Introduction to Focus Blending
Like HDR, “focus blending” is becoming another popular way for photographers to create digital images that are not possible with a single capture in the camera. Focus blending (or focus stacking) uses multiple captures to achieve great depth of field by creating a composite image during post-processing. Focus blending is useful for photography styles requiring great depth of field, such as nature and landscape, architecture and macro.
Before we delve into focus blending, let’s quickly review some fundamentals. “Depth of field” (DOF) refers to the range of distance that shows “acceptable” sharpness in a photograph. The appropriate DOF is a creative decision for the photographer.
With “shallow” DOF, only a short distance is sharply in focus. Both in front of and behind this point of critical focus, the sharpness falls off to blurriness. You can achieve shallow depth of field with wide apertures such as f/2.8, f/4, etc.
Great depth of field, or “deep focus”, is obtained through the use of smaller apertures with higher numbers, like f/16 or f/22. With a wide depth of field, objects that are very near the camera and those all the way out to infinity will be in sharp focus.
DOF is based on five factors, including
1) The distances between the camera and objects in the scene,
2) The selected lens aperture,
3) The focal length of the lens,
4) The distance at which the lens is precisely focused.
The fifth factor is the construction of the lens itself. It’s essential to understand that different lenses have different DOF inherent to the lens design. A wide angle lens of 24mm has much greater depth of field than a long telephoto lens at 200mm. What’s less obvious is that a zoom lens with an available focal length ranging from 24mm to 80mm, when set to 70mm, will usually have a greater depth of field than a lens that goes from 50mm-120mm when also set to 70mm.
You also need to know that most lenses produce the sharpest image at an aperture two full stops down from wide open. For example, on a lens with an f/4 maximum aperture, f/8 is two stops down. This is often referred to as the “sweet spot” of the lens. Due to characteristics of modern lens construction, this aperture provides the absolute sharpest image. However, f/8 of course has much shallower DOF than f/22. So there’s often a compromise between the sharpest focus and most depth of field.
Bearing all this in mind, it’s easy to see that there will be situations in which, with the lens set at the desired focal length for the ideal composition, you cannot possibly attain the depth of field you desire. This is where focus blending comes in.
As mentioned in the introduction, focus blending uses multiple captures combined in the computer to achieve great depth of field. In each capture, the focusing distance is changed slightly, so that the depth of field in each image overlaps the previous and next capture in the sequence. As when making multiple shots for different exposures, this is referred to as “bracketing”. Bracketing for focus ensures that each part of the final image contains the most possible sharpness… most significantly because it allows you to use the sweet spot of the lens.
Later, during post-processing, you then use software to combine the separate captures into one final image. There are specialized software packages available to do this; currently the most popular of these is Helicon Focus, which produces excellent results in a very automated method. Also, beginning with Photoshop CS3, you can easily blend multiple layers together in Photoshop, which I’ll explain in a bit.
First, here’s how to bracket focus when shooting in the field. Note that this technique is easiest with a DSLR but you can do it with a point-and-shoot with manual controls.
1. Use a tripod. (This technique is virtually impossible shooting hand-held.)
2. Create your composition as normal, but leave a little extra around the edges to crop later.
3. Use manual shooting mode if possible – typically, all the captures should have the same exposure settings. (Note: you can combine focus blending with HDR for the most possible control, in which case you might use different exposures for each focus slice. I did this for the top slice of the example photo.)
4. Set your aperture to f/8, or the sweet spot of the lens. (On a 2.8 lens the sweet spot will be around f/5.6.) However, using a wide aperture will mean that you need to use much more slices to get everything in focus. It might be worth the compromise to use f/11 or f/16 to limit the number of required slices. Experiment, your results may vary.
5. Focus first on the nearest part of the scene. If possible, use your camera’s DOF Preview button to check the focus and the depth of field. Note: if your camera has Live View, use it for checking focus and DOF.
6. Take the first shot, focused on the near part of the scene.
7. Repeat Steps 5 and 6, focusing on progressively further distances in the scene. Make sure your bracketed shots all have DOF overlapping so that the entire distance of the scene is covered by sharp focus and adequate DOF.
8. Finish with the focus set to the farthest point in the scene. In a landscape shot this will most often be at or near infinity.
How to Use Photoshop for Focus Blending
1. Get all the captures from the bracketed sequence into one layered Photoshop document. There are a number of ways to do this; the easiest is probably copy/paste. The important thing is that you have all the captures on separate layers within a single document. It also helps if the layers are stacked in order of the progression of focus in the bracketed sequence.
2. Make sure the bottom layer is not Background. If it is, just double-click it to turn it into a regular layer.
3. In the Layers palette, shift-click to select all the layers.
4. Run the Edit > Auto-Align Layers command. This step is essential, because it compensates for differences in scale caused by changing focus distance. I usually use the Auto Projection method, with the Lens Correction options disabled. Photoshop does a fantastic job of scaling, rotating and aligning layers so that the elements in the frames line up properly.
5a. Use the Edit > Auto-Blend Layers command to automatically blend the layers together using the Stack mode with Seamless Tones and Colors checked.
5b. For the most precise results, use Layer masks and the Brush and/or Gradient tools to hide/show the sharply focused part of each layer. This manual method provides the best results but takes the most time and effort.
Tip: Try Auto-Blend first; you might get good results requiring just minor retouching afterward. If Auto-Blend doesn’t produce acceptable results, the manual method with layer masks is your best option.
6. Crop the image.
7. Apply any final image processing appropriate to finish the photo.
Focus blending is a powerful technique that combines precision camera operation with the power of modern imaging software to produce results that would otherwise be impossible. This is just an introduction to the technique; you can find lots more on the Web, including tutorials on the Digital Scheimpflug technique popularized by Tony Kuyper. With practice, you will find that you are no longer limited by the inherent DOF of any lens.
by Scott Bourne