![Histogram-Normal](https://upload.wikimedia.org/wikipedia/commons/3/3c/Histogram-Normal.png)
In
digital photography
Digital photography uses cameras containing arrays of electronic photodetectors interfaced to an analog-to-digital converter (ADC) to produce images focused by a lens, as opposed to an exposure on photographic film. The digitized image is sto ...
, exposing to the right (ETTR) is the technique of adjusting the
exposure of an image as high as possible at base ISO (without causing unwanted saturation) to collect the maximum amount of light and thus get the optimum performance out of the digital
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curr ...
.
The name derives from the resulting
image histogram
An image histogram is a type of histogram that acts as a graphical representation of the tonal distribution in a digital image. It plots the number of pixels for each tonal value. By looking at the histogram for a specific image a viewer will ...
which, according to this technique, should be placed close to the right of its display. Advantages include greater tonal range in dark areas, greater
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
(SNR),
fuller use of the
colour gamut and greater
latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
during
post-production
Post-production is part of the process of filmmaking, video production, audio production, and photography. Post-production includes all stages of production occurring after principal photography or recording individual program segments.
The ...
.
The direction of the relative adjustment to the camera's meter reading depends on the dynamic range (or
contrast ratio
The contrast ratio (CR) is a property of a display system, defined as the ratio of the luminance of the brightest shade (white) to that of the darkest shade (black) that the system is capable of producing. A high contrast ratio is a desired aspec ...
) of the scene. Typically, with low-contrast scenes, an increase in exposure over that indicated by the camera's meter will be required. When attempting a single-exposure of a high dynamic-range scene, a reduction in exposure from the meter's reading may be needed. In the final analysis, however, the camera's meter is irrelevant to ETTR since the ETTR exposure is established, not by a meter reading, but by the camera's exposure indicators, the histogram and/or the highlight-clipping indicators (blinkies/zebras).
ETTR images requiring increased exposure may appear to be
overexposed (too bright) when taken and must be correctly
processed (
normalized) to produce a photograph as envisaged. Care must be taken to avoid
clipping
Clipping may refer to:
Words
* Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement"
* Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel
* Clipping (publications) ...
within any colour
channel
Channel, channels, channeling, etc., may refer to:
Geography
* Channel (geography), in physical geography, a landform consisting of the outline (banks) of the path of a narrow body of water.
Australia
* Channel Country, region of outback Austral ...
, other than acceptable areas such as
specular highlights
A specular highlight is the bright spot of light that appears on shiny objects when illuminated (for example, see image on right). Specular highlights are important in 3D computer graphics, as they provide a strong visual cue for the shape of a ...
.
The principle is also applied in
film
A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
photography in order to maximize the
negative's latitude and
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
and achieve richer blacks when the image is printed slightly down.
With high-dynamic-range scenes
With the advancement of digital
image sensor
An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curr ...
s, the same ETTR technique may be applicable to scenes with a relatively
high dynamic range
High dynamic range (HDR) is a dynamic range higher than usual, synonyms are wide dynamic range, extended dynamic range, expanded dynamic range.
The term is often used in discussing the dynamic range of various signals such as images, videos, au ...
(HDR) (high
contrast within both bright highlights and dark shades in
harshly lit scenes), previously in the domain of
HDR techniques involving
multiple exposure
In photography and cinematography, a multiple exposure is the superimposition of two or more exposures to create a single image, and double exposure has a corresponding meaning in respect of two images. The exposure values may or may not be ide ...
s. , the better of the recent photographic imaging sensors of the 35 mm "full-frame" format can accommodate up to about 14 stops of engineering
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
(DR) or 11.5 stops of useful photographic DR in the
raw
Raw is an adjective usually describing:
* Raw materials, basic materials from which products are manufactured or made
* Raw food, uncooked food
Raw or RAW may also refer to:
Computing and electronics
* .RAW, a proprietary mass spectrometry dat ...
shooting mode.
A complication to using ETTR with higher DR is the fact that the vast majority of photographic cameras can only display a histogram produced by its JPG processing engine. In-camera JPG engines tend to have a narrowed DR than the sensor and does not faithfully represent the underlying raw data. Also, the histogram of the JPG image produced in-camera, highly depends on the camera settings, and is therefore a rather inexact indication of the exposure of the raw image data; the right edge of the raw histogram is not displayed. Several approaches may be used for practicing ETTR in this situation.
* Specific software, for instance RawDigger, FastRawViewer, Histogrammar or the discontinued Photobola Rawnalyze, can be used to compute the raw histogram. If a computer is not available for analysis when shooting, it may be used to pick best exposure from an exposure-bracketing sequence of images.
* An in-camera approximation of the raw histogram is provided by a few cameras, such as Leica M8 and M9, and by
Magic Lantern
The magic lantern, also known by its Latin name , is an early type of image projector that used pictures—paintings, prints, or photographs—on transparent plates (usually made of glass), one or more lenses, and a light source. Because a si ...
, the unofficial firmware "hack" for Canon DSLR cameras (which also includes an ''automatic ETTR'' option).
* Some recent cameras offer settings for the JPG processing engine with higher DR which is somewhat closer to the sensor DR (for instance, S-Log2 gamma of Sony, Flat picture control of Nikon).
* With other cameras, a workaround approach known as "UniWB" may be used. One should be careful when using rough in-camera histogram approximations, particularly with live histograms, because they may fail to display small highlights areas.
In cases where the scene DR is within the sensor DR but much wider than the DR of the JPG engine, ETTR results in the JPG preview image exposed for the highlights and appearing dark, because the midtones range of the scene is recorded in the shadows range of the JPG preview, and the shadows range of the scene is not recorded in the JPG preview. Wide-DR-capable raw processing external software is required (e.g.,
Adobe Camera Raw
Adobe Photoshop is a raster graphics editor developed and published by Adobe Inc. for Windows and macOS. It was originally created in 1988 by Thomas and John Knoll. Since then, the software has become the industry standard not only in raster ...
/ Adobe Photoshop Lightroom,
DxO OpticsPro,
Capture One
Capture One (also known as Capture One Pro) is a photography software suite. Capture One offers Capture One Pro for desktop, a tool to edit, catalog raw image file process, and tether.
Capture One works with raw files from many different digita ...
,
Raw Therapee).
When facing higher-DR scenes, metering on most cameras tends to aim for the midtones, often clipping both extreme highlights and shadows of the JPG preview image (seen as spikes at the right and left edge of the JPG histogram, respectively). One might expect that this would automatically lead towards a raw histogram in which the right (highlights) range is neatly occupied as with ETTR. Unfortunately, metering algorithms in different cameras may produce very different results in such conditions. It is not uncommon that when shooting a scene with DR within the sensor DR, the default camera metering results in a raw image with blown highlights and ample empty space at the left side of the histogram. In such case, the ETTR principle of maximizing exposure to the point where the raw histogram is aligned to its right edge, requires a seemingly counterintuitive ''negative''
exposure compensation
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 u ...
. This may be explained with the ETTR principle: to make best use of the additional DR, available with wide-DR sensors, the extra DR is used to expand the shadows end of the recording range, without increasing the raw
highlight headroom
Highlight headroom is the measure of how much additional dynamic range a given photographic medium (such as film or digital image sensors) has to record the detail within the brightest parts of a scene.
As an example, consider a photograph of a ...
.
If the resulting shadows are unable to be processed to acceptable noise or tonal range, one has simply encountered a situation that cannot be taken with a single shot, and consideration could be given to
HDR (High Dynamic Range) techniques requiring multiple exposures. If the latter is infeasible due to scene or camera motion, one may revert to the technique of exposing to the important highlights (abbr. ETTIH), which is in fact just a slight generalization of ETTR. With ETTR, exposure is maximized up to the constraint of preserving all highlights at the right edge of the histogram (a constraint implied by the hard saturation of the digital sensor). With ETTIH, exposure is maximized a bit further; the constraint is relaxed so that only the ''highlights deemed important'' are preserved before the right edge of the histogram, and the ''highlights deemed unimportant'' may fall into the saturation zone of the sensor. Typical examples of unimportant highlights include the sun, other very bright light-sources, and sharp-edged specular highlights like chrome car bumpers in the sun; however, one should avoid blowing areas with smooth luminosity gradients, for instance the sky around the sun, because these likely lead to visible sensor saturation artefacts (banding). Many cameras have over-exposure "blinkies", showing the location of blown highlights which is not evident from the histogram. Some cameras also have under-exposure "blinkies" as an exposure aid. Mirror-less cameras often have "preview zebras", which is another aid for being aware of and correcting over-exposure.
Raw processing software may feature "highlights reconstruction" algorithms (currently unavailable in-camera) which are able to partially mitigate the negative visual impact of digital hard saturation of the three color channels, thus extending the range of blow-able unimportant highlights slightly.
In general, whether one is dealing with a high-DR scene or a low-DR scene, both the process and aim of ETTR are the same: exposure is adjusted (forgetting the meter) until only dispensable highlights are clipping, thereby resulting in an image with the greatest
signal-to-noise
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in decib ...
(and therefore the greatest
image quality Image quality can refer to the level of accuracy with which different imaging systems capture, process, store, compress, transmit and display the signals that form an image. Another definition refers to image quality as "the weighted combination of ...
). In both cases, the practice of ETTR does not use or depend on the camera's meter. Rather, it uses and depends on the exposure indicators, either the histograms and/or the highlight indictors (blinkies/zebras), which, ideally, have been set to reflect as well as possible the maximal values of the underlying raw data.
Background
![Linear Distribution versus Gamma Corrected Distribution](https://upload.wikimedia.org/wikipedia/commons/a/ae/Linear_Distribution_versus_Gamma_Corrected_Distribution.png)
ETTR was initially espoused in 2003 by
Michael Reichmann
Michael H. Reichmann (25 June 1944 – 18 May 2016), better known by his monogram MR, was a Canadian landscape, travel and street fine-arts photographer, as well as videographer, author, blogger, workshopper, independent technical consultan ...
on his website, after purportedly having a discussion with
software engineer
Software engineering is a systematic engineering approach to software development.
A software engineer is a person who applies the principles of software engineering to design, develop, maintain, test, and evaluate computer software. The term ''p ...
Thomas Knoll
Thomas Knoll is an American software engineer who created Adobe Photoshop. He initiated the development of image processing routines in 1988. After Knoll created the first core routines, he showed them to his brother, John Knoll, who worked at In ...
, the original author of
Adobe Photoshop
Adobe Photoshop is a raster graphics editor developed and published by Adobe Inc. for Microsoft Windows, Windows and macOS. It was originally created in 1988 by Thomas Knoll, Thomas and John Knoll. Since then, the software has become the indu ...
and developer of the Camera Raw
plug-in.
Their rationale was based on the
linearity
Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...
of
CCD and
CMOS
Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
sensors, whereby the
electric charge
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respe ...
accumulated by each
subpixel
In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device.
In most digital display devices, pixels are the sm ...
is proportional to the amount of light it is exposed to (plus
electronic noise
In electronics, noise is an unwanted disturbance in an electrical signal.
Noise generated by electronic devices varies greatly as it is produced by several different effects.
In particular, noise is inherent in physics, and central to the ...
). Although a camera may have a
dynamic range
Dynamic range (abbreviated DR, DNR, or DYR) is the ratio between the largest and smallest values that a certain quantity can assume. It is often used in the context of signals, like sound and light. It is measured either as a ratio or as a base-1 ...
of 5 or more
stops
Stop may refer to:
Places
*Stop, Kentucky, an unincorporated community in the United States
* Stop (Rogatica), a village in Rogatica, Republika Srpska, Bosnia and Herzegovina
Facilities
* Bus stop
* Truck stop, a type of rest stop for truck dri ...
, when image data is
recorded digitally the highest (brightest) stop uses fully half of the discrete tonal values.
This is because a difference of 1 stop represents a doubling or halving of
exposure. The next highest stop uses half of the remaining values, the next uses half of what is left and so on, such that the lowest stop uses only a small fraction of the tonal values available.
This may result in a loss of tonal detail in the dark areas of a photograph and
posterization
Posterization or posterisation of an image is the conversion of a continuous gradation of tone to several regions of fewer tones, causing abrupt changes from one tone to another. This was originally done with photographic processes to create p ...
during post-production. By deliberately exposing to the right and then
stopping down
In photography, stopping down refers to increasing the numerical f-stop number (for example, going from 2 to 4), which decreases the size (diameter) of the aperture of a lens, resulting in reducing the amount of light entering the iris of a lens ...
afterwards (during processing) the maximum amount of information is retained.
It has since been demonstrated that the benefit of more exposure lies not really in the better quantization, because the noise always present in photographic captures renders it invisible tu the human eye
, but solely in the better SNR, particularly in the shadows of a high-contrast scene that need some pushing
.
Limitations
The ETTR exposure is, by its very nature, established with a camera ISO setting that allows the exposure indicators (right edge of histogram or blinkies/zebras) to indicate when the sensor is at or near saturation for desired highlights. Most people will find this to be the camera's base (lowest, not false) ISO. However, depth of field requirements may demand such a high ''f''-ratio, and motion-blur/camera-shake issues may demand such a fast shutter speed, that ETTR is not possible at this ISO setting. When this happens, one retains the spirit of ETTR (maximizing signal-to-noise) by making the exposure as high as possible subject to the shooting conditions. ISO may then be increased to bring the image up to a desired brightness. But, since increasing ISO does not actually increase sensor exposure (instead the sensor's
signal gain
In electronics, gain is a measure of the ability of a two-port circuit (often an amplifier) to increase the power or amplitude of a signal from the input to the output port by adding energy converted from some power supply to the signal. It i ...
is increased), it should be applied only after the actual exposure (set by the ''f''-ratio and shutter speed) has been made as large as possible subject to shooting constraints.
Live histograms and highlight-clipping indicators, which are almost always based on the processed JPEG rather than on the raw data, might indicate highlights are blown when in fact they are not and could be recoverable from a raw file. Therefore, it can be difficult to expose properly to the right without risking inadvertently blown highlights.
This problem can often be mitigated by using camera tonal settings that allow the JPEG histograms and highlight-clipping indictors to best reflect the underlying raw data.
See also
*
High-dynamic-range imaging
In photography and videography, multi-exposure HDR capture is a technique that creates extended or high dynamic range (HDR) images by taking and combining multiple exposures of the same subject matter at different exposure levels. Combining mu ...
References
External links
Optimizing digital exposures using ETTRRestore Those Clipped ChannelsShooting to the Right (and why it can be a bad idea)Exposure vs. Brightening'' – why ISO is not part of exposureETTR Exposed – A complete tutorial for doing ETTRSensor details for various cameras
{{Photography
Photographic techniques
Digital photography