IAI CFPH flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Reciprocal exposure law

Exposure equals intensity times time; halving the aperture area and doubling the shutter time keeps exposure constant, so f/8 at 1/60 s equals f/5.6 at 1/125 s.

Depth of field

The range of subject distances that appears acceptably sharp; it increases with smaller apertures, shorter focal lengths, and greater camera-to-subject distance, and shrinks at high magnification.

Hyperfocal distance

The focus distance H = f squared divided by (N times c) plus f; focusing at H keeps everything from H/2 to infinity acceptably sharp for a given focal length and circle of confusion.

Circle of confusion

The maximum blur spot diameter that still appears sharp at the intended viewing size; it is used to compute depth of field and hyperfocal distance for a given sensor format.

Diffraction limit

The point at which stopping down further softens the image because the aperture approaches the Airy disk scale; on full-frame sensors it typically begins beyond about f/11 to f/16.

Exposure value

EV = log2(N squared divided by t), where N is the f-number and t the shutter time; each unit equals one stop, so EV 12 at ISO 100 matches f/16 at 1/125 s.

Guide number

GN = f-number times distance for a given ISO; a GN 40 flash at f/8 reaches 5 meters, which helps plan flash-lit evidence exposures at different distances.

Inverse square law

Illuminance from a point source falls with the square of distance; moving a flash from 1 m to 2 m quarters the light, changing exposure across a scene.

White balance

The adjustment that makes neutral objects appear neutral under a given light source; tungsten is about 3200 K, daylight about 5500 K, and shade about 7000 K.

Color reference target

A chart of known color patches included in the first frame of a lighting setup; it enables camera profiling and objective color comparison across images.

Raking light

Oblique illumination at a low angle to the surface; it exaggerates surface relief and is used for footwear impressions, tool marks, and indented writing.

Cross-polarization

A polarizing filter on the lens combined with polarized light sources; it removes specular glare from glass, water, and wet surfaces to reveal detail beneath.

Alternate light source

A UV or blue-green excitation source used with barrier filters; it reveals latent stains, biological fluids, and gunshot residue that are invisible under white light.

Infrared reflectography

Imaging with an IR-sensitive camera and IR source; it penetrates some surface layers and reveals carbon-based inks and alterations in documents and paintings.

Ultraviolet fluorescence

Excitation around 415 to 450 nm with a yellow or orange barrier filter; biological fluids fluoresce, but ambient light must be excluded and exposures are long.

Luminol photography

Capturing the faint blue chemiluminescence of luminol in total darkness with a fast lens, high ISO, and a tripod; exposures often exceed 30 seconds.

Macro magnification ratio

The ratio of image size to object size; at 1:1 the object fills the sensor, and effective aperture becomes N times (1 + m), reducing light and deepening diffraction.

Focus stacking

Combining multiple images taken at different focus distances into one sharp composite; the number of slices scales with magnification and required depth, and slices must be registered first.

Photogrammetry overlap

The proportion of shared area between successive images, commonly 60 to 80 percent; sufficient overlap and texture are required for reliable 3D reconstruction and measurement.

Photogrammetric scale bar

A rigid bar of precisely known length placed in the scene; error in its length propagates directly into every dimension measured from the reconstructed model.

ABFO-style scale

An L-shaped forensic scale placed in the same plane as the injury or impression; it allows accurate sizing and prevents perspective error in later measurement.

Perspective control

Converging lines are controlled by camera position, not focal length; keeping the sensor plane parallel to the subject preserves true shape and relative dimensions.

Rolling shutter

A sensor that exposes rows sequentially, which can skew fast-moving objects; a global shutter exposes the whole frame at once and is preferred for measurement-grade imaging.

Lens distortion types

Barrel distortion bows lines outward in wide-angle lenses, pincushion bows them inward in telephoto lenses, and mustache distortion is a complex zoom pattern; all require correction before measurement.

Field of view

FOV is approximately 2 times arctan(sensor dimension divided by 2 times focal length); a 50 mm lens on a 36 mm sensor gives about 39.6 degrees horizontally.

Print resolution

Pixels per inch equals pixel dimension divided by print dimension in inches; a 3000-pixel-wide file prints 10 inches wide at 300 ppi.

EXIF metadata

Capture information embedded in the file, such as camera, lens, settings, and time; it can be altered, so treat it as corroborating rather than conclusive evidence.

File hashing

Computing a digest such as SHA-256 of an image file at ingest and at each transfer; a matching hash verifies the file is unchanged since that point.

Chain of custody

A documented record of who handled evidence, when, and how; for digital images it includes hashes, transfer logs, and retention of the unedited original.

Lossless versus lossy formats

Raw and TIFF preserve image data for re-processing and archival use; JPEG is lossy and should serve only as a presentation copy, since resaving degrades detail.

Generative fill prohibition

Adding or removing content in an evidence image is not acceptable; only disclosed enhancements such as contrast, brightness, and sharpening may be applied to a copy.

Photo log

A record for each exposure listing frame number, location, direction, lighting, and description; it links images to the scene diagram and supports later reconstruction.