The inner workings of a camera are much more complex than most people care to know about, but everyone should have a basic understanding of how digital photographs are created.

The ADC is the *Analog-to-Digital Converter*. After the exposure of a picture ends, the electrons captured in each photosite are converted to a voltage. The ADC takes this analog signal as input, and classifies it into a brightness level represented by a binary number. The output from the ADC is sometimes called an ADU, or *Analog-to-Digital Unit*, which is a dimensionless unit of measure. The darker regions of a photographed scene will correspond to a low count of electrons, and consequently a low ADU value, while brighter regions correspond to higher ADU values.

The value output by the ADC is limited by its *resolution* (or *bit-depth*). This is defined as the smallest incremental voltage that can be recognized by the ADC. It is usually expressed as the number of bits output by the ADC. For example a full-frame sensor with a resolution of 14 bits can convert a given analog signal to one of 2^{14} distinct values. This means it has a tonal range of 16384 values, from 0 to 16,383 (2^{14}-1). An output value is computed based on the following formula:

`ADU = (AVM / SV) × 2`^{R}

where `AVM`

is the *measured analog voltage* from the photosite, `SV`

is the *system voltage*, and `R`

is the *resolution* of the ADC in bits. For example, for an ADC with a resolution of 8 bits, if AVM=2.7, SV=5.0, and 2^{8}, then ADU=138.

Resolution (bits) | Digitizing steps | Digital values |
---|---|---|

8 | 256 | 0..255 |

10 | 1024 | 0.1023 |

12 | 4096 | 0..4095 |

14 | 16384 | 0..16383 |

16 | 65536 | 0..65535 |

*Dynamic ranges of ADC resolution*

The process is roughly illustrated in Figure 1. using a simple 3-bit, system with 2^{3} values, 0 to 7. Note that because discrete numbers are being used to count and sample the analog signal, a stepped function is used instead of a continuous one. The deviations the stepped line makes from the linear line at each measurement is the *quantization* *error*. The process of converting from analog to digital is of course subject to some errors.

Now it’s starting to get more complicated. There are other things involved, like *gain*, which is the ratio applied while converting the analog voltage signal to bits. Then there is the *least significant bit*, which is the smallest change in signal that can be detected.