Exposure vs. Brightening

Started Mar 22, 2013 | Discussions thread
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gollywop
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Exposure vs. Brightening
Mar 22, 2013

Note: A revised version of this article is available at:

Here


Some recent threads have again raised concerns in the minds of some that the term exposure is often incorrectly defined and used to refer to notions outside its proper realm. This situation arises in part, I believe, because no alternative terminology is commonly accepted to replace the term exposure when it is misapplied. I intend to make the case here, following others that have come before me, that the term brightening provides simple, straightforward, and consistent terminology to correct this lack.

I'm sure there will be plenty of knowledgable chaps on this forum who will chime in to help refine what I am about to write, and I welcome that. This forum is blessed with several people who actually know what they're talking about. I am giving what follows as a first-draft, which I nevertheless feel to be correct in its essentials, to get the main idea going -- but please feel free to have at it. I have already benefited greatly from conversations with Great Bustard for which I am extremely grateful -- but I hasten to add the usual disclaimer that I alone am responsible for the content and any possible erroneous constructions -- because Great Bustard is well known to be incapable of error or even unclear explanations.

Let me begin with a simple model of camera behavior that will help motivate the suitability of the suggested terminology. Some may wish to skip the next section and go straight to the summary that follows.

BACKGROUND:

Properly defined, exposure is the amount of light falling per unit area on a sensor: it is determined by the scene illuminance, the f-ratio (more properly the t-ratio – the equivalent f-ratio for a lens with 100% transmittance), and the shutter speed. Note that ISO does not figure in this definition. Let us see why.

Light is made of photons, and the camera's sensor is essentially a photon counter. The photons that fall on the sensor during exposure release electrons, and it is these released electrons that make the signal that is processed, recorded, and eventually becomes the final image. Not all the photons produce a an electron. The sensor's QE (Quantum Efficiency) denotes the average number of photons required to release an electron. For example, a QE of 50% means that two photons, on average, are required to release one electron. So, a sensor with a QE of 50% would only require half the exposure to record the same amount of light as a sensor with a QE of 25%.

In subsequent (and separable) actions, the charges of these electrons are read, amplified, and otherwise transformed in-camera (by hardware and software) as they are converted into the digital numbers (ADUs - analog to digital units) that comprise the recorded raw-data file. It is from these raw data that processed images, such as jpegs, will be made, either in-camera or with a raw processor on a computer. The camera's ISO setting determines this amplification factor in-camera. Raw processors can also manipulate these numbers, effectively allowing one to apply software-based "ISO" with a computer.

Rather generally, let me refer to any such amplification that takes place, either in-camera and/or in the raw processor, as brightening. Sometimes this brightening is referred to as ISO, but that term can be treacherous because it is only partially applicable to the total gain that may be applied and it is a concept used in very separate contexts with very different meanings (some of which are themselves inherently nebulous).

The total brightening applied to an exposure in producing the final image, either in-camera or with a raw processor, determines the brightness of that image, a term that applies only to a final image. Thus, exposure is the input to the process of making an image -- photons falling on the sensor --, and brightness characterizes the final image that is the output -- the level of the data of the final image after brightening is applied to the exposure either by adjusting the ISO in-camera and/or by pushing or pulling in the raw conversion.

In brightening an exposure, it is possible to apply excessive amplification, either in-camera or with the raw processor, that causes either the raw data or the data defining the final image to exceed their characteristic bit-depths, i. e., to cause the data to be clipped. It seems reasonable to refer to any such clipping, whether of the raw data or final-image data, as over-brightening.

In contrast to over-brightening, we can consider the case where the sensor is subjected to so much light per unit area during exposure that some of its photosites (sensels) become over-saturated (blown) and unable to accommodate all the light they receive. Both over-saturation and over-brightening will result in portions of the final image being clipped.

Note, then, that it is possible for an image to be "appropriately" exposed (no blown pixels) but over-brightened (clipped data) because of excessive brightening applied either in-camera or during processing. Such an over-brightened image, even though appropriately exposed, is often incorrectly referred to in the vernacular as over-exposed.

[Indeed, it is because the term over-exposed is so encased in inappropriate connotations that the term over-saturated is employed here instead. Were the term over-exposed understood and used only with reference to the notion of exposure as given here, it would serve perfectly. However, given its history, it is probably wise to wipe the slate clean and begin anew with another, entirely suitable, one.]

SUMMARY AND BASIC DEFINITIONS:

We can view the working of a camera in two steps: (1) Light from the framed scene is applied to a sensor that is able to represent that light as electrons. The density of light falling on the sensor in this step is determined by the illuminance of the scene, the f-ratio (more accurately, the t-ratio), and the shutter speed. (2) The charges of these electrons are then read, amplified, and transformed by the camera's hardware and software to produce the raw data from which an image is formed either in-camera by the camera's jpeg engine or with a computer using an appropriate raw processor.

Step (1) is exposure. It is here that the basic information from which the final image will be made is introduced into the camera through its "exposure" to the light. Step (2) is brightening. It is here that the basic exposure information obtained in step (1) is amplified and transformed by hardware and/or software ultimately into an image of the desired brightness.

Here is a flow-diagram depicting this process.

With the preceding model in mind, we make the following definitions of basic concepts:

Exposure: the amount of light falling per unit area on a sensor: it is determined by the scene illuminance, the f-ratio (more properly the t-ratio), and the shutter speed.

Brightness: the values of the data that characterize a final image after amplification is applied to the exposure either by adjusting the ISO in-camera, or by pushing or pulling in raw conversion. This term refers only to the final image.

Brightening: the process of applying gain to an exposure, either through ISO in-camera or tonal transformations in processing, to increase or alter the values of the raw data and/or the final-image data.

Over-saturated: an exposure in which at least some of the sensor's photosites (sensels) are exposed beyond their capacity (blown pixels). [Over-saturated could also be called over-exposed when exposed is defined as above.]

Over-brightened: an application of gain that leads either the raw data or the final-image data values to exceed their bounds (clipping).

EXTENSIONS:

These basic notions allow for unambiguous descriptions of higher-order concepts, such as:

Brightness vs. brightening: brightness refers only to the level of the values of the final-image data, while brightening refers to any amplification that affects the values of either the raw data or the final-image data.

Over-saturated vs. over-brightened: over-saturation occurs when the bounds of the sensor are exceeded; over-brightening occurs when the bounds of the data are exceeded -- either the raw data or the final-image data. Over-saturation necessarily leads to clipped raw data and, hence, to over-brightening. While it is possible for an image resulting from over-saturation (blown sensor pixels) to be processed so that the final-image data do not exceed their bounds (for example, pulling back a blown sky in a jpeg so its values are less than 255), there will be a pile-up of data on the right-hand side of the histogram that indicate and betray the over-brightening. The converse, however, as we have noted, need not be true: it is possible for a properly exposed image (with no blown pixels) to be over-brightened (having clipped raw or final-image data values).

Raw-data clipping vs. final-image data clipping: Over-saturation and/or excessive application of in-camera ISO can lead to clipped raw data. Likewise, excessive pushing of unclipped raw data can lead to clipped final-image data. In either case, we have over-brightening.

Recoverable highlights: clipped highlight values whose clipping has resulted from over-brightening by the jpeg processor of otherwise properly exposed and brightened (i.e., not clipped) raw values. Alternative tone curves applied to such raw data could result in a final image of appropriate brightness but without these highlights being blown.

SHOOTING CONSIDERATIONS REDUX:

Here are outline statements of various shooting considerations employing the above terminology. Some may find them to have pedagogical value.

Achieving desired brightness shooting jpeg:

The jpeg shooter has two means for establishing the brightness of his/her final image: exposure and brightening. Given the scene, there are two camera controls that affect exposure, f-ratio and shutter speed, and there is one that affects brightening, ISO. For most jpeg shooting, the desired brightness is best achieved first by maximizing the exposure (while avoiding over-saturation) and then adding the minimal amount of ISO required, if any. The combined average effects of the two can be assessed roughly via the camera's metering. The trade-off in exposure between f-ratio and shutter speed must be resolved by considerations of DoF (depth of field) and acceptable motion blur and/or camera shake. An appropriate exposure may also be influenced by other artistic considerations.

The jpeg shooter should also be aware that achieving a desired degree of brightness can be accompanied by over-brightening. This should be assessed using the histograms and/or blinkies. This situation can only be resolved by some compromise, either using less exposure or brightening or by accepting the clipped highlights. Some cameras also incorporate tone-curve (highlight/shadows) settings that can be used to allieviate the situation.

A jpeg final image that has been brightened to the desired brightness without maximizing the exposure (while avoiding over-saturation) may be said to be under-exposed. Under these circumstances it might be difficult to tell from the image itself that it was under-exposed, but if the exposure had instead been maximized, an image of the same brightness could have been achieved with less noise.

A jpeg final image that has been shot with maximum exposure (while avoiding over-saturation) but otherwise looks too dark is not under-exposed: it is under-brightened.

Achieving desired brightness shooting raw:

The raw shooter also has two means for establishing the desired brightness of his/her final image: exposure and brightening, but there are differences from shooting jpeg. Given the scene, exposure is still set using the camera's f-ratio and shutter speed, but brightening can take place either in-camera via the ISO setting or in the raw processor.

Barring overriding artistic considerations, the raw shooter should typically consider exposing to the right (ETTR). ETTR is an attempt to maximize the exposure at base ISO by choosing exposure settings that push the histogram to its right-hand edge without clipping. This has the benefit of maximizing signal and minimizing relative shot noise. If this results in an image that is too bright, one simply pulls the brightening back in raw processing.

Shooting considerations may make ETTR impossible. DoF considerations, for example, may dictate a high f-ratio and camera-shake considerations may dictate a fairly high shutter speed. Together, these settings may entail an exposure that falls short of ETTR. What to do?

BTTR (brightening to the right) is a similar concept to ETTR except that ISO above the base level may be applied to help place the histogram against its right-hand edge. If ETTR can be achieved (at base ISO) additional ISO would be counterproductive, simply resulting in portions of the image being clipped. But if ETTR is not possible, one would typically still want to maximize exposure (i.e., push the histogram as far to the right as possible at base ISO) and then add brightening.
Whether the brightening should be added in-camera or during raw processing (or both) depends on the nature of the camera. With an ISO-less camera (one whose read noise does not change with the camera's ISO setting), one could do either, but there are advantages to shooting dark at the base ISO and brightening during raw processing, which will typically result in a final image with better IQ and less chance of clipped highlights. With a non-ISO-less camera, the benefit is in favor of brightening with added in-camera ISO, which will typically result in less noise than shooting darker and pushing in raw processing. Some cameras become ISO-less only after reaching a given ISO level, say 800 or 1600. Here there are benefits from increasing ISO in-camera, if needed, up to this level and then effecting any further brightening during raw processing.

Various constraints can come into play in determining an optimal exposure. These include DoF considerations affecting the f-ratio, motion-blur and/or camera-shake considerations affecting shutter speed, any desired amount of blown highlights (often none), any desired shadow noise (or lack of it). Sometimes these constraints are incompatible and cannot all be achieved at once. In this case the raw shooter must make compromises. The maximal exposure satisfying these compromises while avoiding unwanted over-saturation is the optimal exposure, even if it is not ETTR. A shot based on any lesser exposure may be said to be under-exposed.

Assessing over-saturation in the camera:

Over-saturation occurs when there are blown photosites (sensels). We can't see the sensor, so there is no direct evidence of this condition. However, blown photosites will manifest themselves at base ISO as clipped raw data. Unfortunately, cameras also do not provide accurate indicators of the raw data. Until this happens, the only way to know for sure if raw-data clipping has occurred is to examine the raw file in a program like RawDigger.

We can, however, get a rough idea of over-saturation by examining the histograms and/or blinkies of a shot taken at base ISO. Some cameras show these indicators in live view before the shot is taken. Virtually all cameras show them after the shot is taken. The post-shot indicators are more accurate than the live-view indicators, although the differences are often (but not always) minor. These indicators do not directly reflect the raw data, but rather, are based on a jpeg rendering of the raw data. When shooting raw, the camera's histograms provide the most accurate depiction of the raw data when using UniWB, but this is a refinement and is not essential. Thus, the best, but not perfect, in-camera indication of over-saturation is a post-shot (at base ISO) histogram that indicates clipping or the presence of post-shot blinkies.

Caution, however, must be exercised when trying to assess over-saturation using the camera's jpeg-based color histograms when shooting ETTR or BTTR. Because the levels for the different underlying raw color pixels can differ greatly, attempting either ETTR or BTTR could lead to some raw color channels being clipped and others not. This situation might go undetected by the camera's histograms because the resulting demosaiced jpeg pixels need not be clipped but could nevertheless have the wrong color because of the one or more clipped raw channels. Conversely, unless the raw shooter is employing UniWB, the jpeg data will reflect the application of the WB multipliers. This could lead to the camera's color histograms, particularly in the blue or red channels, to indicate clipping that does not exist in the underlying raw data.

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