Optics glossary

Short definitions of the terms used across Optixwise tools and articles, written for camera and lens engineers. Each entry links to the tool that calculates it.

Active alignment (AA)

A camera-module assembly process in which the lens is moved in five or six axes while the sensor images a test chart, then fixed with UV-cured adhesive at the sharpest position. Used when the depth of focus is too small for passive mechanical tolerances. See fixed-focus modules.

Airy disk

The bright central spot of the diffraction pattern a perfect lens forms from a point of light. Its diameter to the first dark ring is 2.44 · λ · N, about 2.7 µm at f/2 and 550 nm. Diffraction tool.

Aspect ratio

Width divided by height of the sensor's active area, such as 4:3 or 16:9. Sensors with the same optical format but different aspect ratios have the same diagonal but different widths. Sensor tool.

Back focal length (BFL)

The distance from the vertex of the last lens surface to the image plane when the lens is focused at infinity. It sets how much room the module has for the IR filter, cover glass and sensor package.

Chief ray angle (CRA)

The angle at which the ray through the center of the aperture stop reaches a point on the sensor, measured from the optical axis. Sensor microlenses are shifted for a design CRA; a lens with a different CRA causes corner shading and color shift. See CRA explained.

Circle of confusion (CoC)

The largest blur spot still counted as sharp. Depth of field and depth of focus are both defined by it. For camera modules a practical choice is one to two pixel pitches. See choosing a CoC.

Crop factor

The full-frame diagonal (43.27 mm) divided by a sensor's diagonal. Multiply a focal length by it to get the full-frame lens with the same field of view. Sensor tool.

Depth of field (DoF)

The range of object distances that appear acceptably sharp, with blur below the circle of confusion. Depth-of-field tool.

Depth of focus

The range of sensor positions behind the lens over which the image stays sharp: ±N · c, or 2 · N · c in total. It sets the focus and tilt tolerance in module assembly. Depth-of-focus tool.

Diffraction limit

The resolution limit set by the wave nature of light, independent of lens quality. An incoherent system passes no detail finer than the cutoff frequency 1 / (λ · N). A camera is "diffraction-limited" when this blur, rather than aberrations or pixels, dominates. Diffraction tool.

Distortion

Deviation of image position from the ideal projection, usually given in percent at the image corner. Barrel distortion compresses the edges, pincushion stretches them. "TV distortion" is a different measure based on how much a straight edge bows across the frame.

DORI

Detect, observe, recognize, identify: the IEC 62676-4 pixel-density levels for surveillance, at 25, 62.5, 125 and 250 px/m on the target. See DORI explained and the pixels-on-target tool.

Effective focal length (EFL)

The focal length of the lens as a whole, which sets magnification and field of view. For a rectilinear lens, image height is EFL · tan θ.

f-number (N)

Focal length divided by entrance-pupil diameter. It controls image brightness (proportional to 1/N²), depth of field, depth of focus and the diffraction limit.

f-θ (equidistant) lens

A fisheye projection in which image height is proportional to field angle, r = f · θ. Common in wide-angle surveillance, automotive and robotics cameras because angle maps linearly to pixels. Field-of-view tool.

Field of view (FOV)

The angle of the scene the sensor captures, given as horizontal, vertical and diagonal (HFOV, VFOV, DFOV). It depends on focal length, sensor size and the lens projection. Field-of-view tool.

Hyperfocal distance

The closest focus distance at which everything out to infinity is acceptably sharp, H ≈ f² / (N · c). Focusing at H makes the sharp range run from about H/2 to infinity. Depth-of-field tool.

Image circle

The diameter of the area the lens illuminates with acceptable quality on the image plane. It must be at least the sensor diagonal, or the corners darken.

Magnification (m)

Image size divided by object size. For an object at distance s, m = f / (s − f). At close range the working f-number grows to N · (1 + m).

Modulation transfer function (MTF)

How much contrast an optical system keeps at each spatial frequency, from 1 at coarse detail falling toward 0 at fine detail. MTF at the sensor's Nyquist frequency is a common sharpness specification. Diffraction tool.

Nyquist frequency

The finest line pattern a pixel grid can sample, 1 / (2 · p): about 172 line pairs per mm for 2.9 µm pixels. Sensor tool.

Optical format

A sensor size class written as a fraction of an inch, such as 1/2.8″. It comes from old camera-tube diameters and is not a real dimension. See why 1/2.8″ is not 1/2.8 inch.

Pixel density (px/m, PPM)

Sensor pixels per meter of the scene at the target distance, f / (p · D) for a rectilinear lens. The basis of DORI. Pixels-on-target tool.

Pixel pitch

The center-to-center spacing of sensor pixels, usually 0.6–6 µm. Together with the pixel count it gives the exact active-area size.

Rayleigh criterion

Two points are just resolved when one's peak falls on the other's first dark ring, a separation of 1.22 · λ · N on the sensor. Diffraction tool.

Relative illumination (RI)

Image brightness at a field position relative to the center. It falls toward the corners because of the cos⁴ law, vignetting and pupil effects, and is typically specified at the image corner.

Working f-number

The effective f-number for a near object, N · (1 + m). It, not the marked f-number, sets diffraction and depth of focus at close range.