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Tuesday, March 12, 2013

Unleash Unlimited Depth Of Field

Use stacked focus to create images that overcome the bounds of optics for your macro shots and more

This Article Features Photo Zoom

Finished Image. Craig Blacklock defied the limits of depth of field and diffraction to create this image. He created the series of exposures shown here and on the next page, shooting each one for maximum sharpness in a limited area and then composited the final image together.

For the first time since the invention of the medium, photographers are now able to create images that rival our visual memory. For years, we've been using high dynamic range (HDR) techniques, combining bracketed captures to extend the range of exposure. Using the stacked focus technique, we can extend depth of field, as well.

When we view a composition, our eyes scan the scene, refocusing on every detail, then our brain composites everything we've seen into a visual memory with the entire scene sharp. Painters have depicted the world this way for years, but prior to the advent of stacked focus, photographers were limited to what a lens could resolve in a single capture.

The stacked focus technique allows us to overcome this limitation by making multiple captures, each at a slightly different focus distance, then use software that selects the sharpest pixels from this stack of captures to extend the depth of field through the entire photograph or a specific range within the photograph. In addition to extending the depth of field, the sharpness is enhanced, because each capture creates another focus plane that is in true focus—not just acceptably small circles of confusion, which appear sharp in small prints, but may look mushy in larger ones.

This technique is most commonly applied to close-up work. It's also useful in landscape photography, however, when we want the depth of field to begin closer than that achieved when the lens is focused at the hyperfocal distance for the smallest usable aperture.

Which ƒ-stop Is Best For Creating Stacked Focus Images?
Depth of field is inversely proportional to the square of the magnification. This is why we have such a hard time getting much in focus as we move in closer to a subject or use longer lenses. Of course, we can regain some, or all, of that lost depth of field by closing down the aperture. Depth of field doubles every two stops we close down the aperture. But eventually we run into the fuzzy brick wall of diffraction. As light passes by the aperture blades, it's diffracted. This scattering of the light causes an overall degradation of sharpness. The smaller the aperture, the more pronounced the effect. Images made at ƒ/22 will be slightly softer at the plane of focus than those made at ƒ/16. Those made at ƒ/32 will be dramatically softer than those made at ƒ/22.

So, we must consider both the circle of confusion and the effects of diffraction when selecting which ƒ-stop to use in stacked focus images. Since we're trying to extend depth of field, we're looking for that sweet spot where we get the most depth of field from each capture, without significant loss of sharpness due to diffraction.

For full-frame-sensor cameras, this is an effective aperture of ƒ/22, and for APS-sensor cameras, it's ƒ/16. Keep in mind, the ƒ-stops marked on the lens are based on the lens focused at infinity. As we focus closer, the marked aperture effectively becomes smaller (example: marked aperture ƒ/11 is effectively ƒ/22 when the lens is focused at a 1:1 reproduction ratio).

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