Dynamic Range Explained

When the term “dynamic range” is mentioned, many people will quickly think of HDR, or “High Dynamic Range.” With this term, and technique, so popular in the world of photography, it is rarely ever considered what dynamic range actually is, let alone why you would want it to be “high.” In short, dynamic range describes the measurement between maximum and minimum values. While not specific to photography, in this article on dynamic range in photography we can interpret dynamic range as the measurement between the whitest whites and the blackest blacks in an image, or the lowest and highest values of density and luminance.

Above: A fully tonal black-to-white gradient. 

Before delving in too deeply, consider a black-and-white gradient: a smooth transition from black to white with seemingly innumerable gray tones between the blackest and whitest values of the gradient. Now, with that fully tonal gradient in your mind, try to imagine a smooth gradient that runs from dark gray to light gray. The range of gray tones is much more limited without black or white points and, as such, the dynamic range is shorter. An easier way to recognize this is that the contrast between the maximum and minimum points on the scale is much greater on the black-and-white gradient than it is on the gray-to-gray gradient. This range of contrast, in addition to the greater number of tones between minimum and maximum values, is a higher dynamic range.

Above: A gray-to-gray gradient has a shorter dynamic range. 

This is important to photography, mainly due to the fact that our recording mediums, whether they are a digital sensor, roll of film, digital file, or a print, cannot perceive the same dynamic range that our eyes can. No matter how broad that tonal scale appears on your photographic image, it is being truncated in some way and compensating for the lack of an absolute value of white or black. A printed photograph cannot be any whiter than the white of the paper or any darker than the ink on the paper. Just the same, a digital or film-based image can only record so much detail between the darkest shadows of a scene and the brightest highlights, and eventually will render tones at the end of this scale as an effective black or white simply because there is not enough detail available. Each medium has its own dynamic range, and often the goal is to extend the range of tones in between the maximum and minimum values to create a more full-feeling image, similar to the gradient that runs from pure black to pure white.

This ability to produce a wider range of tones, or to have a greater range of tones available between the medium’s black and white, is what is sought when comparing the dynamic range of different cameras, films, papers, or nearly any sort of constraint that is applied when making a photograph.

In a practical sense, dynamic range is most effectively used and apparent when working in scenes with a great deal of inherent contrast. Instances where there are bright elements and shadows are the most difficult to photograph, due to the inability to record detail in both regions of an image, and often a compromise will have to be made to control the shadows or the highlights. Cameras with a greater dynamic range, measured in stops, will retain detail to a greater extent than a camera whose dynamic range is smaller. For example, if a scene is metered with the highlights at EV (exposure value) 12 and the shadows at EV 1, there is a 12-stop difference in the highlights and shadows, and a camera with a dynamic range of 12 stops would need to be used to record detail in all portions of the image without clipping (the inability to render detail due to the exposure value being beyond the recording limitations of the medium). It is rare that cameras’ dynamic range measurements are given, or are completely accurate; however, as a general rule of thumb, it can be assumed that cameras with larger photosites, or a greater pixel size or pixel pitch, will have the ability to record a greater dynamic range. Larger sensors and lower resolutions can be an indicator of a larger photosite: larger sensors will have room for larger photosites, and sensors with lower resolution will allow for the photosite to be larger than those with a higher resolution. Larger photosites allow for the collection of more light and, subsequently, more detail and a higher contrast ratio to be recorded.

Above: HDR composite image.   Below: Separate photos that create the composite.

Without going further down the scientific road, there are some useful tips for extending the effective dynamic range of an image to visually expand the range of tones you are able to record and avoid losing detail in the highlights and shadows.

As first mentioned, high dynamic range (HDR) imagery is a technique that many use to gain greater detail in the highlights and shadows of a scene, beyond that of what a single exposure can record. Using the previous example as a starting point, if a scene has an exposure value range of 12 stops and you know your camera can comfortably record a 10-stop range, you are now losing a stop of detail in both the high and low regions, or two stops in one or the other region, depending on your exposure bias. To compensate for this by using HDR, you would record three sequential exposures at different exposure settings to ensure that the details of the darkest shadows and the brightest highlights are recorded. For example, if your base exposure is f/5.6 at 1/60-second, you would then also record exposures of f/5.6 at 1/30-second, and f/5.6 at 1/125-second. In post production, you would then merge these three images into a single frame, taking the shadow detail from the 1/30-second exposure, the highlights from the 1/125-second. exposure, and averaging the mid-tones among the three exposures in order to effectively produce an image with a 12-stop range of exposure values.

 

An alternative, and more traditional method for controlling your exposure and lengthening your dynamic range, is through the use of graduated neutral density filters. Favored among landscape photographers and those who commonly photograph large expanses of sky, the graduated neutral density filter’s design allows you to gain full exposure on the shadowed foreground of a scene without the bright sky turning completely white. The design of these filters places a portion of neutral density at one edge of the filter while leaving the remaining half completely clear. From here, you will record your exposure as normal for the foreground and let the density control the highlights of the sky, which gives you a resulting image with an increased dynamic range and full detail in both of the contrasting regions of the scene.

 

Some other practical examples that often come up and relate to dynamic range are the terms Dmax and Dmin, and how they relate to scanning and printing applications, as well as film-based photography. While the terms are applicable to digital photography, as well, they mainly relate to the deepest measurable black point of a print, film, or scan. An example of finding the Dmax would be to take a piece of photographic paper, expose it to room light, and develop. Once developed, the paper will be completely black, and this measurable “blackness” is the Dmax for the given medium. Dmin is the opposite, and is the measurable area of the paper that has not received any exposure (i.e. paper white, or what is commonly referred to as base+fog). Dmax and Dmin are portmanteaus for Density + Maximum or Density + Minimum, and relate to the optical density a medium is able to record. In terms of digital, the Dmax value is commonly given for scanners and relates to the darkest portion of a print or film from which the scanner can still retrieve detail, and the higher the number, the better. Density values are measured in a logarithmic scale with a base of 10, meaning that a scanner with a 3.0 Dmax (a 1000:1 contrast ratio) is capable of recording 10 times more detail than a scanner with a 2.0 Dmax (a 100:1 contrast ratio).

 

This is similar to dynamic range expressed in stops, where exposure values around 0 represent the Dmax of a scene and EVs of +15 relate to the Dmin. The difference between these two values is effectively the dynamic range, the greater the difference equaling a longer dynamic range.

All images © Tim Cooper

Dynamic range is an often overlooked aspect of photography, mainly due to the fact that it’s not something easy or always possible to control. Often creative decisions are made to negate the desire to have a fully controlled range of tones in order to favor a high- or low-key aesthetic, and on the other hand many photographers are very conscious of the apparent dynamic range and go through great lengths to compress as many stops and as much detail as possible into an image. Regardless of the creative avenue, an understanding of your camera, film, or sharing medium allows you to work within its dynamic range limits, or to seek alternatives for expanding this range.

To watch a video of Tim Cooper discussing HDR photography, click here. For an additional video of Cooper discussing realistic HDR photography, click here.