Excuse me: What does this little red line on the lens do?

The small red line on the lens usually indicates that it has some special function. Specifically, this small red line usually represents the camera's autofocus system. When the lens is in autofocus mode, a small red line will automatically appear in the viewfinder to help the photographer determine the focus.

However, different lenses and cameras may have different designs, and this small red line may have different functions. Therefore, to understand the exact function of the small red line on the lens, it is best to consult the manual of the lens or camera.