A long-focus lens is a lens with a focal length significantly greater than the diagonal measurement of the film or sensor format it is used with. These lenses are often referred to as telephoto lenses and are designed to magnify distant subjects while providing a narrow field of view.
Understanding long-focus lenses is important for photographers who want to capture subjects at a distance, such as wildlife, sports, or portrait photography. These lenses allow for selective framing, compression of perspective, and the ability to isolate the subject from the background, making them essential tools for achieving sharp, detailed images in a variety of photographic applications.