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|Figure 1: Waterfall on the Middle Prong of the Little River, Great Smoky Mountains National Park, Tenn. Photographed with a 4x5 view camera with a 120mm lens focused and with the lens tilted.|
Figure 2: Fall color reflected in the Middle Prong of the Little River, Great Smoky Mountains National Park, Tenn. The scene was photographed with a 4x5 view camera with a 120mm lens focused and with the lens tilted.
Depth of field: This has been a concept photographers have grappled with since the advent of the camera. Historically, photographers relied on two basic techniques to maximize depth of field: using hyperfocal focusing techniques with large ƒ-stops and shifting the plane of the lens so the axis of the lens was no longer perpendicular to both the film plane and a focused slice of reality.
Shifting The Lens Plane
A conventional lens has the planes of the sensor (or film), the lens and the focused slice of reality all perpendicular to the axis of the lens. Theodor Scheimpflug (1865-1911) stated: “If the lens plane is tilted down, when the extended lines from the lens plane, the object plane and the film plane intersect at the same point, the entire subject plane is in focus.” This is the Scheimpflug principle.
To the landscape photographer, this means if the lens is tilted forward an appropriate distance, you can adjust the plane of focus to run parallel to the ground. What’s created is a wedge of focused reality that’s projected out from the camera, creating an illusion of great depth of field. The limitation is when vertical structures (tree trunks, tall flowers) extend above the wedge of focused reality and can’t be focused by tilting the lens and keeping the ground plane in focus at the same time.
These two photographs are examples of scenes that were sharply focused using the Scheimpflug principle (Figs. 1 and 2). Tilting the lens worked well because there were no vertical objects in the foreground projecting above the Scheimpflug plane of focus.
For lenses that don’t have tilt capability, photographers have relied on the principle of hyperfocal focus. For a conventional lens (without tilt), the hyperfocal distance is a mathematical computation that’s a function of focal length and ƒ-stop based on the “circle of confusion.” Simply put, circle of confusion describes the smallest image element that retains identifiable details.
Hyperfocal distance is the distance at which, when the lens is focused at that distance, everything will be in “acceptable” focus, based on circle of confusion, from half the hyperfocal distance to the horizon. What’s acceptable focus? That’s a subjective judgment made by the print industry and has been based on viewing an 8x10 print at a distance of approximately one foot.
For example, if you’re using a Canon 24mm lens at ƒ/16 that’s not a full sensor, the hyperfocal distance is 12 feet. Focused at 12 feet, everything from six feet to the horizon will be “acceptably sharp,” based on standards set by the printing industry. The technique produces results satisfactory for most photographic situations, with the only limitation being that the overall image can be made sharper using different techniques.
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Figure 4: Chromatic aberration can be adjusted in Adobe Camera Raw (or Lightroom) using the Lens Corrections tab. Most red/cyan aberration can be greatly improved using the slider as shown, usually in a negative direction.
Helicon Focus was initially designed for microscopists. The idea is to take photographs at many focusing points through a scene to produce a final blended image that’s sharp throughout. While the technique works well, and it’s also useful for the landscape photographer, the technique has some inherent limitations. Helicon Focus is an interpolation technique that can produce some problems such as “ringing artifacts.” These are rings around some structures, especially sharp-edged, bright subjects such as trillium flowers.
Also, if there’s movement in the image (water, moving clouds), serious image degradation occurs. The Helicon Focus designers recognized both problems and have designed a Retouching tool to deal with them; however, I’ve had limited success with retouching.
Focus Slice Auto-Align
Another technique for precisely controlling sharp depth of field uses “focus slicing” as you would do for Helicon Focus, but the images are combined using Auto-Align in Photoshop. The Focus Slice Auto-Align technique has no inherent limitations because the procedure isn’t interpolative, and the focus slices are layered on each other with the sharp zones of each slice “painted” onto the final composite image.
Figure 5: The Foreground slice was opened first with the Horizon slice placed next to the Foreground slice by clicking on Arrange, then Tile in the Auto-Align function of Photoshop CS4.
The limitations that exist are procedural, and there also are some physical limitations inherent in SLR lenses, especially wide-angle lenses. The procedural limitations are: 1) the technique is labor-intensive in contrast to Helicon Focus, which is extremely fast, and; 2) each focused slice must slightly overlap the next slice in focused sharpness. If the focus slices don’t overlap in sharpness, there will be zones of “lack of sharpness” between slices that can’t be corrected. Overlapping zones of focus also are needed for Helicon Focus.
The lens limitations are concerned with diffraction problems caused by small-aperture diameters at larger ƒ-stops in SLR lenses. ƒ-stop is defined by the mathematical equation: ƒ-stop = lens focal length divided by aperture diameter.
For a wide-angle lens such as a 24mm at ƒ/22, the aperture diameter is approximately 1mm. That’s a small opening. So when the lens focuses an image and it passes through an aperture at ƒ/22, there’s considerable diffraction as the image passes through the small aperture and is focused on the sensor (or film). At apertures ƒ/16 and beyond, there’s a progressive degradation in image quality, so SLR lenses should be used at “sweet spots,” ƒ-stops around ƒ/8 to ƒ/11, when possible. Of course, that means a reduction in depth of field, so to use focus-slicing techniques, it’s best to use several focus slices to ensure sharpness throughout the final composite.
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Figure 6: The Horizon slice is highlighted and placed on top of the Foreground slice using the Move tool while holding down the Shift key. Photoshop labels this “Layer 1,” and it’s highlighted.
Another problem with diffraction from a small aperture is chromatic aberration, especially prevalent in wide-angle lenses when used at small apertures (large ƒ-stops). Visible light is made up of variable amounts of red, blue and green light (RGB). When one of the components isn’t focused on the same point as the other two, chromatic aberration occurs. The most common aberration is a red/cyan aberration, which occurs when the red component doesn’t focus on the same point as the green and blue components (cyan). As a result, a red or cyan fringe occurs around some structures with sharp edges. Chromatic aberration usually isn’t as severe when a lens is shot at its “sweet spot,” another reason for avoiding large ƒ-stops when possible.
Chromatic aberration can be adjusted in Adobe Camera Raw (or Lightroom) (Fig. 3). I’ve found that a value of around -30 for red/cyan fringe usually is effective, though the value may differ with each scene. It must be noted that the corrections available now in Adobe Camera Raw and Lightroom aren’t perfect, but improvements are expected in Lightroom 3 and Photoshop CS5 when they’re released.
Figure 7: To align both layers, highlight them in the Layers palette, go to Edit, click on Auto-Align Layers, and check Auto, then OK. This results in the two layers being aligned and ready for painting in the sharp foreground.
Given these physical limitations, limitations a photographer must be aware of, how does the Focus Slice Auto-Align technique work to achieve an image that’s sharp throughout when great depth of field is required? With the lens set at ƒ/11, begin by focusing on a structure in the immediate foreground and photograph it. Touching nothing but the focus ring, focus somewhere beyond the foreground structure initially photographed and photograph it. If the image includes the horizon, set the focus on infinity and make that the final slice. For wide-angle lenses that inherently have great depth of field (17-24mm), usually two to three slices are sufficient. For stronger lenses, more slices will be required to make sure there will be some focus overlap between each slice.
You’re now ready to align the images in Photoshop. To simplify the discussion, I’ll use two examples photographed with a 24mm lens requiring only two slices. For more slices, the procedure is the same; it just requires more steps beginning with the foreground slice and working through the progression of focus slices, one at a time.
To photograph the ice field at Peninsula State Park in Door County, Wis., creating sharp focus throughout the image would be a simple task for a lens with tilt capability. This is because the Scheimpflug wedge of focus runs parallel to the ground with all the ice formations included (Fig. 4). For the straight 24mm lens that was used, photographing the scene required two focus slices, one slice focused on the foreground ice and the second slice focused on the background ice (infinity) with an aperture set at ƒ/11. An intermediate slice was made to make sure there were no gaps in sharpness, but because of the inherent depth of field for the 24mm lens, only two slices were needed.
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The next step is to open all slices in Adobe Camera Raw or Lightroom and process the RAW images. When done, use the Select All and Synchronize features in Adobe Camera Raw to make sure all adjustments apply to all the slices and then save the slices in a Photoshop folder as 16-bit files.
First, open the Foreground slice in Photoshop. It can be confusing because Photoshop opens the Foreground slice that was opened first and labels it “Background.” Then, open the Horizon slice (for a two-slice image). Go to Window, scroll to Arrange, then click on Tile (Fig. 5). Click on the Move tool, highlight the Horizon layer, hold down the Shift key and place the Horizon layer on the Foreground layer. Photoshop labels this “Layer 1,” and it’s highlighted (Fig. 6). Delete the Horizon layer.
In the Layers palette, highlight both layers, go to Edit, click on Auto-Align Layers and check Auto, then OK (Fig. 7). In the Layers palette, highlight Layer 1. Go to Layer, then Layer Mask, Reveal All. In Set Background Color in the Tools palette, set the foreground color to black. The Horizon layer is on top, and we’ll paint in the sharp Foreground layer underneath. Expand the image to 100% and, with the Brush tool set at about 200 and Hardness at 0%, paint in the sharp foreground image underneath. To show where you’ve painted, use the Backslash key to engage the Rubylith mask of the active channel, which is shown as a red mask. This also is convenient for moving to the next quadrant for continuing the painting (Fig. 9). Clicking again on the Backslash key removes the red mask. The final sharp image was shown in Figure 4.
Figure 9: To follow the progress of the painting process, the Rubylith mask of the active channel can be engaged by using the Backslash key, revealing the red mask where painting has occurred. The mask can be removed using the Backslash key.
Completing a sharp image for the ice fields in Door County was fairly simple and a straightforward painting exercise because the foreground and background are distinctly layered. The image of the path in the Great Smoky Mountains National Park (Fig. 8) is more difficult because the posts in the handrails project vertically. This is an image where a tilt/shift lens wouldn’t offer a great advantage because the posts extend well above the Scheimpflug wedge of focus. The near posts focus as a foreground layer, but the posts and handrails at the back of the bridge are focused in the second layer, or horizon layer. The foreground posts and handrails also are surrounded by the vegetation in the horizon layer in the distance. Therefore, the foreground posts need to be carefully painted in with the image expanded to 100% with careful attention paid to the transition zones in the handrails, but the procedure used is exactly as described for the ice field scene.
In conclusion, if a scene requiring great depth of field can be captured using a tilt/shift lens, that would be ideal because the scene can be photographed with a single exposure. Or if the desired depth of field can be accomplished without artifacts with the simpler and faster Helicon Focus, that would also be preferable in the interest of convenience. However, Focus Slice Auto-Align, a procedure made possible by digital technology, is still the most accurate and precise procedure for controlling depth of field. And when no simpler technique will work, this is the procedure of choice.
To see more of Willard Clay’s photography, visit www.willardclay.com.