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Tuesday, November 27, 2012

In Search Of Sharpness


How to take control of depth of field in-camera and in the computer

This Article Features Photo Zoom

Selective Focus. Selective focus is used to make this leopard in Botswana stand out from a busy foreground and background. Note that the precise point of focus is placed on the cat's eyes. Canon EOS-1D Mark III, EF 500mm ƒ/4L set to ƒ/4.5 at 1⁄90 sec. and ISO 400
Image-Stacking For Unlimited Depth Of Field
We keep saying that the digital age of photography has freed us from many of the limitations of film. With DOF, this is especially true; we now have the potential for unlimited sharpness because of the ability to composite large numbers of images. This process is called "stacking," taking a series of images, each at a different but overlapping focus zone, and processing the images in stacking software to output a finished composite image that retains only sharp areas. The resulting image can have unlimited or controlled DOF, depending upon the photographer's intent.

Stacking to improve DOF is useful for all subjects. I regularly use it in landscape compositions to achieve sharp focus from immediately in front of the camera to infinity. Stacking can be especially helpful when working with longer focal-length lenses, where DOF is naturally quite shallow. A limitation with stacking is that neither the subject nor the camera can move during each exposure, so it's not effective on a windy day or with, say, running horses in the composition.

Macro photography presents the most difficulty in attaining DOF, and stacking capabilities have revitalized my interest in high-magnification work. Consider that, at 1X (life-size), the DOF at ƒ/16 is only 2.24mm. At 5X, it's a mere 0.269mm. Not everything is perfect or easy in stacking macro images. Complete elimination of movement of the camera and subject is even more imperative at high magnification, and if a close detail obscures elements behind it in the composition, there can be a blooming effect that causes an out-of-focus halo in the foreground.

Making precise, small adjustments to focus is a big challenge. At lower magnifications, the focus can be changed in incremental, visually monitored movements of the lens focus ring, but as the magnification increases, either the camera/lens assembly or the subject must be moved at fractions of a millimeter. Enter the StackShot (www.cognisys-inc.com), a focusing rail with a step motor at the back that moves the camera in very small, precise adjustments (from 100mm to 1 micron) that are calculated and preset on a controller. Either a tripod or copy stand is necessary to hold the StackShot and camera assembly. A battery system is available for field work.

Basic Steps For Depth-Of-Field Control
The two things we usually want to do with depth of field (DOF) are to maximize it or to place it strategically within an image. Work with these basic steps to become familiar with the ways your camera, lens and exposure choices affect DOF.

1 Choose the lens and focal length needed to properly frame the subject.
Longer focal lengths yield less DOF.
2 Choose an aperture to expand or concentrate the DOF (smaller apertures yield more DOF) so the subject is in focus. Use DOF charts and/or lens markings to assist in these settings.
3 Choose the exposure and ISO required for proper lighting and capture speed.
4 Make adjustments among these variables to achieve the resolution and exposure you seek.
Once captured, the set of images must be composited in stacking software. The first software I used to apply the stacking technique was Helicon Focus, developed some years ago by Danylo Kozub in the Ukraine. The program has been improved regularly and can be found at www.heliconsoft.com. Recently, I discovered another stacking program, Zerene Stacker (www.zerenesystems.com), which I find to be very versatile and especially good for "deep stacking," that is, combining more than 100 images for DOF at high magnifications, from 5X to 10X. Adobe Photoshop CS4 to CS6 (www.adobe.com) also include a stacking feature within the Blend modes.

Shoot It Right
While the digital era gives photographers a whole new range of powerful postprocessing tools that can be used to improve images, depth of field isn't one of those problems that can be "fixed in Photoshop." As always, the capable photographer keeps postprocessing options in mind at capture and also seeks the best quality possible in the camera. The search for sharpness is such a basic element of photography that it's worth the effort to master and apply a full range of techniques.

See more of George Lepp's photography at his website, www.georgelepp.com.

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