ESPARSHOT - Glossary

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).

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 do not "fill up" so quickly, so there is more time to capture the dark pixels 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.