What is the real trick to reduce noise when taking pictures?

What bothers us when taking pictures with a digital camera is that the lens is always not so clear and there is always noise, which makes the effect of taking pictures very unsatisfactory. Digital camera technology is becoming more and more advanced and mature, but many cameras are not satisfactory. The evaluation between cameras focuses on ISO and noise control. If we can provide extremely high ISO and effectively suppress noise, we will get more support from people. The so-called noise is that irregular and rough particles or spots appear in the picture, which destroys the clarity, sharpness and details of the photo and reduces the quality of the work. Therefore, in most cases, we don't like the noise in photos, so we always hope to suppress it. There are many reasons for noise. Many people prefer to take low ISO underexposed photos for post-processing in low light, rather than high ISO, because they think that "ISO= noise" and that sensitivity equals noise, so the higher ISO equals more noise, and then it is simplified to the conclusion that "low ISO will get less noise". Although noise is directly related to ISO, what is the real situation? Let's look at a set of photos. These are two photos taken by photographer Andrew Childress with Canon 5D Mark II at different ISO levels. On the left is a photo taken with ISO6400, and on the right is a "pseudo-ISO 6400" photo taken with ISO 1600, two levels lower, and then two levels higher are exposed with Lightroom. Obviously, the result is that the noise of ISO 6400 is obviously reduced. Therefore, to reduce noise, we need to learn to "properly expose with a camera", and we can't fully believe that low ISO means less noise. Of course, the causes of noise are still complicated, and each camera handles it in a different way, so it is best to do more experiments and knowledge. For example, we shoot a digital camera with the highest pixel up to 3 million pixels at 654.38+0 million pixels, which is equivalent to using three CCD photosensitive units as one, the photosensitive capacity is equivalent to three times, and the image noise is also reduced a lot. Therefore, the small skills of noise reduction when taking pictures need our constant discussion and study.