Cine Lenses Follow
Unlike photo based lenses that use the focal length of the lens relative to the size of the opening to measure the F-stop, cinema lenses use what is known as a 'T-stop', which is a measurement of the amount of light that passes through the lens.
Cinema, or 'cine' lenses also feature a smooth/fluid aperture ring, allowing for smoother exposure adjustments while filming, as opposed to the click/notch aperture featured in photographic lenses that are intended to be adjusted between photos.