Focus stacking

Getting sufficient depth of field can be particularly challenging in macro photography, because depth of field is smaller (shallower) for objects nearer the camera, so if a small object fills the frame, it is often so close that its entire depth cannot be in focus at once. Depth of field is normally increased by stopping down aperture (using a larger f-number), but beyond a certain point, stopping down causes blurring due to diffraction, which counteracts the benefit of being in focus. It also reduces the luminosity of the image. Focus stacking allows the depth of field of images taken at the sharpest aperture to be effectively increased. The images at right illustrate the increase in DOF that can be achieved by combining multiple exposures.

Stacked image of the Curiosity Rover's first sampling hole in Mount Sharp. The hole is 1.6 cm (0.63 in) wide and 6.7 cm (2.6 in) deep.

The Mars Science Laboratory mission has a device called Mars Hand Lens Imager (MAHLI), which can take photos that can later be focus stacked.[6]

In microscopy, high numerical apertures are desirable to capture as much light as possible from a small sample. A high numerical aperture (equivalent to a low f-number) gives a very shallow depth of field. Higher magnification objective lenses generally have shallower depth of field; a 100× objective lens with a numerical aperture of around 1.4 has a depth of field of approximately 1 μm. When observing a sample directly, the limitations of the shallow depth of field are easy to circumvent by focusing up and down through the sample; to effectively present microscopy data of a complex 3D structure in 2D, focus stacking is a very useful technique.

Atomic resolution scanning transmission electron microscopy encounters similar difficulties, where specimen features are much larger than the depth of field. By taking a through-focal series, the depth of focus can be reconstructed to create a single image entirely in focus.[7]

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  Pepper mill, stack of 28 frames
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  Stacked image of 3 × 2.5 mm electric wires
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  Shaver head, stack of 36 frames, retouched
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  Macrolepiota procera, stack of 15 frames
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  Stacked image of the inner ridge of an orchid blossom
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  Stacked image of two Arecaceae viewed through a hole in a tree trunk
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  Pellet, stack of 32 frames
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  Alluaudia comosa, stack of 10 frames
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  Mold on Litchi chinensis, stack of 20 frames
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  Skull, stack of 6 frames
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  Sympetrum flaveolum male, stack of 36 frames

• Video example of how focus stacking is applied to images

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  Software creates from the sharpest areas in a stack of sections.