The EF400mm f/4 DO IS II USM is a professional lens that is particularly suited for wildlife and sports photographers. This lens allows photographers to achieve the reach of a super-telephoto lens, while maintaining superior standards of mobility in the lens through a portable and lightweight form factor. While traditional telephoto lenses are designed with gaps between the optical elements to reduce chromatic aberration, the EF400mm f/4 DO IS II USM uses a gapless dual-layered diffractive optical element, which greatly reduces the ring-shaped diffractive flaring caused by high-intensity light sources. This reduction is taken further with the incorporation of a large diameter ground aspherical lens and a UD lens.
It also contains an improved IS unit that significantly enhances image stabilization when the appropriate IS mode is selected and features a shorter minimum focal distance, down to 3.3m from 3.5m in its predecessor. Maximum magnification has also improved from 0.12x to 0.13x as a result of the superior lens characteristics. The EF400mm f/4 DO IS II USM is highly resistant to harsh environments. This is achieved through a fluorine coating on both the front and rear lens elements, which makes the lens dirt and oil-resistant while easier to clean. All rings and buttons are constructed with a dust and drip-proof structure, preventing dust and liquid from entering the sensitive interior of the lens.