What is ISO sensitivity for?

The popular point is film, or the sensitivity of the photosensitive original of a digital camera to light. The higher the value, the stronger the sensitivity to light, the less light needed (suitable for taking pictures in the dark) and the greater the noise. The smaller the value, the weaker the sensitivity, the more light needed and the less noise.

The following is a professional explanation:

Sensitivity, also known as ISO value, is a measure of the sensitivity of negative film to light, which is determined by the measurement of sensitivity and several values, and has recently been standardized by the International Organization for Standardization. For the negative with low sensitivity, it takes a long exposure time to achieve the same imaging effect as the negative with high sensitivity, so it is usually called slow negative. Highly sensitive negatives are therefore called fast negatives. ISO system closely related to sensitivity is used to measure the sensitivity of digital imaging system. Whether it is digital or negative photography, using higher sensitivity to reduce exposure time usually leads to the decline of image quality (due to thicker negative particles or higher image noise or other factors). Basically, the higher the sensitivity, the worse the photo quality.

The higher the photosensitive speed, the greater the value. In addition to shortening the exposure time, it will also affect the image quality.

Generally speaking, the picture quality at low sensitivity is more delicate and the color is more faithful. When the sensitivity is high, there will be noise and particle problems, which will make the photo unable to be enlarged. This phenomenon holds true whether it is the movies of the past or the digital cameras of today. Especially when digital cameras use high sensitivity, the serious noise produced by CMOS or CCD is unbearable for traditional photographers. This is one of the reasons why many photographers still choose negative cameras to shoot their works today.