I own several Canon lenses, and this lens is the sharpest of all the telephotos. It is sharper than even the 100-400 Type II zoom telephoto, and weighs very little more. I especially like the center of gravity with a 5D4 body attached, it's very nearly right over the handle center. That makes it easy to hand carry as well as balance on my gimbal tripod. Like all long telephotos, it does require some slight microadjustment for the very sharpest pictures, but once done, you will be blown away. Great lens, highly recommend.

I use this lens for wildlife photography and I am amazed at how light and portable it is. I am able to maneuver and hold the camera and lens with one hand. I have it mounted on an R5 and it works flawlessly. The autofocus is fast and locks on. I was amazed at how small the lens is compared to my previous 600mm and 400 f2.8 Nikon. This lens is a keeper and something that I can easily carry around and handhold all day long. The F/4 aperture is not an issue as the noise reduction on the R5 allows for higher ISO levels and lets plenty of light in even in dark forests. I am so glad I made the decision to return the 600mm and go with this lens.

The first version of this lens has been controversial, as some people claimed it wasn't sharp and had poor contrast and flare issues. Rest assured that these issues have been corrected in the new version. Previously I had the 400mm f/5.6 lens. I have used this lens to photograph birds for the past 8 years or so. I thought it was pretty sharp lens, and it is, but it cannot compare to the sharpness of the 400 f/4 DO II lens. The detail it reveals in a bird's feathers is quite amazing. The contrast and color snap of this new version is outstanding, comparable to my other L series lenses. The AF speed is very fast and accurate. Flare is very well controlled. With the 7D MKII, photographing ducks in flight is no problem. The lens works very well with the Canon 1.4X converter, with very little loss of sharpness while retaining fast focus. It can also produce very sharp images with the 2X converter, although the focusing is a bit of an issue with the 7D MKII. Quite often you will have to manually focus a bit to get it close to accurate focus, at which time the AF kicks in to final focus. I should mention that all my testing, even with converters, was done hand held. This makes it particularly useful to those who want to hike some distance from the car.All in all, this lens has exceeded my expectations, and I recommend it to those who want a high quality lens that is much more hand holdable and portable than the longer, heavier glass.

Super sharp, super fast. I'm getting far more difficult handheld wildlife shots with this lens than with my 100-500. I'll keep the zoom for large animals and insect zoom/macro work, but for the rest I'll be using the DO 400mm.

I am a professional who has been in the business 40+ years. I've used a lot of different big glass over this time period. I recently bought and used the 400 f4 for a week long assignment on Polar Bears on the North Slope of Alaska. Needed something I could easily manage and hopefully handhold. The conditions were very cold and harsh and we shot a lot of the time from a moving, rocking boat. I used the lens with both film and digital camera bodies. This is an incredible lens both in terms of sharpness, performance-especially in the cold, quickness of focus, clarity, color and contrast. It is light enough to hand hold comfortably which is difficult to do with any other fast big telephoto lens. If you need a big piece of glass and don't need a 2.8 aperture, this is your lens. When using the 1.4x tele-converter the images were also sharp with good color. I have no complaints and totally recommend this lens for its size, weight and sharpness. You won't be disappointed. I captured some great, very detailed, Polar Bear images with it.

Pros The lightest super-telephoto lenses on the market delivering up to 800mm of stabilized reach with fast AF (pro bodies). Sharp with both EF extender 1.4X III and 2x III with no visible loss in crispness or contrast unlike its predecessor. compact size, it fits in a small backpack. Easy transportation, carry-on compatible with small regional jets. Rapid AF even when attached to the Extender 2X III (EOS-1D X only) Far-focus MFD of 8m at 800mm. Cons Price.

This has been my primary lens for wildlife photography for 1,5 years now and I just love everything about it! I switched from 400 5.6L and this DO II is just on another level. The microcontrast and sharpness puts it in the same league with other mk II super telephoto primes. Its lightweight, works with extenders extremely well. IS is highly effective, I have no problem handholding shots with the 2X III attached at 1/100 sec. It is rarely mentioned, but this lens can natively produce amazing bokeh, as long as your subject is in a relatively close distance. While having a fast 2.8 prime is always tempting, the extra weight, size, cost it comes with makes those lens unreasonable for my wildlife photography.
If you need a portable prime telephoto lens with the highest image quality, look no further!

I just purchased an EF 400mm f/4 IS II lens. My first chance to really try it out was when a brood of American kestrels fledged from a nest box on our pole building. The pictures I got were very sharp and enabled me to identify food items being brought to the young by the adults. Getting closer with a lens like this allows you to learn some of the ins and outs of animal behavior besides getting some really neat photos. Thanks, B and H!

When the first 400mm DO was introduced I bought it. The light weight and small size were quite attractive, but after using the lens on a safari inn Kenya I was disappointed- it just was not as sharp as my Canon 500mm f4 IS lens. So, I sold the 400 and moved on.

The Mk II version of the 400 DO is a great improvement over the lens I sold. It is close in quality to the 300mm f2.8 Mk II which I use extensively (with 1 1.4x Mk III TC) for flight shots.

I have been using this (new for me) lens with the Canon R 5, alone and with the 1.4x Mk III TC, and the results are impressive.