Tuesday, February 25, 2014
Stacks Of Blooms
Images In Bloom • Pictures By Number • Going Long And Light • Cloudy With A Chance Of Storage
Images In Bloom
Q I'm using focus-stacking techniques for my macro images and often I get a halo around some sharp parts of the image. Is there a way to prevent this from happening, or at least dealing with it later in my software?
A Focus-stacking is a technique that expands depth of field in an image. To accomplish it, the photographer frames the image, then captures a series of in-focus slices, moving through the subject and refocusing from foreground to background. When composited in focus-stacking software, only sharp areas are retained, yielding an image that's sharp from foreground to background.
One of the challenges posed by focus-stacking captures is that when an object close to the camera is rendered out of focus, it blooms—that is, as it gets fuzzy, the image gets larger. As they grow, foreground elements can interfere with efforts to attain sharp captures of background areas. This effect is emphasized when using a longer focal-length lens or when trying to focus-stack a relatively large area, and can be exacerbated by significant tonal differences between the foreground and background.
For landscape photography, we use focus stacking in situations where there are important subjects in both foreground and background. Think, for example, of a field of spring wildflowers stretching to a backdrop of snow-covered mountains. To tell the whole story, we want both the flowers and the mountains tack-sharp. If a foreground object, such as a flowering bush, obstructs the distant background, the blooming effect may occur: The foreground object becomes out of focus, obscuring and inhibiting sharp capture of the background behind it. Since chopping down the bush after it's photographed is not an option for responsible nature photographers, the best options are to work around the problem. Use a fairly small ƒ-stop (ƒ/16) so that the foreground object doesn't go completely out of focus when photographing the background. Another choice is to change your perspective; position the foreground subject more into the lower aspect of the composition so that it doesn't intrude into the mountain's space.
When focus-stacking captures in macro photography, the possibility of blooming is greater. Although the size of the subject may be very small, the range of focus between front objects and elements at the back of the composition can be large at high magnification. So when photographing the throat of a flower, for example, the nearest parts, the tips of the stamen and stigma, may bloom out of focus and hide portions of the flower's base as it's being photographed. These problems can be solved in post-capture software featuring retouching. Two programs I use that offer this function are Helicon Focus (www.heliconsoft.com) and Zerene Stacker (www.zerenesystems.com). Once the image is composited, areas of sharp focus can be cloned from individual captures to the final composite; in some cases, the cloning required might be quite detailed and take some practice to accomplish well.
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