[Yes] This lens plus the Canon 5D Mark IV and the 2.x III extender are a great solution for me. I am not very often in a place where I can carry a heavy tripod and this combo is light enough for me to bring in a backpack either on my bike or hiking. Even without a tripod I am getting great results! I am an amateur photographer and mostly interested in birds so the weight of this lens was what caught my attention. I'm glad I didn't spend the extra money on a heavier lens because I would not be able to carry it around like this one. Perfect, fast auto focus, sharp even with the extender, I'm happy!

[Latch] I have the 500 f/4 lens and the 400 f/5.6 lens.� For photographing Peregrine Falcons doing aerial maneuvers and food exchanges, I needed something fast focusing and light since I hand hold these lenses.� I could not swing around the 500 f/4 fast enough and I heard that a couple photographers who have the lens loved it.� So I ordered it even though it's my only NON "L," lens and I LOVE IT!� I use it with my 1Dx Mark II and at times add the 1.4 Extender III or the 2X Extender III.� I am wonderfully surprised at how quickly it focuses even with the Extenders! �
The other thing I love about it is how light weight this lens is!!! � I can shoot all day and not get tired!� I would highly recommend it!

[Latch] This lens is very sharp and fast and is well worth your extra Expense for it. I shoot lot's of Wildlife ( Birds, four legged stuff, etc. ) I did have the Canon 400mm F/5.6 L lens: It is sharp but slow and without Image Stabilizers. This lens has three different Stabilizer modes......What a different's...Even with a extender EF1.4xII, it's just as fast and sharp...It's a Gem.

[Gray] Why is the new �400mm DO II the telephoto lens for everyone else? �Well, when mated with an APS-C camera and a 2X extender (with which it works well), it has the serious reach of 1280mm. � It is the second cheapest of all the expensive telephotos lenses. �It is the lightest of all the telephoto lenses. �It is the smallest, and the easiest to handle of the telephoto lenses. �All of which means this lens is more likely to be attached to your camera as opposed to decorating the place where you store your unused equipment. �Now if you make a living with your camera you should get whatever it takes to get that money shot even if it is cumbersome, heavy and generally unpleasant to use. �Because, well, that’s why they call it work. �But if you are like the rest of us who just enjoy taking pictures and prefer to invest in equipment we will actually use. �And, you need, or just want, a telephoto, this lens deserves your serious �consideration.

[Gray] 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!

[Latch] I own the 400 f/4 DO II and the superlative 500 f/4 L IS II, both purchased from B&H. I just returned (Feb.,2015) from a two week birding trip along the Louisiana and Texas gulf coast. My preference was to have the 7DII paired with the 400 f/4 DO II and the 5DIII paired with the 500 f/4 II. I successfuly used a 1.4 and a 2.0 Extender III with both setups. The 400 DO II acquires focus as well as the 500 II. Both are very quick. The build quality of the 400 DO lens is on a par with my 500 II. I have thousands of images to compare and I can say with complete confidence and all honesty the 400 DO II image quality is equal to the 500 II. I can hear the naysayers now but folks I have nothing to gain by reporting my findings. Heck, I spent the extra money to have the 500 f/4 L IS II and no regrets. IMO Canon DO optics and coatings for the 400 DO II set a new standard in the super telephoto world for light weight with 'L' glass quality. The 7DII and 400 DO II with a 1.4 extender weigh in at about 7.13 lb. and 896mm of reach. There simply is not another kit available to match the image quality along with the portability and reach of this Canon combination. Don't hesitate to own this fine lens. Just an aside IMO the 7DII body is a must have for birding or sports.

[Latch] I recently switched systems from Nikon to Canon and a fellow photographer who shoots Canon has this lens and he recommended I consider it. I rented one to try for a couple of motorsports events I was shooting and it performed outstandingly on my 5D MK4 and especially on my 1Dx MKII. In my previous system I had a 400mm f2.8 and this lens on the Canon bodies and with there ISO performance, I do not even think about it being an f4. The image quality with this piece of glass is phenomenal and pairing it with the 1.4 extender renders no loss of sharpness or image quality. The size and weight on this lens is another plus and it weighs at least half of what my old systems 400mm did. It's a bit smaller than the 300mm f2.8 and I can hand hold this all day and not feel any fatigue at all. If your looking for a 400mm consider this one. Rent one or borrow one and try it. You will be pleasantly surprised. The price point on this lens for what I get is a great deal and value as well. Love it and the images it produces.

[Latch] I bought the lens 6 months ago, primarily for birds in flight photography. It does an outstanding job. I like the lens so much, that I am actually considering getting rid of my 600mm lens. The 400 DO II lens is very sharp, and works great with both teleconverters. Autofocus is a bit slower, but image quality is still impressive. The size & wt. of this lens makes it excellent for hiking, and luggage carry on challenges with the airlines is over!

[Latch] Had a chance to use this side-by-side with the 400 2.8 III, also an outstanding lens. For my use, there was no noticeable difference in sharpness. I don't think I'll miss the extra stop on the 2.8 but I really prefer the handling of the DO.
Paired with a 5D3, very fast focus. Very happy with this purchase

[8.665 lb] 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!