ISO 400 is a standard setting for the sensitivity of a camera's sensor to light. In photography, a higher ISO number allows for better performance in low-light conditions, making it easier to capture images without using a flash. However, increasing the ISO can also introduce noise, which may reduce the overall quality of the image. When using ISO 400, photographers can achieve a balance between light sensitivity and image clarity. This setting is often used in various situations, such as indoor events or cloudy days, where lighting may not be optimal. It is a popular choice among both amateur and professional photographers.