We explain the concept of dynamic range: what it is, how it works and what effect it has on the images you take
The term ‘dynamic range’ is used a lot in camera reviews and features. Along with noise and resolution, it is one of the many considerations of enthusiast photographers when buying a camera, yet there is often little discussion as to what the term actually means and how it can affect the images you take.
Put very simply, the dynamic range of a camera is the range of tones that it can record between black and white. The larger the dynamic range, the more of these tones can be recorded and the more detail can be extracted from the highlight and shadow areas of a scene.
Dynamic range is usually measured in exposure values, or stops. For example, a camera with a dynamic range of 12EV would be one that could record all the details in a scene with a dynamic range of 12EV or less. Given the same scene, a camera with a dynamic range of 10EV would not be able to record all the tones present, so detail would be lost in either highlights or shadows, depending on how the image is exposed.
Although it is important to try to capture as many tones in a scene as possible, for most photographers the goal should actually be to try to create a pleasing image. This doesn’t mean that every last detail must be captured. For example, if both the highlight and shadow details in an image are grey, rather than black or white, then the image will be of very low contrast and rather dull and boring. The key is to understand the boundaries of your camera’s dynamic range and know how it can be used to create an image with a good level of contrast, but without large areas of complete black or white.
What the camera sees
Each pixel in an image represents a single photodiode on the camera’s sensor. The photodiodes collect light photons and turn them into an electrical charge, which is then converted to digital data. The more photons that are collected, the greater the electrical signal and the brighter the pixel in the image will be. If a photodiode does not collect any light photons, then no electrical signal will be created and the representative pixel will be black.
However, sensors come in a variety of different sizes and resolutions, and use different technologies that affect the size of photodiodes on each sensor.
Typically, mobile phones and compact cameras have very small image sensors compared to a DSLR. This means they also have much smaller photodiodes on the sensor. So, even though both a compact camera and a DSLR may have a 16-million-pixel sensor, the dynamic range will be different.
The most common analogy is that each photodiode is like a bucket that collects light. Imagine having 16 million buckets collecting light, compared to 16 million cups. The buckets have a greater capacity and as such can collect more light. The cups have a much lower capacity, which, going back to the photodiodes, means that they reach capacity and produce a completely white pixel with a lower quantity of light photons than the larger photodiodes.
Where a small photodiode may reach capacity and produce a white pixel, the larger photodiode can continue collecting the light photons and may not reach capacity, which means it stands a better chance of retaining detail in the highlights rather than becoming completely white.
What this means in practice is that cameras with smaller sensors, such as smartphones and consumer compacts, have a smaller dynamic range than most compact system cameras or DSLRs, which use the larger sensors. However, it is important to remember that whether this will affect your images depends on the level of contrast in the scene you are photographing.
In a very low-contrast scene there may be little or no difference in the tonal range captured by a mobile phone and a DSLR. The sensors of both cameras may be capable of capturing the complete range of tones of the scene if all of the tones are in the mid-range and the scene is metered and exposed correctly. It is when shooting high-contrast scenes that the greater dynamic range that is generally provided by a larger sensor comes into its own. The larger photodiodes have better ability at recorded a wider range of tones, and thus have a greater dynamic range.
Bit depth explained
Bit depth is closely related to dynamic range and dictates the number of tones that can be shown in an image. Although digital images are full colour, the sensor doesn’t actually record colour, it just records a value for the amount of light. For example, a 1-bit image only contains a simple on or off instruction for each pixel, so in this case there are only two possible outcomes: a black or white pixel.
A 2-bit image is made up of four different levels (2×2). If both bits are on the pixel is white, if both are off then it is black, but there is also the possibility to have one on and one off, which creates another two options that in an image equates to two more tones. This gives you black and white, and two shades of grey. If you have a 4-bit image, there are 16 potential outcomes (2x2x2x2) or different tones.
When it comes to digital imaging and sensors, you will most commonly hear about 12, 14 and 16-bit sensors, each capable of recording 4,096, 16,384 and 65,536 different tones respectively. The larger the bit-depth, the greater the potential number of brightness values, or tones, that can be recorded by the sensor.
However, not all cameras can produce files with the bit depth to match the sensor. For example, on some Nikon cameras it is possible to produce raw files as either 12-bit or 14-bit. The extra data in the 14-bit files means that there is usually more detail in highlight and shadow areas, although the file size is larger and takes the camera longer to process and save, while also taking up more space on a memory card. Saving raw images as 12-bit files is faster, but the tonal range of the image must be compressed, which means that some very dark greys will appear as black and some light tones may become completely blown out to white.
If you shoot JPEGs, your images are compressed even more. JPEG images are 8-bit files made up of 256 different brightness values, so many of the subtle details available for editing in raw files are completely lost in a JPEG file. It should be remembered that in an 8-bit colour image, each pixel in the image is made up of a red, green and blue value, so there are 16,777,216 possible colours (256 x 256 x 256) for each pixel. However, a value of 0 or 255 for each colour will still only ever produce black or white. For example, 0,0,0 is black and 255, 255, 255 is white.
So, while JPEG images are fine for most uses, if you really want to get the most from the dynamic range of your camera, it is best to save images as raw files in the highest bit-depth possible. This means you will have a greatest amount of information in highlight and shadow areas when it comes to editing.