IAI CFPH cheat sheet

Reciprocal exposure law: E = I × t; doubling shutter time and halving aperture area (one f-stop) keeps exposure constant, so f/8 at 1/60 s equals f/5.6 at 1/125 s.

Depth of field increases with smaller aperture (larger f-number), shorter focal length, and greater camera-to-subject distance; it decreases at high magnification, which is why macro evidence shots need small apertures or focus stacking.

Diffraction softening begins when the aperture diameter approaches the Airy disk scale; for a full-frame sensor, stopping beyond about f/11–f/16 trades depth of field for lost resolution.

Hyperfocal distance H = f²/(N × c) + f, where f is focal length, N the f-number, and c the circle of confusion; focusing at H keeps everything from H/2 to infinity acceptably sharp.

Scale documentation: place an L-shaped or ABFO-style scale in the same plane as the injury or impression, fill the frame with the scale, and shoot perpendicular to the surface to avoid perspective error in later measurement.

Photogrammetry requires overlapping images (commonly 60–80% overlap), consistent lighting, fixed focal length, and known scale references; lens distortion must be corrected before 3D reconstruction.

Total pixel count = sensor width in pixels × height in pixels; print resolution in ppi = pixel dimension ÷ print dimension in inches, so a 3000-pixel-wide file prints 10 inches wide at 300 ppi.

Chain of custody for digital images: record camera, lens, settings, and operator; hash the original file (e.g., SHA-256) at ingest; retain the unedited original and work only on verified copies.

Alternate light source (ALS) use: UV reflectance/fluorescence and blue-green excitation with barrier filters reveal latent stains, gunshot residue, and biological fluids invisible under white light.

Color accuracy depends on white balance and a color reference target in frame; without a neutral reference, colorimetric comparison of evidence across scenes is unreliable.

Infrared photography penetrates some surface layers and darkens certain pigments and dyes; it requires an IR-pass filter and a camera with the IR-cut filter removed or a dedicated IR model.

Perspective (converging lines) is controlled by camera position, not focal length; keeping the film/sensor plane parallel to the subject plane preserves true shape and relative dimensions.

Motion blur is avoided when shutter speed is at least 1/(focal length) for handheld work; for critical evidence, use a tripod, remote release, and mirror lock-up.

Macro magnification m = image size ÷ object size; at 1:1 the object fills the sensor, and effective aperture becomes N × (1 + m), reducing light and deepening diffraction.

Archival storage targets for digital evidence commonly follow ISO 18925/18902 for optical media and ISO 18934 for mixed collections; verify integrity by periodic hash checks and migration.

Injury photography: take an orientation (context) frame, a mid-range frame, and a close-up with scale; use oblique lighting to reveal texture, depth, and bite-mark or bruise patterns.

Exposure value EV = log2(N²/t); each unit of EV equals one stop, so EV 12 at ISO 100 corresponds to f/16 at 1/125 s under sunny-16 conditions.

Fingerprint and impression capture: use a copy stand with the lens axis perpendicular to the surface, fill the frame with the impression, and include a scale in the same plane to allow 1:1 reproduction.

Scene documentation sequence: overall, mid-range, and close-up images, plus a photo log recording frame number, location, direction, and description for each exposure.

Digital image authentication examines EXIF metadata, quantization tables, sensor pattern noise (PRNU), and error level analysis; none alone is conclusive, so findings must be corroborated.

Luminol and fluorescein require total darkness and long exposures (often 30 s or more at wide aperture) because the chemiluminescence is faint; a tripod and cable release are mandatory.

Aerial and drone scene imaging must record altitude, camera angle, and ground control points so that orthomosaics can be georeferenced to survey accuracy.

Rendering intent (perceptual, relative colorimetric, absolute colorimetric, saturation) determines how out-of-gamut colors map between device profiles; absolute colorimetric preserves measured color values for comparison.

Bracketing and HDR capture are used for high-contrast scenes such as fire scenes and vehicle interiors; combine exposures only after preserving the original raw files.

Focal length and field of view: FOV ≈ 2 × arctan(sensor dimension ÷ (2 × focal length)); a 50 mm lens on a 36 mm-wide sensor gives about 39.6° horizontal field of view.

Latent print powders and dyes must be photographed before lifting; use a scale, and record the powder type and lighting angle, since contrast depends on the illumination geometry.

Bloodstain pattern documentation requires overall, mid-range, and close-up images with a scale and an angle reference; impact-angle calculations depend on documented ellipse dimensions.

Video evidence: preserve the original container file, note frame rate and resolution, and extract frames with a validated tool; frame timing must be referenced to the source clock.

ISO affects signal-to-noise: raising ISO amplifies signal and noise together, so low-light evidence should favor longer exposure on a tripod over high ISO whenever motion is not a factor.

Reflectance transformation imaging (RTI) captures a subject under varying light directions and computes surface normals, revealing tool marks and indented writing not visible in a single photograph.

3D laser scanning and structured-light scanning produce point clouds; registration error and resolution must be reported with any measurement derived from the model.

Photographic measurement of a scene requires at least one known dimension in the same plane; without it, pixel measurements cannot be converted to real units.

Color management workflow: camera profile → working space (e.g., Adobe RGB or ProPhoto) → output profile; converting rather than assigning profiles preserves appearance.

Ultraviolet fluorescence for biological fluids uses excitation around 415–450 nm with a yellow or orange barrier filter; exposure times are long and ambient light must be excluded.

Documentation of digital manipulation: any enhancement (sharpening, brightness, cropping) must be logged and applied to a copy, with the original retained unaltered for court.

Shutter types affect evidence capture: rolling shutter can skew fast-moving objects, while global shutter captures the whole frame simultaneously and is preferred for measurement-grade imaging.

Lens distortion types: barrel (wide-angle), pincushion (telephoto), and mustache (complex zooms); correct with a calibrated distortion model before photogrammetric measurement.

Lighting geometry for texture: raking or oblique light at a low angle exaggerates surface relief for footwear and tool-mark impressions; diffuse axial light minimizes shadow for color and pattern.

Storage and archive longevity: raw files should be retained alongside processed JPEGs; checksum verification at ingest and at migration detects bit rot and unauthorized alteration.

Forensic video frame extraction must record the exact frame index and timecode; playback software that interpolates frames can fabricate detail not present in the source.

Report writing for imaging: state equipment, settings, lighting, scale, and any processing applied, so another examiner can reproduce the image under the same conditions.

Macro depth of field is extremely shallow; at 1:1 and f/16 on a full-frame camera, usable depth of field is often only a few millimeters, so focus stacking is standard for impression evidence.

Color temperature is measured in kelvin; tungsten ≈ 3200 K, daylight ≈ 5500 K, and shade ≈ 7000 K; mismatched white balance produces color casts that can mimic or mask stains.

Polarizing filters remove specular glare from glass, water, and wet surfaces, revealing detail beneath; cross-polarization (filter on lens and light source) is used for document and painting examination.

Image resolution and evidentiary value: native capture resolution determines detail; upsampling cannot create real detail and must be disclosed if used for presentation.

Photographic scales must be rigid, legible, and placed in the focal plane; flexible tapes can distort and are unsuitable for measurement-grade documentation.

Court presentation of images should include the unprocessed original, the processed derivative, and a description of each step linking the two.

Scene lighting ratios: main-to-fill ratios of 3:1 to 5:1 give modeling without losing shadow detail; flat 1:1 lighting is preferred when color and pattern fidelity matter more than texture.

Focus stacking combines multiple images at different focus distances; the number of slices needed scales with magnification and required depth, and each slice must be registered before merging.

Digital zoom is interpolation, not optics; only optical focal length changes real magnification, so evidence images should avoid digital zoom or disclose its use.

Metadata (EXIF) can be altered; treat it as corroborating rather than conclusive, and verify capture time against independent records such as dispatch logs.

Evidence photography of vehicles and fire scenes often needs wide dynamic range; expose for highlights and lift shadows in raw conversion while retaining the original file.

Photographic documentation of a scene should be completed before any item is moved; later images of moved items must be labeled as such to preserve context.

Angle of incidence equals angle of reflection; for specular surfaces like glass or polished metal, position the light at the mirror angle to control glare or move off-axis to suppress it.

Inverse square law: illuminance falls with the square of distance, so moving a flash from 1 m to 2 m quarters the light; this affects exposure consistency across a scene.

Guide number GN = f-number × distance (for a given ISO); a GN 40 flash at f/8 reaches 5 m, useful for planning flash-lit evidence shots.

Color reference targets (e.g., 24-patch charts) enable profile creation and objective color comparison; include one in the first frame of each lighting setup.

Bitemark and injury photography should include a scale, a color reference, and an orientation frame; distortion from camera angle can alter apparent arch shape.

Preservation of digital originals: write-protect the source card, image it with a validated tool, and hash both the image and the working copy before analysis.

Infrared and ultraviolet capture require modified or dedicated cameras; standard sensors filter these bands, so unmodified cameras cannot record true UV or IR reflectance.

Photographic reconstruction of a scene can test visibility and sightlines; it must document camera height, lens, and position so that the reconstruction is verifiable.

Multiple flash exposures for large scenes: balance ambient and flash by adjusting shutter (ambient) and aperture/ISO (flash), since flash duration is shorter than most shutter speeds.

Image file formats: raw preserves sensor data for re-processing; TIFF is lossless for archival derivatives; JPEG is lossy and should be a presentation copy only.

Measurement uncertainty in photogrammetry depends on camera calibration, baseline, and scale accuracy; report a confidence interval with any derived dimension.

Documentation of lighting diagrams helps reproducibility; record light positions, distances, modifiers, and power settings for each evidence setup.

Latent print development by lighting: oblique white light for dust prints on smooth surfaces, and UV or ALS for fluorescent powders and dyes; photograph before any lifting attempt.

Photographic comparison of footwear and tire impressions requires 1:1 reproduction with a scale in the same plane, plus consistent lighting to preserve tread detail.

Digital watermarking and hashing serve different purposes: hashing detects alteration, while watermarking can embed provenance but may be removed by re-encoding.

Scene diagram and photo cross-reference: number each photo in the log and mark camera positions and directions on the sketch to reconstruct the sequence later.

Exposure for fluorescent evidence is long and dim; use a wide aperture, high ISO only as needed, and a tripod; test exposures and bracket to avoid losing faint signal.

Court admissibility of imaging often turns on authentication and reproducibility; the photographer must be able to explain every setting and processing step applied.

Lens aperture and exposure: f-numbers form a geometric sequence (1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22); each step halves the light reaching the sensor.

Shutter speed and motion: 1/60 s freezes slow movement, 1/250 s or faster is needed for walking subjects, and 1/1000 s for fast action; evidence should be sharp or the blur explained.

White balance presets and custom balance: a custom balance from a neutral card in the scene lighting gives the most accurate color for evidence comparison.

Photographic documentation of fingerprints on curved surfaces needs even illumination; a ring flash or diffuse source reduces specular hotspots that obscure ridge detail.

Data integrity: store originals on write-once media where possible, maintain an access log, and verify hashes at each transfer between examiners.

Report language should distinguish observation from interpretation; describe what the image shows before drawing conclusions about its significance.

Photographic scale calibration: photograph a ruler at the same distance and plane as the evidence to derive pixels per millimeter; any change in distance invalidates the calibration.

Aerial photography for scene mapping uses ground control points and overlapping nadir images; without control points, the mosaic cannot be georeferenced accurately.

Light source color rendering index (CRI) affects color fidelity; high-CRI sources (90+) are preferred when color comparison matters, such as paint or fiber evidence.

Digital image enhancement for court: contrast, brightness, and sharpening are acceptable if disclosed; adding or removing content is not, and generative fill must never be used on evidence.

Photographing documents: use even, diffuse lighting, keep the camera parallel to the page, include a scale, and avoid flash hotspots that wash out ink detail.

Retention schedules for case images should follow agency policy and legal hold requirements; deletion before the retention period ends can constitute spoliation.

Macro lighting: a ring flash gives flat, shadowless illumination good for color and pattern; a twin flash gives directional modeling good for texture and depth.

Exposure triangle: aperture, shutter speed, and ISO interact; changing one by a stop requires compensating another by a stop to maintain the same exposure.

Depth of field and sensor size: smaller sensors give greater depth of field at the same field of view and aperture, which is why compact cameras are sometimes used for documentation.

Photogrammetric scale bars must be rigid and precisely known; error in the scale propagates directly into every measured dimension in the model.

Scene photographs should be taken before evidence collection; if items must be moved, photograph them in place first and record the original position.

Authentication of digital images can use embedded capture signatures or trusted timestamping; these are stronger than metadata alone, which is easily edited.

Latent evidence under ALS: use the excitation wavelength matched to the fluorophore and a barrier filter that blocks the excitation light; mismatched filters reduce contrast.

Focal plane and subject plane alignment: tilt or swing movements can extend depth of field along a plane, but they distort measurement, so keep the camera axis perpendicular for evidence.

Color space and bit depth: 8-bit JPEG limits tonal adjustment; 16-bit TIFF or raw retains more data for enhancement without posterization.

Photographic documentation of bloodstain patterns includes overall, mid-range, and close-up frames plus a scale and angle reference; the pattern's geometry drives reconstruction.

Video and image evidence should be copied with a write-blocker or validated acquisition tool, and the copy hash compared to the source before analysis.

Report reproducibility: another examiner using the documented equipment, settings, and lighting should be able to produce a comparable image.

Flash synchronization: the sync speed is the fastest shutter speed at which the whole frame is exposed to flash; exceeding it produces a dark band from the focal-plane shutter.

Ambient and flash balance: shutter speed controls ambient exposure while aperture and ISO control flash exposure, allowing the background to be darkened or lightened independently.

Photographic evidence of wounds should be taken at the same distance and angle on repeat visits to allow comparison of healing and size over time.

Image integrity checks include comparing file hashes, examining metadata for inconsistencies, and looking for resaving artifacts such as quantization table changes.

Lens flare and stray light can obscure evidence; use a lens hood and control light direction, especially when photographing shiny or reflective surfaces.

Documentation should include a lighting diagram and a photo log, because memory alone is insufficient to reconstruct the setup months later in court.

Photographic measurement of tire and footwear impressions requires a scale in the same plane and the camera axis perpendicular; oblique angles introduce foreshortening error.

Forensic image comparison uses class and individual characteristics; the image must be of sufficient resolution to resolve individual detail, or the comparison is limited to class features.

Ultraviolet reflectance photography of documents can reveal erased or altered writing; it requires a UV source, a UV-pass filter on the lens, and a camera sensitive to UV.

Color temperature mismatch between flash and ambient light causes mixed-light color casts; use gels to match sources or exclude ambient by underexposing it.

Metadata should be preserved through the workflow; stripping EXIF during export destroys capture information needed for authentication.

Evidence images should be stored with a documented chain of custody, including hash values and transfer records, to support admissibility.

Exposure compensation adjusts the meter's suggested exposure; it is needed when the scene is dominated by bright or dark areas that bias the meter, such as snow or night scenes.

Histogram interpretation: a histogram clipped at either end loses detail; expose to preserve highlight detail and recover shadows in raw conversion when possible.

Photographic documentation of a fire scene requires wide dynamic range and often supplemental lighting; record conditions such as smoke and water that affect visibility.

Macro focusing is best done manually because autofocus hunts at high magnification; use a focusing rail for precise, repeatable focus placement.

Digital evidence should be analyzed on a copy, with the original preserved and hashed; any derived image must be traceable to the original through a documented process.

Court presentation should use images at native resolution or clearly labeled derivatives; enlarging beyond native resolution can mislead about detail.

Lighting for impression evidence: oblique or raking light reveals depth in footwear and tool marks; diffuse light is used for color and pattern documentation.

Photographic scale selection: use a scale long enough to span the evidence and place it adjacent, not overlapping, so the evidence is not obscured.

Image compression artifacts (JPEG blocking, ringing) can mimic or obscure fine detail; retain lossless originals and avoid repeated resaving.

Photogrammetry software requires camera calibration data and sufficient overlap; insufficient overlap or textureless surfaces cause reconstruction gaps and measurement error.

Scene photography should include a compass or directional reference so that image orientation can be related to the scene diagram.

Documentation of digital processing should list software, version, and parameters, so that the enhancement can be reproduced and evaluated.

Aperture and diffraction: very small apertures (f/22 and beyond) reduce sharpness through diffraction; for evidence requiring maximum detail, use the largest aperture that still gives adequate depth of field.

Shutter speed and camera stability: use a tripod for any exposure slower than 1/(focal length); image stabilization helps but does not replace a tripod for critical evidence.

Color reference and white balance: a neutral gray card in the scene allows accurate white balance in post; without it, color comparison across images is unreliable.

Digital image authentication should consider the whole workflow, from capture to court, because a single hash verifies a file at one point in time, not its history.

Photographic documentation of a scene should record the time and date of each image, ideally synchronized to a reliable clock, to support timeline reconstruction.

Report writing should state limitations, such as resolution or lighting constraints, that affect what the images can and cannot show.

Photographic comparison of questioned and known images requires similar lighting, angle, and scale; differences in these conditions can create false dissimilarities.

Infrared reflectography is used to see beneath surface layers in documents and paintings; it requires an IR-sensitive camera and an IR source, and it reveals carbon-based inks.

Luminol photography requires a dark environment and long exposure; the faint blue chemiluminescence is best captured with a fast lens, high ISO, and a tripod.

Digital image evidence should be described in the report by its file name, hash, and the tool used to view or process it, so that the exact file can be identified later.

Scene lighting should be documented because it affects visibility and color; later recreations must match the original conditions to be comparable.

Photographic scales should be traceable to a known standard; an uncalibrated scale introduces systematic error into every measurement derived from the image.

Exposure for fluorescent evidence: the signal is faint, so use a wide aperture, a long exposure on a tripod, and the correct barrier filter; bracket exposures to avoid missing faint detail.

Photographic documentation of a scene should include images of the overall context, the item in place, and a close-up with scale, in that order, before collection.

Digital image enhancement should be applied to a copy and documented; the original must remain unaltered to preserve its evidentiary value.

Color management across devices requires calibrated monitors and profiles; uncalibrated displays can make colors appear different from the captured evidence.

Photogrammetric measurement error increases with distance and decreases with baseline and resolution; report uncertainty rather than a single number without qualification.

Court presentation of video should use the original file and a player that does not interpolate frames, so that the displayed detail matches the source.

Lighting ratio and contrast: a 3:1 ratio gives moderate contrast, 5:1 strong contrast; for evidence where texture matters, higher ratios reveal surface detail but may hide color.

Photographic documentation of a scene at night requires artificial lighting; record the type and position of lights so that visibility at the time can be reconstructed.

Image file naming and organization should be consistent and linked to the photo log, so that each image can be located and identified in court.

Digital image analysis should use validated tools and documented parameters; the same tool and settings should be used for questioned and known images when comparing.

Photographic scale placement should avoid obscuring evidence; place the scale adjacent and in the same plane, and take a second frame if the scale covers detail.

Report language should be precise about what the image shows and what it does not; overstating the evidentiary value of an image undermines credibility.

Depth of field and magnification: at high magnification, depth of field shrinks rapidly, so focus stacking or a smaller aperture is needed, but diffraction limits how far the aperture can be stopped down.

Photographic documentation of a scene should include a scale and a color reference in at least one frame, so that size and color can be verified later.

Digital evidence should be stored with a hash and a chain-of-custody record; any transfer should be logged with the hash verified at each step.

Photogrammetry requires a calibrated camera and known scale; without calibration, lens distortion introduces systematic error into measurements.

Scene photography should be completed before any alteration, and any later images should be labeled to distinguish them from the original scene.

Report writing should include the equipment, settings, lighting, and processing used, so that the images can be evaluated and reproduced.

Flash exposure and distance: flash falls off with the square of distance, so subjects at different distances in the same frame receive different exposure; use manual flash and position accordingly.

Photographic documentation of a scene should record the camera position and direction for each image, so that the sequence can be reconstructed on a diagram.

Digital image authentication should consider the possibility of manipulation; metadata, hashes, and visual inspection together provide stronger evidence than any one alone.

Color reference targets should be included in the first frame of each lighting setup, so that color can be balanced consistently across the series.

Photographic scale calibration should be repeated if the camera-to-subject distance changes; a calibration from one distance does not apply to another.

Court presentation should distinguish the original image from any processed derivative, and the processing should be described in the report.

Lighting for latent prints on porous surfaces: use oblique light for dust prints and ALS for fluorescent treatments; photograph before lifting to preserve the original appearance.

Photographic documentation of a scene should include overall, mid-range, and close-up images with scale, plus a photo log linking each image to its location.

Digital image processing should be applied to a copy, with the original preserved and hashed; any enhancement should be documented and reproducible.

Photogrammetric measurement requires a scale in the same plane as the evidence; without it, pixel measurements cannot be converted to real units.

Scene photography should record the time and date, ideally synchronized to a reliable clock, to support timeline reconstruction.

Report writing should state limitations, such as resolution or lighting constraints, that affect what the images can and cannot show.

Exposure and motion: a fast shutter speed freezes motion but requires more light; for evidence in low light, use a tripod and a longer exposure rather than a high ISO that adds noise.

Photographic documentation of a scene should include a scale and a color reference in at least one frame, so that size and color can be verified later.

Digital evidence should be stored with a hash and a chain-of-custody record; any transfer should be logged with the hash verified at each step.

Photogrammetry requires a calibrated camera and known scale; without calibration, lens distortion introduces systematic error into measurements.

Scene photography should be completed before any alteration, and any later images should be labeled to distinguish them from the original scene.

Report writing should include the equipment, settings, lighting, and processing used, so that the images can be evaluated and reproduced.

Flash exposure and distance: flash falls off with the square of distance, so subjects at different distances in the same frame receive different exposure; use manual flash and position accordingly.

Photographic documentation of a scene should record the camera position and direction for each image, so that the sequence can be reconstructed on a diagram.

Digital image authentication should consider the possibility of manipulation; metadata, hashes, and visual inspection together provide stronger evidence than any one alone.

Color reference targets should be included in the first frame of each lighting setup, so that color can be balanced consistently across the series.

Photographic scale calibration should be repeated if the camera-to-subject distance changes; a calibration from one distance does not apply to another.

Court presentation should distinguish the original image from any processed derivative, and the processing should be described in the report.

Lighting for latent prints on porous surfaces: use oblique light for dust prints and ALS for fluorescent treatments; photograph before lifting to preserve the original appearance.

Photographic documentation of a scene should include overall, mid-range, and close-up images with scale, plus a photo log linking each image to its location.

Digital image processing should be applied to a copy, with the original preserved and hashed; any enhancement should be documented and reproducible.

Photogrammetric measurement requires a scale in the same plane as the evidence; without it, pixel measurements cannot be converted to real units.

Scene photography should record the time and date, ideally synchronized to a reliable clock, to support timeline reconstruction.

Report writing should state limitations, such as resolution or lighting constraints, that affect what the images can and cannot show.

Exposure and motion: a fast shutter speed freezes motion but requires more light; for evidence in low light, use a tripod and a longer exposure rather than a high ISO that adds noise.

Photographic documentation of a scene should include a scale and a color reference in at least one frame, so that size and color can be verified later.

Digital evidence should be stored with a hash and a chain-of-custody record; any transfer should be logged with the hash verified at each step.

Photogrammetry requires a calibrated camera and known scale; without calibration, lens distortion introduces systematic error into measurements.

Scene photography should be completed before any alteration, and any later images should be labeled to distinguish them from the original scene.

Report writing should include the equipment, settings, lighting, and processing used, so that the images can be evaluated and reproduced.