How does a digital camera work?
For full control over the process of obtaining a digital image, it is necessary, at least in general terms, to imagine the device and the principle of operation of a digital camera.
The only fundamental difference between a digital camera and a film camera is the nature of the photosensitive material used in them. If in a film camera this is a film, then in a digital one – a photosensitive matrix. And just as the traditional photographic process is inseparable from the properties of the film, so the digital photoprocess largely depends on how the matrix converts the light focused on it by the lens into a digital code.
The principle of the photomatrix
The photosensitive matrix or photosensor is an integrated microcircuit (in other words, a silicon wafer), consisting of the smallest photosensitive elements – photodiodes.
The matrix of the camera Nikon D4
There are two main types of sensors: CCD (Charge-Coupled Device, aka CCD – Charge-Coupled Device) and CMOS (Complementary Metal-Oxide-Semiconductor, aka CMOS – Complementary Metal-Oxide-Semiconductor). Both types of matrices convert the photon energy into an electrical signal, which is then to be digitized, however, if in the case of a CCD matrix, the signal generated by the photodiodes arrives in the camera processor in analog form and only then is centrally digitized, then in the CMOS matrix each photodiode is equipped with an individual analog digital converter (ADC), and the data goes into the processor in a discrete form. In general, the differences between CMOS and CCDs, although fundamental to the engineer, are absolutely insignificant for the photographer. For photographic equipment manufacturers, the fact that CMOS arrays, being more complex and more expensive than CCD arrays in development, is also more profitable than the latter in mass production, is also important. So the future is likely to be based on CMOS technology for purely economic reasons.
The photodiodes that make up any matrix have the ability to convert the energy of the light flux into an electric charge. The more photons a photodiode picks up, the more electrons it produces. Obviously, the larger the total area of all photodiodes, the more light they can perceive and the higher the photosensitivity of the matrix.
Unfortunately, photodiodes cannot be located close to each other, since then there would be no space left on the matrix for the electronics accompanying photodiodes (which is especially important for CMOS arrays). Sensitive to light, the sensor surface averages 25–50% of its total area. To reduce light losses, each photodiode is covered with a microlens that exceeds its area and is actually in contact with microlenses of neighboring photodiodes. Microlenses collect the light incident on them and direct it into the photodiodes, thereby increasing the photosensitivity of the sensor.
The basis of any photograph is light. It penetrates the camera through a lens, the lenses of which form an image of an object on a photosensitive matrix. When you press the shutter button, the camera shutter opens (usually for a split second) and the frame is exposed, i.e. lighting the matrix with a stream of light of a given intensity. Depending on the desire to take a light or dark picture, a different amount of light may be required, i.e. different exposure.
Upon completion of exposure, the electric charge generated by each photodiode is read, amplified, and, using an analog-to-digital converter, is converted into a binary code of a given bit depth, which is then fed to the camera processor for further processing. Each matrix photodiode corresponds (although not always) to one pixel of the future image.
Bit depth determines the number of shades, i.e. gradations of brightness for each pixel. The higher the bit depth, the more smooth tonal transitions the camera can capture. Most digital SLR cameras are capable of storing 12 or 14 bits of information for each pixel. 12 bits means 212 = 4096 shades, and 14 bits means 214 = 16384 shades.
Under the dynamic range of the matrix, we mean the relationship between the maximum signal level of the photodiodes and the background noise level of the matrix, i.e., in essence, the ratio between the maximum and minimum light intensities that the matrix is able to perceive.
The more photons the photodiode is able to catch before it reaches saturation, the greater the dynamic range the sensor as a whole will possess. The capacity of photodiodes is proportional to their physical size, and therefore, ceteris paribus, a camera with a larger matrix, and hence with larger photodiodes, will have a large dynamic range and lower noise level.
In addition, a larger matrix usually means a higher maximum ISO speed for a particular camera model.