focus with each adjoining frame.
4) Exposure settings (aperture, shutter speed, ISO and, if not shooting RAW, white balance) should be identical for each frame in the stack. If this isn’t the case, the final stacked frame will likely have issues with uneven tones. The only thing that should change from frame to frame is the point of focus.
5) Typically, it’s critical that the subject matter itself not move during the process of producing the exposures, though there are some exceptions (patterned moving water, such as a waterfall or stream is one—waves are not), which I’ll discuss in greater detail below.
Manual Focus Stacking
I perform manual focus stacking in Photoshop, but any program with layers and masks is suitable. I have, on rare occasions, done manual stacking work with three frames, but I typically use this approach with only two shots.
For landscape shots, the key to using this approach is to have a scene where the upper and lower parts of a frame are at distinct, discrete distances from the camera. For example, the image below is a two-frame manual stack. Note how the elements—trees, rocks, etc.—in the upper half of the frame are significantly more distant from the shooting position than those in the lower part of the frame.
This image was photographed at f/7.1, and at this aperture, I couldn’t get the entire scene sharp in a single frame. (The camera I was using becomes diffraction limited at apertures smaller than f/7.1.) So, I found a focal point that made the foreground and mid-ground sharp (the background was soft) and clicked the shutter. I then refocused so that the mid-ground and background were sharp, and clicked the shutter again. I converted both RAW images with identical settings, brought the frames into Photoshoop, and stacked the two shots on top of one another (with the mid-ground/background sharp image on the top), and added a layer mask. I found the spot—very near the middle of the frame vertically—where both images were sharp and, and painted black over the lower half of the mask to reveal the lower half of the image on the bottom of the stack—the one with the sharp foreground and mid-ground. And it was that simple—I now had a single sharp frame, from front to back.
Below is another image that I did a two-frame stack of. In this case, it was very easy to visualize how to pull it off; the only sharp areas were the extreme foreground (the rocks) and the far background (the trees). Everything in between was naturally soft (the blurred water). After establishing the exposure criteria I simply focused on the rocks and clicked the shutter, then focused on the trees and clicked the shutter. The process of assembling the image was essentially the same as with the creek shot above, with the mask line established somewhere in the water. It didn’t really matter where.
Note that with both of these images, subject movement between shots wasn’t an issue. The water movement itself was so tightly patterned and naturally blurred that it’s impossible to detect the masking line between frames. But beyond that, any foliage movement between frames wouldn’t be detectable since all of the foliage lies in either Frame A or Frame B; there’s no need to line things up since these frames are distinct elements of the final product. This is not the case with the automated focus stacking approach discussed below.
A more commonly applied approach—both for close-up photography, where depth of field is often extremely limited (sometimes measured in millimeters) and for landscapes and still life shots—is a more automated, software-based stacking process. There are a number of different packages that you can use, including Helicon Focus and Zerene Stacker. (I use Helicon Focus myself, but I’ve heard very good things about Zerene.) I almost always use this approach when the number of shots needed to provide the final image is greater than two.
In this instance, within the software package of your choice, you select the frames that you want to be part of the stack and then let the software do its thing. You typically do have some options for setting parameters—I advise messing around with the various options (which may include specific processing algorithms, radius settings and thresholds) to see how they impact the final result. Over the years that I’ve been using Helicon Focus I have changed the default settings to obtain what I consider to be better results.)
I have, at times, combined as many as 20 images in a single stack, with extremely magnified close-up images. A more common number, particularly with landscape images, is 5-8.
Note that with this approach, it’s absolutely critical that the subjects don’t move. Even slight image movement will often result in “ghosting”—a kind of multiple outline artifacting that will show up around subject edges. Over the years, stacking software has gotten better and better at eliminating this ghosting (which, to a degree, inevitably results because changing focus points in the field literally changes the size of the subject as recorded by the camera’s sensor), but movement ghosting of any significance is very hard to eliminate. This problem can often be cleaned up manually in postprocessing, but it can be a tedious exercise indeed it if exists in any quantity.
The best part of focus stacking is realizing that it exists, because realizing that this approach is available can open up an entirely new way of thinking about imagery. Images that are optically impossible in a single frame can be obtained using this technique and recognizing this can be a breath of fresh air to your photographic creativity.
Thursday Tips is written by Kerry Mark Leibowitz, a guest blogger on 1001 Scribbles, and appears every other Thursday. To read more of his thoughts on photography, please visit his blog: Lightscapes Nature Photography.