In photography, the term «millimeters» refers to the focal length of a lens. The focal length is a measure of how much the lens is able to magnify an image. A lens with a shorter focal length, such as a wide-angle lens, has a wider field of view and is able to capture more of the scene in the photograph. A lens with a longer focal length, such as a telephoto lens, has a narrower field of view and is able to magnify the image more, allowing you to get closer to distant subjects.
The size of the lens is also often expressed in millimeters, with a larger number indicating a longer lens. For example, a 50mm lens is a standard lens that is typically used for everyday photography, while a 300mm lens is a telephoto lens that is often used for sports or wildlife photography.
In general, the focal length of a lens is an important factor to consider when choosing a lens for a particular type of photography. It can affect the perspective of the photograph, as well as the amount of light that the lens is able to gather.