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Tuesday, August 31, 2010

Control Your Depth of Field


Try the Focus Slice Auto-Align technique to get tack-sharpness from near to far

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This Article Features Photo Zoom

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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