The function described controls both exposure and focus within a camera system. It allows the user to fix these settings independently of each other, preventing automatic adjustments that might be undesirable in specific shooting conditions. For instance, when photographing a subject that is backlit, locking the exposure and focus on the subject ensures it is properly illuminated and sharp, even if the background is much brighter.
This feature is valuable for creating consistent results in challenging lighting situations or when precise control over the final image is required. Its roots lie in the need for photographers to override automatic camera functions to achieve a desired aesthetic or technical outcome. By separating the control of these two crucial aspects of image creation, greater creative freedom is provided to the photographer.