Focus Stacking
Definition
A computational technique combining multiple images focused at sequential depths into a single all-in-focus image; used for three-dimensional evidence with relief exceeding single-exposure DoF.
- Technique
- Combines images focused at sequential depths into one all-in-focus image
- Category
- Computational post-processing technique
- Use case
- Three-dimensional evidence with relief exceeding single-exposure depth of field
- Input
- A sequence of images at incrementally shifted focus distances
Common questions
Why is focus stacking necessary for macro photography of items like tool marks or bite marks?+
At the close working distances used for macro forensic photography, depth of field becomes extremely shallow, often only a fraction of a millimetre, so a single exposure cannot keep an entire three-dimensional surface sharp. Stacking multiple frames at different focus points recovers detail across the full depth of the impression.
Does focus stacking alter the evidentiary content of the resulting image?+
The technique combines only the in-focus regions from each source frame using established software algorithms, without adding or removing surface detail, so the composite is generally treated as a faithful representation, provided the process and original frames are documented and retained for review.
What can go wrong with a focus-stacked image if the subject moves between exposures?+
Because the technique relies on aligning a sequence of frames, any movement of the subject, camera or lighting between shots can produce blending artefacts, ghosting, or misregistered detail at the seams between stacked regions, so the setup must stay rigid throughout the capture sequence.
Related terms
- ABFO No. 2 Scale
- The right-angle L-shaped reference scale with circular targets used in bite mark photography. Its design enables post-capture correction of lens distortion and...
- CCD (Charge-Coupled Device)
- A sensor architecture that transfers accumulated pixel charge across the array to a single output amplifier, producing uniform noise characteristics suitable for...
- CMOS (Complementary Metal-Oxide Semiconductor)
- A sensor architecture with per-pixel amplifiers, enabling fast readout and low power consumption; BSI-CMOS variants now dominate professional forensic cameras.
- Depth of Field (DoF)
- The zone of acceptably sharp focus around the focused plane; controlled by aperture, focal length, and subject distance. Critical for close-up evidence...
- Diffraction Limit
- The aperture threshold below which wave diffraction at the aperture edges degrades resolution more than the narrower aperture improves depth of field....
- Dynamic Range
- The ratio in exposure stops between the brightest recordable highlight and the darkest shadow detail above the noise floor; typically 12-14 stops...
- F-Stop
- The ratio of focal length to aperture diameter, expressed as f/number. Lower numbers mean wider aperture, more light, and shallower depth of...
- ISO
- A standardised sensitivity rating for digital sensors; higher ISO values amplify the output and allow lower-light capture but introduce random noise that...
- Oblique Lighting
- A lighting geometry with the source at 5-30 degrees to the evidence surface, casting long shadows that reveal surface relief. Standard for...
- Ring Flash
- A circular flash tube mounted around the lens that illuminates from all azimuths simultaneously, producing uniform shadowless illumination used for bite-mark and...
- White Balance
- A camera setting that corrects for the colour temperature of the ambient illuminant, ensuring neutral colours render as neutral. Manual white balance...