Light meter

Last updated
Use of a light meter for portrait cinematography in a Turkish music video set Erkan Umut shoots Sibel Can.jpg
Use of a light meter for portrait cinematography in a Turkish music video set

A light meter (or illuminometer) is a device used to measure the amount of light. In photography, an exposure meter is a light meter coupled to either a digital or analog calculator which displays the correct shutter speed and f-number for optimum exposure, given a certain lighting situation and film speed. Similarly, exposure meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.

Contents

Light meters also are used in the general field of architectural lighting design to verify proper installation and performance of a building lighting system, and in assessing the light levels for growing plants.

If a light meter is giving its indications in luxes, it is called a "luxmeter". [1]

Evolution

Watkins Standard Exposure Meter - a type of actinometer Watkins Standard Exposure Meter.jpg
Watkins Standard Exposure Meter - a type of actinometer
Watkins Bee Meter - a type of actinometer Watkins Bee Meter.jpg
Watkins Bee Meter - a type of actinometer

Actinometers

The earliest exposure meters were called actinometers (not to be confused with the scientific instrument with the same name), first developed in the late 1800s after commercial photographic plates became available with consistent sensitivity. These photographic actinometers used light-sensitive paper; the photographer would measure the time required for the paper to darken to a control value, providing an input to a mechanical calculation of shutter speed and aperture for a given plate number. [3] :69 They were popular between approximately 1890 and 1920. [4]

Extinction types

Dremo extinction meter Dremo Light Meter (1931) (15494003563).jpg
Dremo extinction meter

The next exposure meters, developed at about the same time but not displacing actinometers in popularity until the 1920s and 1930s, are known as extinction meters, evaluating the correct exposure settings by variable attenuation. [4] One type of extinction meter contained a numbered or lettered row of neutral density filters of increasing density. The photographer would position the meter in front of their subject and note the filter with the greatest density that still allowed incident light to pass through. In another example, sold as Heyde's Aktino-Photometer starting from the early 1900s, the photographer views the scene through an eyepiece and turns the meter to vary the effective density until the scene can no longer be seen. [6] The letter or number corresponding to the filter strength causing the "extinction" of the scene was used as an index into a chart of appropriate aperture and shutter speed combinations for a given film speed. [3] :72

Extinction meters tended to provide inconsistent results because they depended on subjective interpretation and the light sensitivity of the human eye, which can vary from person to person. [7]

Photoelectric types

Analog handheld light meter - Gossen Lunasix 3 (in US: Luna Pro S); available from 1961 to 1977 Gossen Lunasix 3 front.jpg
Analog handheld light meter - Gossen Lunasix 3 (in US: Luna Pro S); available from 1961 to 1977

Later[ when? ] meters removed the human element and relied on technologies incorporating selenium, CdS, and silicon photodetectors.

Selenium and silicon light meters use sensors that are photovoltaic: they generate a voltage proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter; they need no battery to operate and this made them very convenient in completely mechanical cameras. Selenium sensors however cannot measure low light accurately (ordinary lightbulbs can take them close to their limits) and are altogether unable to measure very low light, such as candlelight, moonlight, starlight etc. Silicon sensors need an amplification circuit and require a power source such as batteries to operate. CdS light meters use a photoresistor sensor whose electrical resistance changes proportionately to light exposure. These also require a battery to operate. Most modern light meters use silicon or CdS sensors. They indicate the exposure either with a needle galvanometer or on an LCD screen.

An automatic light meter/exposure unit from an 8 mm movie camera, based on a galvanometer mechanism (center) and a CdS photoresistor, in opening at left. Autoexpmeter.JPG
An automatic light meter/exposure unit from an 8 mm movie camera, based on a galvanometer mechanism (center) and a CdS photoresistor, in opening at left.

Many modern consumer still and video cameras include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that. Photographers working with controlled lighting and cinematographers use handheld light meters to precisely measure the light falling on various parts of their subjects and use suitable lighting to produce the desired exposure levels.

Reflected and incident measurements

Exposure meters generally are sorted into reflected-light or incident-light types, depending on the method used to measure the scene.

Reflected-light meters measure the light reflected by the scene to be photographed. All in-camera meters are reflected-light meters. Reflected-light meters are calibrated to show the appropriate exposure for "average" scenes. An unusual scene with a preponderance of light colors or specular highlights would have a higher reflectance; a reflected-light meter taking a reading would incorrectly compensate for the difference in reflectance and lead to underexposure. Badly underexposed sunset photos are common exactly because of this effect: the brightness of the setting sun fools the camera's light meter and, unless the in-camera logic or the photographer take care to compensate, the picture will be grossly underexposed and dull.

This pitfall (but not in the setting-sun case) is avoided by incident-light meters which measure the amount of light falling on the subject using a diffuser with a flat or (more commonly) hemispherical field of view placed on top of the light sensor. Because the incident-light reading is independent of the subject's reflectance, it is less likely to lead to incorrect exposures for subjects with unusual average reflectance. Taking an incident-light reading requires placing the meter at the subject's position and pointing it in the general direction of the camera, something not always achievable in practice, e.g., in landscape photography where the subject distance approaches infinity.

Another way to avoid under- or over-exposure for subjects with unusual reflectance is to use a spot meter: a specialized reflected-light meter that measures light in a very tight cone, typically with a one degree circular angle of view. An experienced photographer can take multiple readings over the shadows, midrange, and highlights of the scene to determine optimal exposure, using systems like the Zone System.

Many modern cameras include sophisticated multi-segment metering systems that measure the luminance of different parts of the scene to determine the optimal exposure. When using a film whose spectral sensitivity is not a good match to that of the light meter, for example orthochromatic black-and-white or infrared film, the meter may require special filters and re-calibration to match the sensitivity of the film.

There are other types of specialized photographic light meters. Flash meters are used in flash photography to verify correct exposure. Color meters are used where high fidelity in color reproduction is required. Densitometers are used in photographic reproduction.

Exposure meter calibration

In most cases, an incident-light meter will cause a medium tone to be recorded as a medium tone, and a reflected-light meter will cause whatever is metered to be recorded as a medium tone. What constitutes a "medium tone" depends on meter calibration and several other factors, including film processing or digital image conversion.

Meter calibration establishes the relationship between subject lighting and recommended camera settings. The calibration of photographic light meters is covered by ISO 2720:1974.

Exposure equations

For reflected-light meters, camera settings are related to ISO speed and subject luminance by the reflected-light exposure equation:

where

For incident-light meters, camera settings are related to ISO speed and subject illuminance by the incident-light exposure equation:

where

Calibration constants

Determination of calibration constants has been largely subjective; ISO 2720:1974 states that

The constants and shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.

In practice, the variation of the calibration constants among manufacturers is considerably less than this statement might imply, and values have changed little since the early 1970s.

ISO 2720:1974 recommends a range for of 10.6 to 13.4 with luminance in cd/m2. Two values for are in common use: 12.5 (Canon, Nikon, and Sekonic [8] ) and 14 (Minolta, [9] Kenko, [9] and Pentax); the difference between the two values is approximately 16 EV.

The earliest calibration standards were developed for use with wide-angle averaging reflected-light meters (Jones and Condit 1941). Although wide-angle average metering has largely given way to other metering sensitivity patterns (e.g., spot, center-weighted, and multi-segment), the values for determined for wide-angle averaging meters have remained.

The incident-light calibration constant depends on the type of light receptor. Two receptor types are common: flat (cosine-responding) and hemispherical (cardioid-responding). With a flat receptor, ISO 2720:1974 recommends a range for of 240 to 400 with illuminance in lux; a value of 250 is commonly used. A flat receptor typically is used for measurement of lighting ratios, for measurement of illuminance, and occasionally, for determining exposure for a flat subject.

For determining practical photographic exposure, a hemispherical receptor has proven more effective. Don Norwood, inventor of incident-light exposure meter with a hemispherical receptor, thought that a sphere was a reasonable representation of a photographic subject. According to his patent (Norwood 1938), the objective was

to provide an exposure meter which is substantially uniformly responsive to light incident upon the photographic subject from practically all directions which would result in the reflection of light to the camera or other photographic register.

and the meter provided for "measurement of the effective illumination obtaining at the position of the subject."

With a hemispherical receptor, ISO 2720:1974 recommends a range for of 320 to 540 with illuminance in lux; in practice, values typically are between 320 (Minolta) and 340 (Sekonic). The relative responses of flat and hemispherical receptors depend upon the number and type of light sources; when each receptor is pointed at a small light source, a hemispherical receptor with = 330 will indicate an exposure approximately 0.40 step greater than that indicated by a flat receptor with = 250. With a slightly revised definition of illuminance, measurements with a hemispherical receptor indicate "effective scene illuminance."

Calibrated reflectance

It is commonly stated that reflected-light meters are calibrated to an 18% reflectance, [10] but the calibration has nothing to do with reflectance, as should be evident from the exposure formulas. However, some notion of reflectance is implied by a comparison of incident- and reflected-light meter calibration.

Combining the reflected-light and incident-light exposure equations and rearranging gives

Reflectance is defined as

A uniform perfect diffuser (one following Lambert's cosine law) of luminance emits a flux density of ; reflectance then is

Illuminance is measured with a flat receptor. It is straightforward to compare an incident-light measurement using a flat receptor with a reflected-light measurement of a uniformly illuminated flat surface of constant reflectance. Using values of 12.5 for and 250 for gives

With a of 14, the reflectance would be 17.6%, close to that of a standard 18% neutral test card. In theory, an incident-light measurement should agree with a reflected-light measurement of a test card of suitable reflectance that is perpendicular to the direction to the meter. However, a test card seldom is a uniform diffuser, so incident- and reflected-light measurements might differ slightly.

In a typical scene, many elements are not flat and are at various orientations to the camera, so that for practical photography, a hemispherical receptor usually has proven more effective for determining exposure. Using values of 12.5 for and 330 for gives

With a slightly revised definition of reflectance, this result can be taken as indicating that the average scene reflectance is approximately 12%. A typical scene includes shaded areas as well as areas that receive direct illumination, and a wide-angle averaging reflected-light meter responds to these differences in illumination as well as differing reflectances of various scene elements. Average scene reflectance then would be

where "effective scene illuminance" is that measured by a meter with a hemispherical receptor.

ISO 2720:1974 calls for reflected-light calibration to be measured by aiming the receptor at a transilluminated diffuse surface, and for incident-light calibration to be measured by aiming the receptor at a point source in a darkened room. For a perfectly diffusing test card and perfectly diffusing flat receptor, the comparison between a reflected-light measurement and an incident-light measurement is valid for any position of the light source. However, the response of a hemispherical receptor to an off-axis light source is approximately that of a cardioid rather than a cosine, so the 12% "reflectance" determined for an incident-light meter with a hemispherical receptor is valid only when the light source is on the receptor axis.

Cameras with internal meters

Calibration of cameras with internal meters is covered by ISO 2721:1982; nonetheless, many manufacturers specify (though seldom state) exposure calibration in terms of , and many calibration instruments (e.g., Kyoritsu-Arrowin multi-function camera testers [11] ) use the specified to set the test parameters.

Exposure determination with a neutral test card

If a scene differs considerably from a statistically average scene, a wide-angle averaging reflected-light measurement may not indicate the correct exposure. To simulate an average scene, a substitute measurement sometimes is made of a neutral test card, or gray card .

At best, a flat card is an approximation to a three-dimensional scene, and measurement of a test card may lead to underexposure unless adjustment is made. The instructions for a Kodak neutral test card recommend that the indicated exposure be increased by 12 step for a frontlighted scene in sunlight. The instructions also recommend that the test card be held vertically and faced in a direction midway between the Sun and the camera; similar directions are also given in the Kodak Professional Photoguide. The combination of exposure increase and the card orientation gives recommended exposures that are reasonably close to those given by an incident-light meter with a hemispherical receptor when metering with an off-axis light source.

In practice, additional complications may arise. Many neutral test cards are far from perfectly diffuse reflectors, and specular reflections can cause increased reflected-light meter readings that, if followed, would result in underexposure. It is possible that the neutral test card instructions include a correction for specular reflections.

Use in illumination

Light meters or light detectors are also used in illumination. Their purpose is to measure the illumination level in the interior and to switch off or reduce the output level of luminaires. This can greatly reduce the energy burden of the building by significantly increasing the efficiency of its lighting system. It is therefore recommended to use light meters in lighting systems, especially in rooms where one cannot expect users to pay attention to manually switching off the lights. Examples include hallways, stairs, and big halls.

There are, however, significant obstacles to overcome in order to achieve a successful implementation of light meters in lighting systems, of which user acceptance is by far the most formidable. Unexpected or too frequent switching and too bright or too dark rooms are very annoying and disturbing for users of the rooms. Therefore, different switching algorithms have been developed:

Other uses

In Scientific Research & Development uses, a light meter consists of a radiometer (the electronics/readout), a photo-diode or sensor (generates an output when exposed to electromagnetic radiation/light) a filter (used to modify the incoming light so only the desired portion of incoming radiation reaches the sensor) and a cosine correcting input optic (assures the sensor can see the light coming in from all directions accurately).

When the word light meter or photometer is used in place of radiometer or optometer, or it is often assumed the system was configured to see only visible light. Visible light sensors are often called illuminance or photometric sensors because they have been filtered to be sensitive only to 400-700 nanometers (nm) mimicking the human eyes' sensitivity to light. How accurately the meter measures often depends on how well the filtration matches the human eyes' response.

The sensor will send a signal to the meter that is proportional to the amount of light that reaches the sensor after being collected by the optics and passing through the filter. The meter then converts the incoming signal (typically current or voltage) from the sensor into a reading of calibrated units such as Foot-Candles (fc) or Lux (lm/m^2). Calibration in fc or lux, is the second most important feature of a light meter. It not only converts the signal from V or mA, but it also provides accuracy and unit to unit repeatability. National Institute of Standards and Technology (NIST) traceability and ISO/IEC 17025 accreditation are two well known terms that verify the system includes a valid calibration.

The meter/radiometer/photometer portion may have many features including:

Zero: subtracts ambient/background light levels, or stabilize the meter to the working environment

Hold: freezes the value on the display.

Range: for systems that are not linear and auto ranging this function allows the user to select the portion of the meter electronics that best handles the signal level in use.

Units: For illuminance the units are typically only lux and foot-candles but many light meters can also be used for UV, VIS and IR applications so the readout could change to W/cm^2, candela, Watts etc.

Integrate: sums up the values into a dose or exposure level i.e. lux*sec or J/cm^2.

HortiPower spectrum meter to measure Photon Flux Density (light for plants) HortiPower Smart Spectrum Meter.png
HortiPower spectrum meter to measure Photon Flux Density (light for plants)

Along with having a variety of features, a light meter may also be usable for a variety of applications. These may include the measurement of other bands of light such UVA, UVB, UVC and Near IR. For example, UVA and UVB light meters are used for phototherapy or treatment of skin conditions, germicidal radiometers are used for measuring the UVC level from lamps used for disinfection and sterilization, luminance meters are used to measure the brightness of a sign, display or exit sign, PAR quantum sensors are used to measure how much of a given light source's emission will help plants grow, and UV-curing radiometers test how much of the lights emission is effective for hardening a glue, plastic, or protective coating.

Some light meters also have the ability to provide a readout in many different units. Lux and footcandles are the common units for visible light, but so are Candelas, Lumens, and Candela per square meter. In the realm of disinfection, UVC is typically measured in watts per square centimeter, or watts for a given individual lamp assembly, whereas systems used in the context of the curing of coatings often provide readouts in Joules per Square centimeter. Regular measurements of UVC light intensity thus can serve to provide assurance of proper disinfection of water and food-preparation surfaces, or reliable coating hardness in painted products.

Although a light meter can take the form of a very simple handheld tool with one-button operation, there are also many advanced light-measurement systems available for use in numerous different applications. These can be incorporated into automated systems that can, for example, wipe lamps clean when a certain reduction in output is detected, or that can trigger an alarm when lamp-failure occurs.

See also

Notes

  1. Merriam-Webster Dictionary - luxmeter
  2. "Leudi, 1934". Early Photography. Retrieved 8 September 2023.
  3. 1 2 Fraprie, Frank R., ed. (1915). The Secret of Exposure. Practical Photography. Vol. 1. Boston, Massachusetts: American Photographic Publishing Company. Retrieved 8 September 2023. In [the Heyde] instrument, blue-glass prisms are used to cut out the light reflected from the object. One looks through the eyepiece and turns the thicker portion of the prisms (one or both, according to the luminosity of the object) into position until the shadow details are suppressed. By reference to Tables, the necessary exposure may readily be found.
  4. 1 2 "Exposure Meters". Early Photography. Retrieved 8 September 2023.
  5. "Dremo, 1931". Early Photography. Retrieved 8 September 2023.
  6. "Heyde Aktino-Photometer, 1904". Early Photography. Retrieved 8 September 2023.
  7. Dunn, Jack F.; Wakefield, George L. (1974). "3: Extinction Meters". Exposure Manual (Third ed.). Hertfordshire, England: Fountain Press. pp. 82–86. ISBN   0-85242-361-6 . Retrieved 8 September 2023.
  8. Specifications for Sekonic light meters are available on the Sekonic web site under "Products."
  9. 1 2 Konica Minolta Photo Imaging, Inc. left the camera business on March 31, 2006. Rights and tooling for Minolta exposure meters were acquired by Kenko Co, Ltd. in 2007. Specifications for the Kenko meters are essentially the same as for the equivalent Minolta meters.
  10. Some authors (Ctein 1997, 29) have argued that the calibrated reflectance is closer to 12% than to 18%.
  11. Specifications for Kyoritsu testers are available on the C.R.I.S. Camera Services web site under "kyoritsu test equipment."

Related Research Articles

<span class="mw-page-title-main">Luminance</span> Photometric measure

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle.

<span class="mw-page-title-main">Exposure (photography)</span> Amount of light captured by a camera

In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor. It is determined by shutter speed, lens F-number, and scene luminance. Exposure is measured in units of lux-seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.

f-number Measure of lens speed

An f-number is a measure of the light-gathering ability of an optical system such as a camera lens. It is calculated by dividing the system's focal length by the diameter of the entrance pupil. The f-number is also known as the focal ratio, f-ratio, or f-stop, and it is key in determining the depth of field, diffraction, and exposure of a photograph. The f-number is dimensionless and is usually expressed using a lower-case hooked f with the format f/N, where N is the f-number.

<span class="mw-page-title-main">Film speed</span> Measure of a photographic films sensitivity to light

Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system introduced in 1974. A closely related system, also known as ISO, is used to describe the relationship between exposure and output image lightness in digital cameras. Prior to ISO, the most common systems were ASA in the U.S. and DIN in Europe.

<span class="mw-page-title-main">Exposure value</span> Measure of illuminance for a combination of a cameras shutter speed and f-number

In photography, exposure value (EV) is a number that represents a combination of a camera's shutter speed and f-number, such that all combinations that yield the same exposure have the same EV. Exposure value is also used to indicate an interval on the photographic exposure scale, with a difference of 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a stop.

<span class="mw-page-title-main">Light value</span>

In photography, light value has been used to refer to a "light level" for either incident or reflected light, often on a base-2 logarithmic scale. The term does not derive from a published standard, and has had several different meanings:

  1. An arbitrary value indicated by an exposure meter such as the Weston Master V, discussed in Adams. This may have been the origin of the term. The indicated light value was transferred to the meter's exposure calculator, which then was used to determine camera settings. Ray (2000) uses the term, with the acronym 'LV', in this sense. The Honeywell/Pentax 1°/21° spot meter indicated in "light level" ("LL"), with LL essentially exposure value (EV) for ISO 100 film speed. The later Pentax Spotmeter V and Digital Spotmeter indicated directly in EV for ISO 100, but they made no mention of "light level", "light value", or LV.
  2. A synonym for incident light value, from the Additive system of Photographic EXposure (APEX). Zakia and Stroebel (1993) and Stroebel, Compton, Current, and Zakia (2000) used the term in this sense. They used the APEX symbol , normally used for luminance value.
  3. An apparent synonym for luminance value, from APEX. Stroebel, Compton, Current, and Zakia (2000) referred to "scene illuminance" in the text of the article, but the example used units of luminance. The defining equation used the symbol . The table at the end of the article used units of illuminance and the symbol , as noted above.
  4. A synonym for exposure value (EV).
  5. A synonym for "EV at ISO 100 film speed". This usage appears on many web pages, usually without attribution of an authoritative source. Eads (2000) proposed a revised APEX in which luminance value was equal to EV for ISO 100 speed, but he did not use the term light value.
  6. Literally speaking, the English term light value could be translated as "Lichtwert" in German language, however, this is not what the term Lichtwert, as it was introduced by the German shutter manufacturer Friedrich Deckel in 1954 and defined as Belichtungswert in DIN 19010, is used for in German-speaking countries. Instead, the established term Lichtwert describes what became known as exposure value (EV) elsewhere.
<span class="mw-page-title-main">Illuminance</span> Luminous flux incident on a surface per area

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.

The Zone System is a photographic technique for determining optimal film exposure and development, formulated by Ansel Adams and Fred Archer. Adams described the Zone System as "[...] not an invention of mine; it is a codification of the principles of sensitometry, worked out by Fred Archer and myself at the Art Center School in Los Angeles, around 1939–40."

<span class="mw-page-title-main">Reciprocity (photography)</span> A photography term

In photography, reciprocity is the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material. Within a normal exposure range for film stock, for example, the reciprocity law states that the film response will be determined by the total exposure, defined as intensity × time. Therefore, the same response can result from reducing duration and increasing light intensity, and vice versa.

<span class="mw-page-title-main">Spectral power distribution</span> Measurement describing the power of an illumination

In radiometry, photometry, and color science, a spectral power distribution (SPD) measurement describes the power per unit area per unit wavelength of an illumination. More generally, the term spectral power distribution can refer to the concentration, as a function of wavelength, of any radiometric or photometric quantity.

A spectroradiometer is a light measurement tool that is able to measure both the wavelength and amplitude of the light emitted from a light source. Spectrometers discriminate the wavelength based on the position the light hits at the detector array allowing the full spectrum to be obtained with a single acquisition. Most spectrometers have a base measurement of counts which is the un-calibrated reading and is thus impacted by the sensitivity of the detector to each wavelength. By applying a calibration, the spectrometer is then able to provide measurements of spectral irradiance, spectral radiance and/or spectral flux. This data is also then used with built in or PC software and numerous algorithms to provide readings or Irradiance (W/cm2), Illuminance, Radiance (W/sr), Luminance (cd), Flux, Chromaticity, Color Temperature, Peak and Dominant Wavelength. Some more complex spectrometer software packages also allow calculation of PAR μmol/m2/s, Metamerism, and candela calculations based on distance and include features like 2- and 20-degree observer, baseline overlay comparisons, transmission and reflectance.

The science of photography is the use of chemistry and physics in all aspects of photography. This applies to the camera, its lenses, physical operation of the camera, electronic camera internals, and the process of developing film in order to take and develop pictures properly.

<span class="mw-page-title-main">Guide number</span>

When setting photoflash exposures, the guide number (GN) of photoflash devices is a measure photographers can use to calculate either the required f‑stop for any given flash-to-subject distance, or the required distance for any given f‑stop. To solve for either of these two variables, one merely divides a device's guide number by the other.

APEX stands for Additive System of Photographic Exposure, which was proposed in the 1960 ASA standard for monochrome film speed, ASA PH2.5-1960, as a means of simplifying exposure computation.

Exposure compensation is a technique for adjusting the exposure indicated by a photographic exposure meter, in consideration of factors that may cause the indicated exposure to result in a less-than-optimal image. Factors considered may include unusual lighting distribution, variations within a camera system, filters, non-standard processing, or intended underexposure or overexposure. Cinematographers may also apply exposure compensation for changes in shutter angle or film speed, among other factors.

<span class="mw-page-title-main">Gray card</span> Reflectance reference used in photography

A gray card is a middle gray reference, typically used together with a reflective light meter, as a way to produce consistent image exposure and/or color in video production, film, and photography.

<span class="mw-page-title-main">Image sensor format</span> Shape and size of a digital cameras image sensor

In digital photography, the image sensor format is the shape and size of the image sensor.

Contrast, in physics and digital imaging, is a quantifiable property used to describe the difference in appearance between elements within a visual field. It is closely linked with the perceived brightness of objects and is typically defined by specific formulas that involve the luminances of the stimuli. For example, contrast can be quantified as ΔL/L near the luminance threshold, known as Weber contrast, or as LH/LL at much higher luminances. Further, contrast can result from differences in chromaticity, which are specified by colorimetric characteristics such as the color difference ΔE in the CIE 1976 UCS.

As visual perception varies logarithmically, it is helpful to have an appreciation of both illuminance and luminance by orders of magnitude.

References