Dynamic Range

Vincent Bockaert, 123di.com

Dynamic Range of a Sensor

The dynamic range of a sensor is defined by the largest possible signal divided by the smallest possible signal it can generate. The largest possible signal is directly proportional to the full well capacity of the pixel. The lowest signal is the noise level when the sensor is not exposed to any light, also called the "noise floor".

Practically, cameras with a large dynamic range are able to capture shadow detail and highlight detail at the same time. Dynamic range should not be confused with tonal range.

Dynamic Range of an Image

When shooting in JPEG, the rather contrasty tonal curves applied by the camera may clip shadow and highlight detail which was present in the RAW data. RAW images preserve the dynamic range of the sensor and allow you to compress the dynamic range and tonal range by applying a proper tonal curve so that the whole dynamic range is represented on a monitor or print in a way that is pleasing to the eye. This is similar to the more extreme example in the tonal range topic which shows how the larger dynamic range and tonal range of a 32 bit floating point image were compressed.

Pixel Size and Dynamic Range

We learned earlier that a digital camera sensor has millions of pixels collecting photons during the exposure of the sensor. You could compare this process to millions of tiny buckets collecting rain water. The brighter the captured area, the more photons are collected. After the exposure, the level of each bucket is assigned a discrete value as is explained in the analog to digital conversion topic. Empty and full buckets are assigned values of "0" and "255" respectively, and represent pure black and pure white, as perceived by the sensor. The conceptual sensor below has only 16 pixels. Those pixels which capture the bright parts of the scene get filled up very quickly.

Once they are full, they overflow (this can also cause blooming). What flows over gets lost, as indicated in red, and the values of these buckets all become 255, while they actually should have been different. In other words, detail is lost. This causes "clipped highlights" as explained in the histogram section. On the other hand, if you reduce the exposure time to prevent further highlight clipping, as we did in the above example, then many of the pixels which correspond to the darker areas of the scene may not have had enough time to capture any photons and might still have value zero (hence the term "clipped shadows" as all the values are zero, while in reality there might be minor differences).

One of the reasons that digital SLRs have a larger dynamic range is that their sensors have larger pixels. All things equal (in particular fill factor, "bucket" depth, and exposure time), pixels with a larger exposed surface can collect more photons in the shadow areas than small pixels during the exposure time that is needed to prevent the bright pixels from overflowing.

It is easy to understand that one of the reasons digital SLRs have a larger dynamic range is that their pixels are larger. Larger pixels can collect more photons in the shadow areas before the bright ones start to overflow.

Some Dynamic Range Examples

The dynamic range of the camera was able to capture the dynamic range of the scene. The histogram indicates that both shadow and highlight detail is captured.
Here the dynamic range of the camera was smaller than the dynamic range of the scene. The histogram indicates that some shadow and highlight detail is lost.
The limited dynamic range of this camera was used to capture highlight detail at the expense of shadow detail. The short exposure needed to prevent the highlight buckets from overflowing gave some of the shadow buckets insufficient time to capture any photons.
The limited dynamic range of this camera was used to capture shadow detail at the expense of highlight detail. The long exposure needed by the shadow buckets to collect sufficient photons resulted in overflowing of some of the highlight buckets.
Here the dynamic range of the scene is smaller than the dynamic range of the camera, typical when shooting images from an airplane. The histogram can be stretched to cover the whole tonal range with a more contrasty image as a result, but posterization can occur.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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