Exposure vs. Brightening
There has been an understandably growing concern among some in the forums and the literature that the term exposure in photography is often incorrectly defined and used to refer to notions outside its proper realm, specifically with respect to the inclusion of ISO in the so-called "exposure triangle" along with f-ratio and shutter speed. This situation arises in part, I believe, because it has only been with the relatively recent rise of raw processing – and the fresh perspective it offers – that the inappropriateness of including ISO as part of exposure has become apparent and significant, and in part because no alternative terminology is commonly accepted to replace the term exposure when it is misapplied. In this article I intend to make clear the fundamental separateness of ISO from exposure and to make the case, following others that have come before me, that the term brightening provides the simple, straightforward, and consistent terminology needed to deal with ISO while allowing the term exposure to stand unsullied on a correct and proper footing.
Let me begin with a simple model of camera behavior that will help motivate the suitability of the suggested notions and terminology. End Notes, denoted in the text with a numbered *, are provided on page 2 of this article to deal with useful details not essential to its flow.*1
Properly defined, exposure is the amount of light falling per unit area on a sensor: it is determined by the scene luminance, 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 transformed into the final image.*2
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 and image editors can also transform 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 or image editor, as brightening. Sometimes this brightening is broadly 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). In what follows, the term "in-camera ISO" is used to emphasize when it is only the camera's ISO setting that is being indicated.
The fundamental information, then, that underlies a photograph resides in the electrons created on the sensor during exposure. All of the brightening that follows, including ISO, merely transforms that fundamental information but cannot alter it. Depth of field (DoF) and motion blur, for example, are established by the exposure settings (f-ratio and shutter speed), and cannot be affected by subsequent brightening processes such as ISO. As such, all such subsequent brightening processes are essentially separable from exposure and are not a part of it.
The total brightening applied to an exposure in producing the final image, either in-camera or with a computer, 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 perceived lightness of the final image after brightening is applied to the exposure either by adjusting the ISO in-camera and/or by pushing or pulling during raw-processing or post-processing.*3
In brightening an exposure, it is possible to apply excessive amplification, either in-camera or with the computer, 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 during exposure that some of its photosites (sensels) become exposed beyond their capacity to contain all the electrons released by the light they receive (blown). It seems reasonable to refer to this situation as over-exposure. Both over-exposure and over-brightening will result in portions of the final image being clipped.*4
It is entirely possible, then, for an image to be "appropriately" exposed (no blown sensels) but over-brightened (clipped data) because of excessive brightening applied either in-camera or with the computer. Such an image is sometimes inappropriately called "over-exposed" in common parlance (and even in much of the literature), but this is an improper use of the term. The image is correctly exposed, but over-brightened.
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 luminance from 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 appropriate software.
Step (1) is exposure. It is here that the basic information (signal) 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.
click image to see larger view
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 luminance, the f-ratio (more properly the t-ratio), and the shutter speed.
Brightness: the perceived lightness of an image resulting from amplification of any form applied to a given exposure either in-camera (including ISO), or by "pushing" or "pulling" in a raw processor or image editor, or by adjusting appropriate controls on the output medium. This term applies only to an image as viewed on a chosen medium: print or monitor.
Brightening: increasing the brightness of an image by any means (in-camera, in the computer, or in the output medium) other than those determining exposure (scene luminance, f-ratio, and shutter speed).
Over-exposed: an exposure in which at least some of the sensor's photosites (sensels) are exposed beyond their capacity (blown pixels).*4
Over-brightened: an application of gain that causes either the raw data or the final-image data values to exceed their bounds (clipping).
These basic notions allow for unambiguous descriptions of higher-order concepts, such as:
Brightness vs. brightening: brightness refers only to the perceived lightness of the final-image data, while brightening refers to any amplification, regardless of where it occurs after exposure, that affects the brightness of the final-image.
Over-exposed vs. over-brightened: over-exposure 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-exposure necessarily leads to clipped raw data and, hence, to over-brightening. While it is possible for an image resulting from over-exposure (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 indicates and betrays 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 sensor pixels) to be over-brightened (having clipped raw or final-image data values).
Raw-data clipping vs. final-image data clipping: Over-exposure 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 highlight clipping.
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-exposure) 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 image brightness (average tonal value) can be accompanied by over-brightening (clipped highlights). Any such over-brightening should be assessed using the histograms and/or blinkies. This situation can only be resolved by some compromise, either using less exposure, less brightening, or by accepting the clipped highlights. Some cameras also incorporate tone-curve settings (highlight/shadows, D-lighting, auto-lighting, and the like) that can be used to alieviate this situation.
A jpeg final image that has been brightened to the desired brightness without maximizing the exposure (while avoiding over-exposure) may be said to be under-exposed. Under these circumstances it might sometimes 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-exposure) 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 is an important difference 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.*5
Shooting considerations may make ETTR impossible. DoF considerations, for example, may dictate a high f-ratio, and camera-shake considerations may dictate a fairly fast shutter speed. Together, these settings may entail an exposure that falls short of ETTR. What to do?
If ETTR is not possible, it is still important to maximize exposure, i.e., to push the histogram as far to the right as possible at base ISO even if not to the right-hand edge. This allows capturing the greatest possible signal with least relative noise and creates the best foundation for the final image. Brightening can then be added to achieve the desired brightness. But where should this brightening be done: using in-camera ISO or during raw processing or both?
The answer to this question depends on the "ISO-nature" of the camera. With an ISO-invariant camera (one whose read noise does not change with the camera's ISO setting), one could do either (brighten in-camera or during raw processing), but there are advantages to shooting dark (letting your image remain unbrightened) at the base ISO and brightening during raw processing. This will typically result in a final image with better IQ and less chance of clipped highlights. With an ISO-variant camera (one whose read noise decreases with increased ISO), the benefit is in favor of brightening with added in-camera ISO, which will typically result in less read noise than shooting darker and pushing in raw processing. Some cameras are partly-ISO-invariant, becoming ISO-invariant 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, if required, during raw processing.*6
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, but not always, 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-exposure 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-exposure in the camera
Over-exposure 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-exposure 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 (a useful refinement, but not essential*7) and the camera's widest color space, usually Adobe RGB. Thus, the best, but not perfect, in-camera indication of over-exposure 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-exposure using the camera's jpeg-based color histograms when shooting ETTR or applying in-camera ISO. Because the levels for the different underlying raw color pixels can differ greatly, attempting either ETTR or adding in-camera ISO 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 due to 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 cause the camera's color histograms, particularly in the blue or red channels, to indicate clipping that does not exist in the underlying raw data.