Focus stacking, Depth of Field

Depth of Field (DOF) is often a problem in photography. Only a distance range within the image is sharp, while objects outside that area are blurred. In particular, telephoto shots, macro work, and tabletop photography are hampered by this problem. The traditional remedy is to stop down (or, in the case of view camera and tabletop photography, to tilt the camera’s back). Remember Group f/64—Ansel Adams and friends? The name was derived from the fact that you had to stop down the large view cameras to f/64 to obtain sharpness from near to far. Large cameras with long focal lengths are, in fact, more affected by this problem than small cameras with short focal lengths.

Nevertheless, small cameras also have their problems. If they have a diaphragm, you cannot (and should not) stop down too much. Lenses with short focal lengths are subject to diffraction problems when stopped down too much. Apertures such as f/64 are definitely out of the question. Therefore, compact cameras usually can only be stopped down to f/16. Many small cameras don’t have a diaphragm at all, but use a neutral density (ND) filter in case the scene is too bright. So the camera is always working at maximum aperture with the lowest possible depth of field (DOF).

Focus stacking is a technique used to solve Depth of Field problems. Images taken with varying subject distances are combined in such a way that the sharp areas from the single images are visible and the unsharp areas are invisible. The result is a composite image with a very large DOF. This technique is especially popular in macro photography.

The images can be combined manually by masking the unsharp areas of each image and then stacking the images together. However, the results are often unsatisfying because the transitions are not good. Heavy retouching is required. A better option is to use specialized programs, so-called focus stackers, for this task. Programs such as the free command line programs enfuse and tufuse, the free programs CombineZPand Picolay, and also the commercial programs Helicon Focus, PhotoAcuteStudio, and Zerene Stacker can be used. Photoshop CS4 now offers focus stacking, too.

Taking the series of single photos requires a bit of planning. You must make sure that the different DOF areas of the images overlap nicely so that they can easily be combined into one large DOF area. Fortunately, the CHDK offers a DOF Calculator (section 4.2.8). Switch it on by going to ALT > MENU > OSD Parameters > DOF Calculator and enabling the entry Show DOF Calculator. To be on the safe side, enable the entry Use EXIF Subj. Dist. (PC65), too. You will probably get a reading that is a little too short, but that is better than a reading that is a little too long. Then put your camera on a tripod, point the camera at the closest point of your subject, focus, and note down the following readings:

Also, measure the distance to the farthest point of your subject (F). F-S is your desired depth of field. Now you know the number of required shots. It can be computed from the formula 2*(F-S)/D0F. For example: given a distance of 60 mm to the nearest point of your subject, a distance of 120 mm to the farthest point of your subject, and a DOF of 12 mm, you would end up with 2*(l20-60)/12 = 10 exposures.

If you don’t use manual focus to do the measurements, you may want to switch your AF Frame to Center for precise focusing.

Now you can configure the camera. If your camera features a manual focusing mode, switch it on. Now dial in the following values:

Figure 4.39 Magnolia. Canon Digital Elph 1100SD (Ixus 80 IS). 1/60 sec, f/2.8, ISO 160. f=6.2 mm (~38 mm). Top left is the first shot in a series of nine images; top right is the last. At a near distance of 142 mm, the measured DOF was 15 mm. The Subj.Dist.Bracket Value was therefore set to 7 mm. The whole covered range of sharpness is 10 x 7 = 70 mm. Image composition was performed with CombineZP. No artifacts were created during composition, so retouching could remain at a minimum (some dust spots on the surface).

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