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 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 strike the sensor during exposure release electrons, and it is the electrical charges created on the sensor by 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 from the sensor, amplified, and otherwise transformed in-camera (by electronics and software) as they are converted into the digital numbers (DNs or ADUs – analog-to-digital units) that comprise the raw-data. It is from these raw data that processed images, such as JPEGs, will be made, either in-camera by the camera's JPEG engine or with a raw processor on a computer. The camera's ISO setting determines the overall scale of this transformation in-camera. Raw processors and image editors can also transform these numbers, effectively allowing one to apply software-based "ISO" with a computer. Further transformation can be made with the controls of the output medium, either printer or monitor.
Rather generally, let me refer to any such amplification or transformation that takes place, either in-camera, or in the raw processor or image editor, or in the output medium, 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 transformation 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 setting of the camera's ISO control that is being indicated.
The fundamental information, then, that underlies a photograph resides in the electrical charges created on the sensor during exposure. All of the brightening that follows, including ISO, merely transforms that fundamental information but cannot add to 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, or with the controls of the output medium, determines the brightness of that image, a term that applies only to a final image as perceived by the viewer. Thus, exposure is the input to the process of making an image – photons striking 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, or by pushing/pulling during raw-processing or post-processing, or by adjusting the brightness of the output medium.*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 (pixels, sensels) become exposed beyond their capacity to contain the entire charge of the 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 electrical charges. 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 electrical charges are then read, amplified, and transformed by the camera's electronics 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, or by controls on the output medium.
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 electronics 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 as produced by a given exposure combined with any subsequent amplification and/or transformation applied to it either in-camera (including adjusting in-camera 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 (pixels, sensels) are exposed beyond their capacity (blown pixels).*4
Over-brightened: an application of brightening 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 (and the role of exposure): brightness refers only to the perceived lightness of the final-image as achieved by any means, including exposure and subsequent brightening, while brightening refers specifically to amplification and/or transformation applied subsequently to exposure. Note then: while exposure affects the brightness of the final image, it is not itself included with - and is indeed separable from - any subsequent acts of brightening leading to that brightness.
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 the condition of 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 and poor or no color in the blown highlights 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 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: Recoverable highlights are those that have been clipped by the JPEG processor through over-brightening of otherwise properly exposed and brightened (i.e., not clipped) raw values. Alternative tone curves applied to such raw data can result in a final image of appropriate brightness but without highlight clipping. Some raw processors are also able to recover tonal information (and even guess at some color information) from raw data that are clipped in only one or two of the three color channels. Success is not assured.
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 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 in-camera ISO required, if any, without allowing the maximal exposure to be altered (as can happen when using semi-automatic programmed modes like A and S). 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 highlight indicators (blinkies//zebras). 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 degree using in-camera ISO without first maximizing the exposure may be said to be under-exposed. Under-exposure, then, occurs when brightening substitutes for exposure. 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 otherwise identical looking image could be 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 or both.
Barring overriding artistic considerations, the raw shooter should typically consider exposing to the right (ETTR). ETTR is an attempt to maximize the exposure (via f-ratio and shutter speed) so that the brightest desired highlights just approach sensor saturation. This condition is assessed, not with the camera's meter, but with the camera's exposure indicators: the histogram(s) and/or the highlight-indicators, often called the blinkies or zebras. In practice, exposure is set to push the histogram as far right as possible without indications of clipping (thus ETTR) or until the highlight indicators are just appearing. This should be done with the camera's ISO set to the value that allows the histogram(s) and highlight-indicators to provide the most reliable information about the state of the sensor. For most cameras this will be the camera's base ISO (the lowest, non-extended ISO) setting. The benefit of ETTR is that it maximizes the captured signal relative to shot noise, and hence leads to an image with maximal image quality (IQ). Since the ETTR exposure is not determined using the camera's metering to obtain an image of appropriate average brightness, the resulting image may often appear initially to be too dark or too light. This is all right; one simply makes the necessary tonal adjustments during raw processing. More on ETTR can be found in ETTR Exposed.
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 setting) 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 dark" and pushing in raw processing. This benefit will be noticed primarily in the shadow regions – but care must be taken to avoid inadvertent clipping of desired highlights. Many cameras are partly-ISO-invariant, becoming ISO-invariant only after reaching a given ISO level – some (often called ISO-invariant) as low as ISO 200 or 400, but more typically at values upwards of ISO 800 or 1600. Here there are benefits from increasing ISO in-camera, if needed, up to this level and then effecting the additonal brightening during raw processing. For greater detail on this important topic, see *5.
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, allowable blown highlights, and allowable shadow noise. 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.
If ETTR is not possible and it is decided to add in-camera ISO after a maximal exposure has been determined, take care when using programmed modes like A or S that the ISO adjustment does not alter the maximal exposure settings (f-ratio and shutter speed) that you've already determined, as it is likely to do. This is not an issue with M mode, where the exposure settings are unaffected by changes in ISO.
Assessing over-exposure in the camera
Over-exposure occurs when there are blown photosites (pixels, 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 fully 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(zebras) of a shot taken at the base ISO setting. 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 somewhat 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 and/or blinkies(zebras) provide the most accurate depiction of the raw data when using UniWB (a useful, but not essential, refinement *7) and the camera's widest color space, usually Adobe RGB. Other camera settings can often help these indictors be more informative.*6 Thus, the best, but not perfect, in-camera indicators of over-exposure are either a histogram that indicates clipping or the presence of (unwanted) blinkies(zebras) when these exposure indicators have been appropriately regulated and are assessed at the camera's base ISO setting.
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.