Glossary

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An objective measure of sharpness which takes into account the sensitivity of the human visual system to specific spatial frequencies and the viewing distance of an image. Edge acutance refers to the ability of a photographic system to show a sharp edge between contiguous areas of low and high illuminance. Texture acutance refers to the ability of a photographic system to show details without noticeable degradations.

Artifact created by the presence of frequencies in the image that are too high compared to the sampling frequency of the sensor. Staircasing-effects or Moiré patterns are two different forms of aliasing.

Ratio of effective focal length and entrance pupil diameter of a lens. Same as f-Number or f# or f/d. The smaller the aperture, the “faster” (brighter) the lens.

Image sensor approximately the size of an APS (Advanced Photo System) negative (roughly 25.1x16.7 mm.)

Conspicuous elements in the image that do not exist in the real scene. They may be caused by any part of the image acquisition system (optics, sensors or processing).

Illumination of a subject from the back. Usually the subject is dark on a bright background.

The most used pattern of photosites for color cameras, named after its creator, a researcher at Kodak in 1976. Photosites are grouped by four, with two sensitive to green wavelengths, one to blue wavelenghts, and one to red wavelengths.

Change of a standard behavior. Usually Exposure Bias or White Balance Bias.

Set of phenomena following which a point-wise light source is not imaged on the sensor on a single pixel. It is due primarily to optical causes and sensor integration. If the light source moves during the integration time, the phenomenon is known as motion blur.

Estimation of the specific parameters of a device; for example, the main calibration parameters of a sensor are sensitivity, spectral responsitivity, and noise; the main parameters of a lens are distortion, vignetting, chromatic aberration and MTF.

CCD

Charge-Coupled Device: one family of electronic image sensors that output an analog signal. In a digital camera, this analog signal is converted to digital by a separate component.

Complementary metal–oxide–semiconductor: one family of electronic image sensors that output a digital signal

Light source or beam whose rays are parallel. A point-wise light source positioned at a lens focal distance provides a collimated beam. A very far point-wise light source, such as a star other than the sun, can also be considered as a collimated beam.

The number of colors that can be coded as digital values. A color channel coded on 8 bits can represent 256 different values.

A two-dimensional array of colored filters placed on top of a sensor that allow the color of the input light to be recorded. Each photosite outputs a value that depicts the light in a subset of the visible spectrum. The most-used color filter array is the Bayer array.

Characteristic of a reflection or transmission material that does not change the spectrum (though the color) of the illuminant.

Process that transforms the color values of the captured scene into digital RGB values displayed on an output device (screen, printer). By extension, also designates the process that transforms raw sensor values into displayed RGB values.

Numerical values defined by DxO Labs researchers that represent the numbers of colors that a sensor can distinguish, up to noise. Read more

Non-uniformity of color rendering in the field: an object may not have the same color all across the image field (see Vignetting).

Coordinate system representing the visible colors, under a given illuminant. The L* coordinate represents the luminance, while a* and b* represent color (respectively the green-magenta axis and the blue-yellow axis). The Lab color space was designed to be perceptually uniform, meaning that a same distance between two colors should be perceived as the same difference whatever those colors are.

Numerical value characterizing the radiation spectrum of a perfect black body. High color temperature leads to a bluish light, low color temperature leads to a reddish light.

CSF is a functional description of the human visual system threshold sensitivity to the contrast (i.e. peak-to-peak luminance difference) of sine wave patterns of a range of spatial frequencies. The Contrast Sensitivity Function (CSF) is dependent on the viewing conditions, i.e. viewing distance, size of the displayed image and luminance viewing conditions.

Statistical numerical value describing the degree of dependence of two random variables.

Noise measured on a sensor in the absence of light. A perfect sensor should have a uniform signal in the dark, meaning no noise. See Noise, thermal [electronic]

Logarithmic scale used to describe phenomena taking a large range of values. Decibels are not a unit and always describe the ratio to a reference value like Signal to Noise Ratio.

Numerical algorithm used to reconstruct a multi-channel image from single-value pixels that can be read on a sensor behind a color filter array (typically a Bayer sensor).

Algorithm that attenuates noise in a signal to improve its quality.

Optical elements that absorb light used to simulate different levels of illumination. They are usually chosen to be as neutral as possible (i.e. their absorption is independent of wavelength) so that they do not change colors but only intensity.

Signal composed of numerical value or digit (or bits). Digital signals are now widespread since they can be easily stored, copied and transmitted with no signal loss on any media.

Distortion is the variation of the magnification of the lens throughout the image field. In practical terms, photographic distortion describes the degree to which a lens cannot render straight lines in a scene as straight lines in the final image. Read more about how distortion is defined, measured and scored.

Algorithm used to reduce the dimension of an image. According to information theory, down-sampling needs to be performed so that it cancels frequencies too high (over Nyquist frequency) to avoid aliasing.

dpi

Dots per inch, describing the precision of a display device (screen, printer, fax).

Digital Single Lens Reflex: digital camera with a removable unique lens used as a viewfinder and for image capture.

DxO Labs transmission chart with density filters used to measure noise. This type of chart has a high dynamic, and no graininess that could be measured as camera noise. Read more

The DxOMark Score reports average lens-camera performances over the whole focal length and aperture ranges.

The DxOMark Score is reported using a gauge that shows the score itself as well as the range of scores over the focal range. WIth this gauge, photographers can view the homogeneity of the lenses image quality over their focal range.

The DxOMark Score is measured for defined exposure conditions corresponding to low-light scene with 150 lux illumination and an exposure time of 1/60s. These conditions were chosen as we believe low-light performances are very important for today’s photography and it is also important for photographers to know how well lenses perform at the widest aperture.

Read more about the DxOMark Score for lenses and how it is designed.

A numerical value describing the ratio between the highest and lowest luminance values that can be accurately measured on a sensor. Read more