Focus Stacking in Affinity Photo

FOCUS STACKING IS A TECHNIQUE in which you take several shots of a subject from the same standpoint—usually a tripod-mounted camera—but with the lens’s focus ring set to different distances every time you open the shutter. This way, a macro photograph that would usually have a naturally shallow depth of field can be manipulated to have the whole subject in focus.

You do need to put a bit of work in to create the raw materials for this one before you start processing. You need a lens that can be manually focused, a way of securing your camera so it doesn’t move and ruin everything (as much as you think you might, you won’t be able to achieve this handheld), and enough patience to make a really good job of it. As you will see, we lacked one of those things.

The ability to focus stack is built into many image-editing apps, including Photoshop and Affinity Photo—we’re using version 1.7.3 of the latter. It’s a similar technique to HDR stacking, but instead of merging the color information from your files, you’re sticking together the bits that are in focus. –IAN EVENDEN

1 TAKE THE PICTURES

We mounted a full-frame DSLR on a tripod, attached a 100mm macro lens, and set it to manual focus and f/2.8. Using the screen on the back of the camera, we racked the focus back so the closest end of the DIMM to the camera was in focus. With the camera in single-shot mode, we fired a frame and moved the focus point slightly farther away before repeating the process. We learned something from this: Be patient. Move the point of sharpest focus very, very slowly away from the camera, because too much of a jump results in a blurry patch in the final merged image. It took us 11 images to travel the length of the DIMM, and this wasn’t really enough. For a second try, we used a cable release and set the camera to its fastest frame rate, jamming down the button and very slowly adjusting the focus while the camera machine-gunned away. This time we created 32 files, each looking much the same [ Image A], but with the focus point in a different place. However, we hit a problem with the camera’s buffer, meaning the capture wasn’t as smooth as we would have liked. We shot in JPEG here, not raw as we usually do, because there seemed little point in creating a lot of data to move around when we would be throwing most of it away. The greater number of files resulted in a better merged result, but there are still some soft spots.

2 MERGE THE IMAGES

Create a new folder in your Photos folder or elsewhere, and put all the images in there. Open Affinity Photo and select “File > New Focus Merge.” In the window that appears, click “Add” and navigate to the folder you created. Click on an image, then press Ctrl-A to select every file in the folder. Hit “Open.” Back in the Merge window, check no rogue files have crept in (select them with the mouse and press “Remove” if they have), then hit “OK.” The process is automatic, and can take a long time if you’re merging a lot of high-megapixel images on a slow CPU. You get an idea of how things are progressing thanks to a progress bar and the changes made to the image on screen in real time [ Image B].

3 SOURCES

When finished, you’ll see your new image on a single layer, plus a palette named “Sources,” in which you’ll find the files that were used to build the composite [ Image C]. You can use these to touch up bits of your image that aren’t quite right. Note that even if you save as an .afphoto file, the Sources list isn’t preserved, so this is your only chance to fix bits that don’t look right.

4 CLONING PREPARATION

Affinity Photo’s Clone brush works in the same way as the one in Photoshop. You hold Alt to define a source, then as you paint, that source is duplicated. If you want to clone from the Sources window, make sure you have the Clone Brush selected (it looks like a rubber stamp, two thirds of the way down the toolbar), then on the Sources palette, double-click images until you find the one you’re looking for—the one that’s sharp where the composite is soft. Spots like these happen fairly commonly, and if you can’t find a way of cloning in some sharper pixels, you will need to shoot all the images again, this time with a slower focus rack, or using a smaller aperture to increase the depth of field in the individual frames. Specular highlights, in which balls of white are formed by shiny bits that go out of focus, seem particularly able to fool the merging algorithm, forming rings of white around otherwise perfectly sharp features. It’s worth identifying the source of these and painting over them. Likewise, if your lens is prone to focus breathing, where objects change size as they go in and out of focus, you can get a soft border around high-contrast areas that needs to be carefully brushed away [ Image D].

5 CLONING

Once you’ve found a suitable source, leave it selected, and find the eye icon hiding at the bottom-left of the Sources palette. Choose this, and you’re returned to the composite, but now your Clone Brush is loaded with the single source frame as though you’d Alt+clicked it. Start painting. It’s a good idea to keep the edges of your brush fairly soft, so your new strokes blend in with what’s already there. There’s a “Hardness” slider at the top-left of the interface, along with brush size and opacity, or you can hit the “More” button for a floating palette you can change on the fly [ Image E].

6 FINISHING UP

Each brush stroke appears in the History palette, so paint in smaller strokes if you want to be able to undo mistakes without having to redo large areas. Save as an .afphoto file to preserve layers, but remember that your Sources will be gone as soon as you close the file, so make sure that you’re really happy before you close the image.