Those looking to invest in a 400mm telephoto lens have a several options available for consideration. One such option available is the Canon EF 100-400mm f/4.5-5.6L IS II USM Lens. And while the telephoto zoom is certainly a versatile option, opting for a 400mm prime lens over the zoom alternative will typically either a) give you a wider maximum aperture at that particular focal length or b) save you some cash (but unfortunately, those benefits seem to be mutually exclusive).
Before we get started, it's important to note that I wouldn't necessarily consider wide-open image quality to be a differentiating factor among the 400mm prime lenses in this comparison. While there are certainly differences, all perform very well. With that in mind, let's take a look at the notable benefits/drawbacks associated with each of the 400mm prime candidates.
The Canon EF 400 f/2.8L IS II USM is the biggest, heaviest and priciest 400mm prime option. However, there's another "-est" descriptor that justifies this lens' price tag for many professional photographers – "widest." The 400L IS II's f/2.8 maximum aperture is 2-stops wider than most zoom lenses including the 400mm focal length, and 1-stop wider than the formidable EF 400mm DO IS II model. This lens is unparalleled when action stopping shutter speeds are necessary, especially in locations where the available light is less than abundant. Of course, the background blur at f/2.8 is noticeably stronger than it is at f/4 or f/5.6, with the benefit of stronger subject isolation. As you might expect, this lens is built for the needs of professionals with weather sealing and excellent AF performance.
Speaking of AF, a 3-position focus limiter switch allows focusing distances to be limited to a specific distance range - or to be unlimited: 8.85' - 23' (2.7m - 7m), 23' (7m) - ∞, 8.85' (2.7m) - ∞. Limiting the focus distance range can improve focus lock times and reduce focus hunting. Autofocus Stop buttons near the objective lens allow autofocus to be temporarily stopped. The Autofocus Stop feature makes it easy to obtain focus lock, turn off autofocus and recompose for a framing that places the active focus point(s) off of the subject.
The 400L IS II features a 4-stop IS (image stabilization) system with normal, panning and tracking modes available (Modes 1, 2 & 3 respectively). And while I mentioned image quality wasn't necessarily a differentiating factor in this comparison, the 400 f/2.8L IS II edges out the other two in most regards.
As noted, a high price isn't the only compromise one must accept to gain a wide, f/2.8 aperture at this focal length; consequently, this lens is neither small nor light. With the hood installed, the 400L IS II weighs in at 143.7 oz (4070g) with and is 7.68 x 13.8" (195.1 x 350.5mm) with the hood reversed for storage. While this lens can be used handheld, most will likely need a solid tripod or, at the very least, a monopod for comfortable medium-to-long-term use. Support/stabilizing gear necessary for long-term use will of course add to the size and weight storage/transport requirements when traveling with the lens.
If price is no object and small size and weight are not priorities, this is the ultimate 400mm option.
It's not often that a lens measuring 6.32 x 9.45” (160.45 x 240.13mm) and weighing in at 80 oz (2265g) (with hood) can be considered small and lightweight, but... everything is relative. Compared to its massive f/2.8 big brother, the Canon EF 400mm f/4 DO IS II USM Lens' size and weight are quite manageable considering its focal length and f/4 max. aperture, allowing for much longer periods of handheld use before fatigue takes its toll. This lens' diffractive optics elements allow for a very compact design that sports and wildlife photographers will especially appreciate.
The 400 DO IS II features a 3-position focus limiter switch with the following settings: 10.8' - 26.2' (3.3m - 8m), 26.2' - 8 (8m - ∞) and 10.8' - ∞ (3.3m - ∞). Autofocus Stop buttons are also included near the objective lens and allow autofocus to be temporarily stopped. Like the 400 f/2.8L IS II, the 400 DO IS II is weather sealed and features a 3-Mode, 4-stop IS system.
While an f/4 maximum aperture may not be considered "wide," the moderately-wide max. aperture combined with this lens' IS system makes for a very versatile tool that's reasonably sized for handheld shooting. Bird, wildlife and sports photographers will often forgo the f/2.8 maximum aperture to enjoy the smaller size, weight and price benefits associated with the f/4 DO IS II option.
Representing the lowest tier in Canon's 400mm primes in size, weight, max. aperture and price is the Canon EF 400mm f/5.6L USM. Note that I didn't list "image quality" in the preceding list as the 400 f/5.6L is no slouch from an image quality perspective. That a lens designed nearly 25 years ago can perform compete so well with the 400L IS II (released only 6 years ago) is impressive, to say the least.
The most alluring aspects of the 400 f/5.6L, aside from the focal length shared by the other lenses in this comparison, are its small size, light weight and comparatively low price. The 400 f/5.6L measures 3.54 x 10.44” (90.04 x 265.17mm) (with hood unextended) and weighs in at only 47.7 oz (1351g). While the 400 DO IS II is shorter with its hood reversed, the 400 f/5.6L's diameter is significantly smaller and it weighs 32.3 oz (915.7g) less than the DO II model, making it significantly easier to travel with and use handheld for long periods of time.
On the downside, image stabilization is not a feature of this lens and it is only partially weather sealed (a lens mount gasket is not present, but the switches and focusing ring have moderate dust and moisture resistance). The lack of IS means that notably higher shutter speeds (up to 4-stops greater) will need to be utilized to negate camera shake compared to the other lenses mentioned above.
And speaking of the sealed switches, this lens only has two: an autofocus/manual focus switch and a focus limiter switch with settings of 11.48' (3.5m) - ∞ and 27.89' (8.5m) - ∞.
Without IS and a wider maximum aperture, the f/5.6 model's high image quality combined with its low price will represent the primary reasons why photographers choose it over one of the other 400mm prime options as well as the EF 100-400 f/4.5-5.6L IS II USM (which features the same max. aperture at 400mm).
Size Comparison Images
While I mentioned the sizes of the lenses detailed above, the comparison images below will put those numbers into relative context. The first image (also displayed atop this post) shows the lenses in their ready-for-the-gear-bag form with hoods reversed (or not extended).
As I mentioned in the introductory paragraph, while there are certainly small differences in image quality (including sharpness, vignetting, flare and [less so] distortion), most will not consider IQ to be a differentiating factor between the 400mm prime candidates listed above. Instead, the significant differences in price found in the available choices directly correlates to the max. apertures available, the inclusion of image stabilization and (to some extent) the amount of weather sealing featured in the lens' design. Ultimately, your max. aperture needs, size/weight requirements and budget limitations will be the most important factors in determining which of these lenses is the ideal addition for your kit.