Exposure (photography)

"Overexposed" redirects here. For other uses, seeOverexposed (disambiguation).

Photographic image taken using a variety of exposures

Inphotography,exposureis the amount oflightper unitareareaching aframeofphotographic filmor the surface of an electronicimage sensor.It is determined byshutter speed,lensF-number,and sceneluminance.Exposure is measured inunitsoflux-seconds(symbol lx ⋅ s), and can be computed fromexposure value(EV) and scene luminance in a specified region.

An "exposure" is a singleshutter cycle.For example, along exposurerefers to a single, long shutter cycle to gather enough dim light, whereas amultiple exposureinvolves a series of shutter cycles, effectively layering a series of photographs in one image. The accumulatedphotometric exposure(H_v) is the same so long as the total exposure time is the same.

Definitions

Radiant exposure

Radiant exposureof asurface, denotedH_e( "e" for "energetic", to avoid confusion withphotometricquantities) and measured inJ/m²,is given by

$H_{\mathrm {e} }=E_{\mathrm {e} }t,$

where

• E_eis theirradianceof the surface, measured inW/m²;
• tis theexposure duration,measured in s.

Luminous exposure

Luminous exposureof asurface,denotedH_v( "v" for "visual", to avoid confusion withradiometricquantities) and measured inlx⋅s,is given by

$H_{\mathrm {v} }=E_{\mathrm {v} }t,$

where

• E_vis theilluminanceof the surface, measured in lx;
• tis the exposure duration, measured in s.

If the measurement is adjusted to account only for light that reacts with the photo-sensitive surface, that is, weighted by the appropriatespectral sensitivity,the exposure is still measured in radiometric units (joules per square meter), rather than photometric units (weighted by the nominal sensitivity of the human eye).Only in this appropriately weighted case does theHmeasure the effective amount of light falling on the film, such that thecharacteristic curvewill be correct independent of the spectrum of the light.

Many photographic materials are also sensitive to "invisible" light, which can be a nuisance (seeUV filterandIR filter), or a benefit (seeinfrared photographyandfull-spectrum photography). The use of radiometric units is appropriate to characterize such sensitivity to invisible light.

Insensitometricdata, such as characteristic curves, thelog exposureis conventionally expressed as log₁₀(H). Photographers more familiar with base-2 logarithmic scales (such asexposure values) can convert usinglog₂(H) ≈ 3.32 log₁₀(H).

Table 2. SI radiometry units

Quantity		Unit		Dimension	Notes
Name	Symbol	Name	Symbol
Radiant energy	Qe	joule	J	M⋅L2⋅T−2	Energy of electromagnetic radiation.
Radiant energy density	we	joule per cubic metre	J/m3	M⋅L−1⋅T−2	Radiant energy per unit volume.
Radiant flux	Φe	watt	W= J/s	M⋅L2⋅T−3	Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power", and calledluminosityin Astronomy.
Spectral flux	Φe,ν	watt perhertz	W/Hz	M⋅L2⋅T−2	Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm−1.
	Φe,λ	watt per metre	W/m	M⋅L⋅T−3
Radiant intensity	Ie,Ω	watt persteradian	W/sr	M⋅L2⋅T−3	Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is adirectionalquantity.
Spectral intensity	Ie,Ω,ν	watt per steradian per hertz	W⋅sr−1⋅Hz−1	M⋅L2⋅T−2	Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1.This is adirectionalquantity.
	Ie,Ω,λ	watt per steradian per metre	W⋅sr−1⋅m−1	M⋅L⋅T−3
Radiance	Le,Ω	watt per steradian per square metre	W⋅sr−1⋅m−2	M⋅T−3	Radiant flux emitted, reflected, transmitted or received by asurface,per unit solid angle per unit projected area. This is adirectionalquantity. This is sometimes also confusingly called "intensity".
Spectral radiance Specific intensity	Le,Ω,ν	watt per steradian per square metre per hertz	W⋅sr−1⋅m−2⋅Hz−1	M⋅T−2	Radiance of asurfaceper unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅m−2⋅nm−1.This is adirectionalquantity. This is sometimes also confusingly called "spectral intensity".
	Le,Ω,λ	watt per steradian per square metre, per metre	W⋅sr−1⋅m−3	M⋅L−1⋅T−3
Irradiance Flux density	Ee	watt per square metre	W/m2	M⋅T−3	Radiant fluxreceivedby asurfaceper unit area. This is sometimes also confusingly called "intensity".
Spectral irradiance Spectral flux density	Ee,ν	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Irradiance of asurfaceper unit frequency or wavelength. This is sometimes also confusingly called "spectral intensity". Non-SI units of spectral flux density includejansky(1 Jy=10−26W⋅m−2⋅Hz−1) andsolar flux unit(1 sfu=10−22W⋅m−2⋅Hz−1=104Jy).
	Ee,λ	watt per square metre, per metre	W/m3	M⋅L−1⋅T−3
Radiosity	Je	watt per square metre	W/m2	M⋅T−3	Radiant fluxleaving(emitted, reflected and transmitted by) asurfaceper unit area. This is sometimes also confusingly called "intensity".
Spectral radiosity	Je,ν	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Radiosity of asurfaceper unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1.This is sometimes also confusingly called "spectral intensity".
	Je,λ	watt per square metre, per metre	W/m3	M⋅L−1⋅T−3
Radiant exitance	Me	watt per square metre	W/m2	M⋅T−3	Radiant fluxemittedby asurfaceper unit area. This is the emitted component of radiosity. "Radiant emittance" is an old term for this quantity. This is sometimes also confusingly called "intensity".
Spectral exitance	Me,ν	watt per square metre per hertz	W⋅m−2⋅Hz−1	M⋅T−2	Radiant exitance of asurfaceper unit frequency or wavelength. The latter is commonly measured in W⋅m−2⋅nm−1."Spectral emittance" is an old term for this quantity. This is sometimes also confusingly called "spectral intensity".
	Me,λ	watt per square metre, per metre	W/m3	M⋅L−1⋅T−3
Radiant exposure	He	joule per square metre	J/m2	M⋅T−2	Radiant energy received by asurfaceper unit area, or equivalently irradiance of asurfaceintegrated over time of irradiation. This is sometimes also called "radiant fluence".
Spectral exposure	He,ν	joule per square metre per hertz	J⋅m−2⋅Hz−1	M⋅T−1	Radiant exposure of asurfaceper unit frequency or wavelength. The latter is commonly measured in J⋅m−2⋅nm−1.This is sometimes also called "spectral fluence".
	He,λ	joule per square metre, per metre	J/m3	M⋅L−1⋅T−2
See also: SI Radiometry Photometry|(Compare)

Table 1. SI photometry quantities

Quantity		Unit		Dimension	Notes
Name	Symbol	Name	Symbol
Luminous energy	Qv	lumen second	lm⋅s	T⋅J	The lumen second is sometimes called thetalbot.
Luminous flux,luminous power	Φv	lumen(= candelasteradian)	lm (= cd⋅sr)	J	Luminous energy per unit time
Luminous intensity	Iv	candela(= lumen per steradian)	cd(= lm/sr)	J	Luminous flux per unitsolid angle
Luminance	Lv	candela per square metre	cd/m2(= lm/(sr⋅m2))	L−2⋅J	Luminous flux per unit solid angle per unitprojectedsource area. The candela per square metre is sometimes called thenit.
Illuminance	Ev	lux(= lumen per square metre)	lx(= lm/m2)	L−2⋅J	Luminous fluxincidenton a surface
Luminous exitance,luminous emittance	Mv	lumen per square metre	lm/m2	L−2⋅J	Luminous fluxemittedfrom a surface
Luminous exposure	Hv	lux second	lx⋅s	L−2⋅T⋅J	Time-integrated illuminance
Luminous energy density	ωv	lumen second per cubic metre	lm⋅s/m3	L−3⋅T⋅J
Luminous efficacy(of radiation)	K	lumen perwatt	lm/W	M−1⋅L−2⋅T3⋅J	Ratio of luminous flux toradiant flux
Luminous efficacy(of a source)	η	lumen perwatt	lm/W	M−1⋅L−2⋅T3⋅J	Ratio of luminous flux to power consumption
Luminous efficiency,luminous coefficient	V			1	Luminous efficacy normalized by the maximum possible efficacy
See also: SI Photometry Radiometry|(Compare)

Optimum exposure

Main article:sensitometry

"Correct" exposure may be defined as an exposure that achieves the effect the photographer intended.

A more technical approach recognises that a photographic film (or sensor) has a physically limiteduseful exposure range,sometimes called itsdynamic range.If, for any part of the photograph, the actual exposure is outside this range, the film cannot record it accurately. In a very simple model, for example, out-of-range values would be recorded as "black" (underexposed) or "white" (overexposed) rather than the precisely graduated shades of colour and tone required to describe "detail". Therefore, the purpose of exposure adjustment (and/or lighting adjustment) is to control the physical amount of light from the subject that is allowed to fall on the film, so that 'significant' areas of shadow and highlight detail do not exceed the film's useful exposure range. This ensures that no 'significant' information is lost during capture.

The photographer may carefully overexpose or underexpose the photograph toeliminate"insignificant" or "unwanted" detail; to make, for example, a white altar cloth appear immaculately clean, or to emulate the heavy, pitiless shadows offilm noir.However, it is technically much easier to discard recorded information duringpost processingthan to try to 're-create' unrecorded information.

In a scene with strong or harsh lighting, theratiobetween highlight and shadow luminance values may well be larger than theratiobetween the film's maximum and minimum useful exposure values. In this case, adjusting the camera's exposure settings (which only applies changes to the whole image, not selectively to parts of the image) only allows the photographer to choose between underexposed shadows or overexposed highlights; it cannot bring both into the useful exposure range at the same time. Methods for dealing with this situation include: using what is calledfill lightingto increase the illumination in shadow areas; using agraduated neutral-density filter,flag, scrim, orgoboto reduce the illumination falling upon areas deemed too bright; or varying the exposure between multiple, otherwise identical, photographs (exposure bracketing) and then combining them afterwards in anHDRIprocess.

Overexposure and underexposure

"Underexposure" redirects here. For the 2005 film by Oday Rasheed, seeUnderexposure (film).

White chair: Deliberate use of overexposure for aesthetic purposes

A photograph may be described asoverexposedwhen it has a loss of highlight detail, that is, when important bright parts of an image are "washed out" or effectively all white, known as "blown-out highlights" or "clipped whites".A photograph may be described asunderexposedwhen it has a loss of shadow detail, that is, when important dark areas are "muddy" or indistinguishable from black,known as "blocked-up shadows" (or sometimes "crushed shadows", "crushed blacks", or "clipped blacks", especially in video).As the adjacent image shows, these terms are technical ones rather than artistic judgments; an overexposed or underexposed image may be "correct" in the sense that it provides the effect that the photographer intended.Intentionally over- or underexposing(relative to a standard or the camera's automatic exposure) is casually referred to as "exposing to the right"or" exposing to the left "respectively, as these shift the histogram of the image to the right or left.

Exposure settings

Two similar images, one taken in auto mode (underexposed), the other with manual settings

Manual exposure

In manual mode, the photographer adjusts thelens apertureand/orshutter speedto achieve the desired exposure. Many photographers choose to control aperture and shutter independently because opening up the aperture increases exposure, but also decreases thedepth of field,and a slower shutter increases exposure but also increases the opportunity formotion blur.

"Manual" exposure calculations may be based on some method oflight meteringwith a working knowledge ofexposure values,theAPEX systemand/or theZone System.

Automatic exposure

Buildings and trees photographed with anautoexposure timeof 1/200 s

A camera inautomatic exposureorautoexposure(usuallyinitializedasAE) mode automatically calculates and adjusts exposure settings to match (as closely as possible) the subject's mid-tone to the mid-tone of the photograph. For most cameras, this means using an on-boardTTLexposure meter.

Aperture priority(commonlyabbreviatedasA,orAvforaperture value) mode gives the photographer manual control of the aperture, whilst the camera automatically adjusts the shutter speed to achieve the exposure specified by the TTL meter.Shutter priority(often abbreviated asS,orTvfortime value) mode gives manual shutter control, with automatic aperture compensation. In each case, the actual exposure level is still determined by the camera's exposure meter.

Exposure compensation

Main article:Exposure compensation

A street view ofTaka-Töölö,Helsinki,Finland,during a very sunny winter day. The image has been deliberately overexposed by +1 EV to compensate for the bright sunlight and the exposure time calculated by the camera's program automatic metering is still 1/320 s.

The purpose of anexposure meteris to estimate the subject's mid-toneluminanceand indicate the camera exposure settings required to record this as a mid-tone. In order to do this it has to make a number of assumptions which, under certain circumstances, will be wrong. If the exposure setting indicated by an exposure meter is taken as the "reference" exposure, the photographer may wish to deliberatelyoverexposeorunderexposein order to compensate for known or anticipated metering inaccuracies.

Cameras with any kind of internal exposure meter usually feature an exposure compensation setting which is intended to allow the photographer to simply offset the exposure level from the internal meter's estimate of appropriate exposure. Frequently calibrated in stops,also known asEV units,a "+1" exposure compensation setting indicates one stop more (twice as much) exposure and "–1" means one stop less (half as much) exposure.

Exposure compensation is particularly useful in combination with auto-exposure mode, as it allows the photographer tobiasthe exposure level without resorting to full manual exposure and losing the flexibility of auto exposure. On low-end video camcorders, exposure compensation may be the only manual exposure control available.

Exposure control

A 1/30 s exposure showing motion blur on fountain atRoyal Botanic Gardens, Kew

A 1/320 s exposure showing individual drops on fountain atRoyal Botanic Gardens, Kew

An appropriate exposure for a photograph is determined by the sensitivity of the medium used. For photographic film, sensitivity is referred to asfilm speedand is measured on a scale published by theInternational Organization for Standardization(ISO). Faster film, that is, film with a higher ISO rating, requires less exposure to make a readable image.Digital camerasusually have variable ISO settings that provide additional flexibility. Exposure is a combination of the length of time and theilluminanceat the photosensitive material. Exposure time is controlled in acamerabyshutter speed,and the illuminance depends on the lensapertureand the sceneluminance.Slower shutter speeds (exposing the medium for a longer period of time), greater lens apertures (admitting more light), and higher-luminance scenes produce greater exposures.

An approximately correct exposure will be obtained on a sunny day using ISO 100 film, an aperture off/16and a shutter speed of 1/100 of a second. This is called thesunny 16 rule:at an aperture off/16on a sunny day, a suitable shutter speed will be one over the film speed (or closest equivalent).

A scene can be exposed in many ways, depending on the desired effect a photographer wishes to convey.

Reciprocity

Main article:reciprocity (photography)

Further information:aperture,exposure range,andf-number

An important principle of exposure isreciprocity.If one exposes the film or sensor for a longer period, a reciprocally smaller aperture is required to reduce the amount of light hitting the film to obtain the same exposure. For example, the photographer may prefer to make his sunny-16 shot at an aperture off/5.6(to obtain a shallow depth of field). Asf/5.6is 3stops"faster" thanf/16,with each stop meaning double the amount of light, a new shutter speed of (1/125)/(2·2·2) = 1/1000 s is needed. Once the photographer has determined the exposure, aperture stops can be traded for halvings or doublings of speed, within limits.

A demonstration of the effect of exposure in night photography. Longer shutter speeds result in increased exposure.

The true characteristic of most photographic emulsions is not actually linear (seesensitometry), but it is close enough over the exposure range of about 1 second to 1/1000 of a second. Outside of this range, it becomes necessary to increase the exposure from the calculated value to account for this characteristic of the emulsion. This characteristic is known asreciprocity failure.The film manufacturer's data sheets should be consulted to arrive at the correction required, as different emulsions have different characteristics.

Digital cameraimage sensorscan also be subject to a form of reciprocity failure.

Determining exposure

A fair ride taken with a 1/3 s exposure

TheZone Systemis another method of determining exposure and development combinations to achieve a greater tonality range over conventional methods by varying the contrast of the film to fit the print contrast capability. Digital cameras can achieve similar results (high dynamic range) by combining several different exposures (varying shutter or diaphragm) made in quick succession.

Today, most cameras automatically determine the correct exposure at the time of taking a photograph by using a built-inlight meter,or multiple point meters interpreted by a built-in computer, seemetering mode.

Negative and print film tends to bias for exposing for the shadow areas (film dislikes being starved of light), with digital favouring exposure for highlights. See latitude below.

Latitude

Example image exhibiting blown-out highlights. Top: original image, bottom: blown-out areas marked red

Latitude is the degree by which one can over, or under expose an image, and still recover an acceptable level of quality from an exposure. Typically negative film has a better ability to record a range of brightness than slide/transparency film or digital. Digital should be considered to be the reverse of print film, with a good latitude in the shadow range, and a narrow one in the highlight area; in contrast to film's large highlight latitude, and narrow shadow latitude. Slide/Transparency film has a narrow latitude in both highlight and shadow areas, requiring greater exposure accuracy.

Negative film's latitude increases somewhat with high ISO material, in contrast digital tends to narrow on latitude with high ISO settings.

Highlights

Main article:Clipping (photography)

Areas of a photo where information is lost due to extreme brightness are described as having "blown-out highlights" or "flared highlights".

In digital images this information loss is often irreversible, though small problems can be made less noticeable usingphoto manipulation software.Recording to RAW format can correct this problem to some degree, as can using a digital camera with a better sensor.

Film can often have areas of extreme overexposure but still record detail in those areas. This information is usually somewhat recoverable when printing or transferring to digital.

A loss of highlights in a photograph is usually undesirable, but in some cases can be considered to "enhance" appeal. Examples includeblack and white photographyand portraits with an out-of-focus background.

Blacks

Areas of a photo where information is lost due to extreme darkness are described as "crushed blacks". Digital capture tends to be more tolerant of underexposure, allowing better recovery of shadow detail, than same-ISO negative print film.

Crushed blacks cause loss of detail, but can be used for artistic effect.

See also

• Bulb (photography)
• Exposure bracketing
• Exposure value
• Film speed
• Gray card
• Multi-exposure HDR capture
• Light painting
• Light value
• Night photography
• Multiple exposure
• Sensitometry(and Hurter–Driffield curves)
• Shutter speed(also calledexposure time)
• Zebra patterning

External links

• Media related toExposureat Wikimedia Commons