The dynamic range or the photographic latitude of the photographic material is the ratio between the maximum and minimum exposure values that can be correctly captured on the image. In relation to digital photography, the dynamic range is actually equivalent to the ratio of the maximum and minimum possible values of the useful electrical signal generated by the photosensor during exposure.
Dynamic range is measured in steps of exposure (EV). Each step corresponds to a doubling of the amount of light. So, for example, if a certain camera has a dynamic range of 8 EV, this means that the maximum possible value of the useful signal of its matrix refers to the minimum as 28: 1, which means that the camera is capable of capturing objects within one frame that differ in brightness not more than 256 times. More precisely, it can capture objects with any brightness, however, objects whose brightness exceeds the maximum allowable value will appear dazzlingly white in the image, and objects whose brightness is below the minimum value will be charcoal black. Details and texture will be distinguishable only on those objects whose brightness fits into the dynamic range of the camera.
To describe the relationship between the brightness of the lightest and the darkest of the objects being shot, the not quite correct term “dynamic range of the scene” is often used. It would be more correct to talk about the brightness range or the level of contrast, since the dynamic range is usually a characteristic of a measuring device (in this case, the matrix of a digital camera).
Unfortunately, the brightness range of many beautiful scenes that we encounter in real life can significantly exceed the dynamic range of a digital camera. In such cases, the photographer is forced to decide which objects should be worked out in all details and which can be left outside the dynamic range without prejudice to the creative idea. In order to maximize the use of the dynamic range of your camera, sometimes you may need not so much a thorough understanding of the principle of the photosensor as a developed artistic sense.
Dynamic Range Constraints
The lower limit of the dynamic range is set by the intrinsic noise level of the photosensor. Even an unlit matrix produces a background electrical signal called dark noise. Noise also occurs when the charge is transferred to an analog-to-digital converter, and the ADC itself introduces a certain error into the digitized signal – the so-called noise sampling.
If you take a picture in total darkness or with a lens cap, the camera will record only this meaningless noise. If a minimum amount of light is allowed to enter the sensor, the photodiodes will begin to accumulate an electric charge. The magnitude of the charge, and hence the intensity of the useful signal, will be proportional to the number of photons caught. In order for the picture to reveal at least any meaningful details, it is necessary that the level of the useful signal exceeds the level of background noise.
Thus, the lower boundary of the dynamic range or, in other words, the threshold of the sensor sensitivity can be formally defined as the level of the output signal at which the signal-to-noise ratio is greater than unity.
The upper limit of the dynamic range is determined by the capacity of a single photodiode. If during the exposure any photodiode accumulates an electric charge of a limit value for itself, then the image pixel corresponding to the overloaded photodiode will turn out to be completely white, and further irradiation will not affect its brightness in any way. This phenomenon is called clipping. The higher the overload capacity of the photodiode, the greater the signal it can give to the output before it reaches saturation.
For greater clarity, we turn to the characteristic curve, which is a graph of the dependence of the output signal on exposure. The binary logarithm of the radiation received by the sensor is plotted on the horizontal axis, and the binary logarithm of the magnitude of the electrical signal generated by the sensor in response to this radiation is plotted on the vertical axis. My drawing is largely conditional and is for illustrative purposes only. The characteristic curve of a real photosensor has a slightly more complex shape, and the noise level is rarely so high.
Digital characteristic curve
Two critical tipping points are clearly visible on the graph: in the first of them, the level of the useful signal crosses the noise threshold, and in the second, the photodiodes reach saturation. The exposure values between these two points make up the dynamic range. In this abstract example, it is equal, as you can see, 5 EV, i.e. the camera is capable of digesting five doubles of exposure, which is equivalent to a 32-fold (25 = 32) difference in brightness.
The exposure areas that make up the dynamic range are not equivalent. The upper zones are characterized by a higher signal to noise ratio, and therefore look cleaner and more detailed.