Sensitivity (ISO)
ISO stands for "International Organization for Standardization," and it is also the abbreviation for sensitivity. It is this organization that has quantified sensitivity. Sensitivity measures the degree of responsiveness of a photographic film to light, determined by measuring several numerical values. The ISO standard is ISO 6. Less sensitive films require longer exposure times to achieve the same imaging as more sensitive films, hence they are commonly referred to as slow-speed films. Highly sensitive films are referred to as fast-speed films. Additionally, an ISO system closely related to sensitivity is used to measure the sensitivity of digital imaging systems. Whether digital or film photography, increasing sensitivity often leads to a decrease in image quality (due to larger film grain or higher image noise or other factors).
Interpretation
ISO sensitivity is a unified international standard for measuring the sensitivity speed of traditional camera films, reflecting the speed of film sensitivity (actually the speed of the chemical reaction between silver elements and light). Traditional cameras can select low, medium, or high-speed films with different ISO sensitivities according to the specific shooting conditions. In traditional film cameras, ISO represents the standard for sensitivity speed, and in digital cameras, ISO is defined the same as film, representing the sensitivity speed of CCD or CMOS photosensitive components. A higher ISO value indicates greater photosensitivity of the photosensitive material. The calculation formula for ISO is S=0.8/H (where S is the sensitivity and H is the exposure). From the formula, we can see that the higher the sensitivity, the less demand for exposure. ISO 200 film has twice the sensitivity speed of ISO 100 film. In other words, under the same conditions, the exposure time for ISO 200 film is half that of ISO 100 film. In digital cameras, by adjusting the equivalent sensitivity, the amount of light source and the brightness of the image can be changed. Therefore, sensitivity has become a numerical value indirectly controlling the brightness of the image.
Principle
The sensitivity of traditional film is achieved by changing the chemical composition of the film to change its sensitivity to light. In contrast, the sensitivity element of a digital camera remains constant. Therefore, digital cameras generally use electronic signal amplification gain technology. The corresponding value to the ISO number is the electronic signal amplification gain value. For example, setting it to the standard value provides the same gain amplitude as ISO 100. The gain values corresponding to ISO 200 and 400 can be achieved by increasing the gain amplitude. Providing high sensitivity naturally requires providing a corresponding gain amplitude. Before outputting the image signal, the CCD or CMOS's original image signal must undergo corresponding signal amplification because the output level of the CCD or CMOS is low, especially when the ambient light is dim. In order to cause a change in the image, the amplifier increases the gain amplitude according to the corresponding ISO value. Additionally, increasing the number of pixels within a given CCD or CMOS area makes it difficult to maintain sensitivity. As the area per pixel decreases, the incident light weakens. Increasing the ISO value by calling a higher gain value will degrade image quality.
Therefore, manufacturers have provided a solution, which is to relatively increase sensitivity by adding the original image signals stored in the CCD or CMOS during image signal reading. By adding the signals of two pixels and transmitting them, twice the sensitivity can be obtained. However, this working method also has its drawbacks and is only suitable for smaller image modes because the number of pixels is halved in processing.
The third solution in the imaging industry to provide high ISO values is to use multiple pixel points as one pixel point for light sensitivity. For example, the standard ISO100 senses each pixel of the photosensitive element. To increase the sensitivity to ISO200, only two pixel points need to be sensed as one point to obtain twice the sensitivity speed. If you want to increase to ISO400, you just need to sense four pixel points as one point to obtain four times the sensitivity speed. Unlike the second method, which senses the merged pixels, this method senses the pixels after merging, so this working method is more advantageous for high-pixel models. However, the generation of noise is more obvious than the first two methods.
Application of Sensitivity Levels
Low Sensitivity
Below ISO800 (as of 2017) is low sensitivity.
In this range, extremely smooth and delicate photos can be obtained. As long as conditions permit and the photo can be taken clearly, try to use low sensitivity. For example, if you can ensure depth of field, it's better to open the aperture by one stop rather than increase the sensitivity by one stop.
Medium Sensitivity
ISO800 to 6400 (as of 2017) belongs to medium sensitivity.
In this range, you need to carefully consider what this photo will be used for and to what extent it will be enlarged. If you can accept noise, medium sensitivity settings reduce the difficulty of shooting handheld cameras and increase the safety factor for shooting under low-light conditions, increasing the success rate.
High Sensitivity
Above ISO6400 (as of 2017) is high sensitivity.
In this range, noise is obvious. When using such settings, the importance of the subject matter of the photo often exceeds the quality of the image. After all, sometimes the shooting conditions are very poor, and it's better to take a slightly lower-quality photo than to not capture any image at all.
Advantages and Disadvantages of High Sensitivity
Advantages
The impact of sensitivity on photography is reflected in two aspects. Firstly, speed: higher sensitivity allows for faster shutter speeds, which is relatively easy to understand. Secondly, image quality: lower sensitivity brings finer image quality, while higher sensitivity results in larger noise in the image. Speaking of this, let's introduce a concept — noise, mainly refers to the rough part of the image generated during the process where the CCD receives and outputs light as a received signal. So, how is noise generated? First of all, it's important to understand that for digital cameras as electronic products, the internal image sensor will inevitably be subject to varying degrees of electromagnetic interference from surrounding circuits and between pixels during operation. In other words, the images taken will inevitably contain noise to varying degrees, and what we see is just the degree of severity.
Disadvantages
For the first solution to provide high ISO values, as the gain amplitude is increased, noise information is correspondingly amplified, so noise in high ISO images becomes more pronounced. For the second method, as described earlier, to achieve high ISO, the number of pixels associated with imaging is also halved, and to ensure the original image information, pixel interpolation is required, which results in noise. For the third solution, due to the reduction in sensitive pixels, complete images can only be obtained by performing pixel interpolation during white balance. I believe that these three explanations are not difficult to understand. The "noise reduction function" popular in digital cameras is designed to reduce the noise introduced by the third solution. I believe that after the discussion in this article, it is no longer a secret that digital cameras have variable ISO sensitivity. Although high ISO brings concerns about noise, it is still a compromise solution without increasing costs to obtain higher shutter speeds and brighter images.
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