I mention photography equipment on this site only when I find something exceptionally good, bad, unusual, or unexpected. The words “unexpectedly good” might be used to describe the Canon EF 70-200 F4.0 IS L lens I recently purchased. I already owned a Canon EF 70-200 F2.8 L (non-IS) lens when I acquired the EF 70-200 F4.0 IS L lens. I made the purchase because I wanted something smaller and lighter in my backpack, and thought that the newer optical design might let me accomplish that goal without loss of image quality. Since the the 70-200 F4.0 IS L is about 1.5 pounds lighter and nearly an inch shorter, it fit my needs in that regard. The new lens also turned out to be an unexpected and significant upgrade in terms of optical quality. What follows is a general discussion of my purchasing research, user experience, and comparison of these lenses. If you are looking for 100% crops and test charts, you will have to look elsewhere.

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The Field
Canon currently markets four different L series 70-200 mm zoom lenses. There are two with a maximum aperture of F2.8, one with image stabilization and one without. Similarly, there are two with a maximum aperture of F4.0, one with image stabilization and one without. There is a tendency to believe that lenses in this set with the same maximum aperture are indeed the same, except for the image stabilization. That is definitely not the case. Not only does image stabilization introduce additional optical elements, but eleven years passed between the introduction of the 70-200 F2.8 L and the 70-200 F4 IS L lenses. During those years technological progress occurred. The F4.0 IS lens has the newest optical design and Canon’s MTF charts show this lens to be the best performing lens of the bunch, especially out at the edges and corners of the frame.

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Lack of Fanfare

Canon’s 70-200mm F4.0 IS L lens has received relatively little fanfare. This seems odd for a lens that has eclipsed the sharpness of Canon's highly regarded 70-200 F2.8 zooms. I suspect this lens has gotten so little attention because it is smaller and less expensive than its more physically impressive F2.8 siblings. A number of online photography forums make it apparent that many confuse the size of their lenses with the size of something else, but that’s fodder for a different story. In short, it seems the lenses some refer to as Canon’s “flagship zooms” have been sunk by a dinghy, and few have noticed.

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Sharpness

My own sharpness comparison testing of the 70-200 F4.0 IS and 70-200 F2.8 non-IS lens models only confirmed what Canon’s MTF charts predicted. I used both real world photos and shots of newspaper classified ads taped to a wall. One sheet was placed at the center and another was placed near the corner of the frame. While this is not quantifiable scientific testing, it was well enough controlled that I am confident that the comparisons are accurate. This testing showed the F4.0 IS lens to have substantially greater contrast and sharpness than my F2.8 lens at the apertures I tested. In my tests at F4.0, the F4.0 IS lens was visibly sharper across the entire image, including in the center. By F8 I could not tell any difference in the center sharpness, but the F4.0 IS lens was still visibly sharper at any point further than half way to the frame edge from the center, in the long dimension of the frame. In other words, in terms of a 24x36 mm frame, everything more than 9mm from the center was visibly sharper with the 70-200 F4.0 IS lens. The difference in edge contrast and sharpness is so substantial that in A/B comparisons I could tell which lens was used for any given shot without referring to my notes. The sharpness difference is somewhat less pronounced at smaller apertures, but it is still quite apparent even at F11 and F16.

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That’s not all there is to say about the outstanding sharpness and contrast of the Canon 70-200 F4.0 IS L lens. I also compared it at 70mm to my 24-70 F2.8 L lens, and at 90mm to my 90mm F2.8 Tilt/Shift lens (with no tilt or shift applied). The results show the EF 70-200 F4.0 IS L zoom lens to have somewhat better sharpness and contrast than the EF 24-70 F2.8 L lens, and nearly identical sharpness and contrast when compared to the 90mm F2.8 TS-E lens, which is remarkably sharp.

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These last results were not completely expected, so I wanted to see if they could be confirmed elsewhere. This lead me to a wonderful Canon lens comparison tool at www.the-digital-picture.com. This site enables you to directly compare 100% crops of an ISO 12233 resolution chart for any two lenses you pick. After picking the lenses, just move your cursor over the crops to switch between lenses. The crops are taken from three areas, near center, near the corner of a 1.6X sensor, and near the corner of a full frame sensor. Comparisons are available for nearly any Canon lens, as well as a few that are made by other manufacturers. This comparison tool confirms all of my test results, and it also shows the 70-200 F4.0 IS lens to be in the same general contrast and sharpness ballpark with the best prime lenses in the 70mm - 200mm focal length range. Take a look for yourself.

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Distortion

My own comparison of the distortion produced by the 70-200 F2.8 L and the 70-200 F4.0 IS L lenses showed that they have identical barrel distortion at 70mm. At 200mm, pincushion distortion of the F4.0 IS model is very slightly worse than that of the F2.8 non-IS model. For practical purposes both lenses exhibit essentially the same amount of distortion.

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Light Fall-off or Vignetting
The F4.0 IS model has more vignetting at F4.0 than does the F2.8 model at the same aperture. This is to be expected, since the faster zoom must have an acceptable amount of vignetting at F2.8, and vignetting decreases as apertures become smaller. Vignetting in the F4.0 IS model at wide apertures is not severe, but it is more pronounced than in the F2.8 model at the same aperture. As expected, vignetting decreases markedly as the lens is stopped down. At typical “landscape apertures” of F8 and smaller it is not an issue. When present, issues like vignetting and distortion can be corrected in Photoshop and other image editing programs, but it is nice when the added steps are not required.

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Image Stabilization
The 4-stop image stabilization used in the EF 70-200 F4.0 IS L zoom is remarkable. Although I seldom take hand-held photographs, the possibility of getting good hand-held images at 200mm with shutter speeds in the neighborhood of 1/15 second opens up numerous possibilities. Beyond making sure it works, I did no extensive testing of the image stabilization on the new lens. It does, however, seem noticeably steadier than the stabilization on our other Canon IS "L" series lenses. 
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Tripod Ring
Now we come to the tripod ring, specifically Canon's "Tripod Mount Ring A2". The fact that it is not included with either of the F4.0 lenses is the subject of a great many complaints in Internet photography forums. The cost for a genuine Canon ring is between $130 and $140, depending on where one looks. There are apparently cheaper third party rings available, but there are also some random problems reported with them, including casting voids which have caused at least one to snap, dropping both camera and lens to the ground. The Canon ring is nicely finished to match the lens and it clamps onto the lens barrel more firmly with less required finger effort than the ring on many older Canon lenses, including the F2.8 lens. Loosening the ring is as easy. Those who struggle to adequately tighten and loosen other Canon tripod rings will welcome this. The foot of the ring could be a little larger. This is the part that contacts the tripod, or the plate in an Arca-style mounting system. The surface area in contact with the tripod or plate seems a bit small for use with a very heavy equipment like a Canon 1-Series body with an attached flash unit. When mounted metal against metal to an Acra-style plate the ring is solid enough, but a slightly bigger foot would give more confidence. The ring itself uses a recessed felt cloth as a bearing surface against the lens barrel. Its feel when rotating the lens with camera attached is not as precise or smooth as the ring on the F2.8 lens, which uses a nylon bearing surface. It feels somewhat sloppy by comparison, but not terrible. The ring still allows for much easier movement of the camera between vertical and horizontal positions than would otherwise be the case. Unlike some of Canon's tripod rings, Canon's "Tripod Mount Ring A2" allows easy removal the ring without taking the lens off of the camera, and without removing the ring from the tripod if it is mounted. Given the light weight and extremely good image stabilization of the EF 70-200 F4.0 IS L lens, this could be a real benefit on some occasions.

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I should mention here that the 70-200 F4 lenses do not really require the tripod ring for shooting from a tripod. These lenses are about half a pound lighter than Canon’s 24-70mm F2.8 zoom, which has no tripod mounting ring available, and which presents no problem simply hanging off the front of most cameras. Hanging either lens off the front of a 1Ds Mark II body mounted on a good tripod works just fine. In fact, when using the tripod ring to mount the 70-200mm F4.0 IS lens to a Kirk BH-1 head, most cameras will be off balance, being much heavier on the camera side. On smaller heads with shorter clamps that allow positioning the camera closer to the post connecting the ball to the clamp, this may not be as severe. I still use the tripod ring in spite of this for the ease it affords in switching between vertical and horizontal camera positioning, and I have encountered no significant problems in doing so.

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Lens Hood

I have seen a number of complaints in photography forums about the hood Canon provides with the F4.0 lens. One called it "an embarrassment". Many said it should be of the "petal" design, like the hood provided with the F2.8 models. I can't imagine being embarrassed by a lens hood and I don't really care whether "petals" or bow ties are in fashion. It should be noted that a traditional lens hood design can provide the same, more, or less lens flare protection than a "petal" design. It all depends on how the lens and hood are designed to operate together. Regardless, the provided lens hood seems quite effective at preventing flare, which is what matters most to me. It is also quite sturdy. Its traditional design better allows the lens to be set hood down on any convenient surface without a lens cap, with less fear of toppling. I do not make a habit of doing this, but have done so occasionally when I need to switch lenses in a big hurry and have no time to put the other one away until after the shooting is done. My only minor complaint about the hood is that it is heavily flocked inside. This makes it much less reflective and thus better at preventing lens flare, but the flocking also tends to pick up all manner of dirt. The petal type hood on the F2.8 models has a smooth matt finish inside, which is more easily cleaned and does not pick up dirt. But overall, the lens hood is just fine unless you're trying to make a fashion statement. In the end, Canon's lens designers probably know more than do most about making a hood that works well with a given lens design.
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No discussion of lens hoods would be complete without stating how important it is to use one when you shoot, always, no matter what. Lens hoods are not just for those times when direct rays of the sun are striking the front element of your lens and causing obvious flare problems. Even when the sun is at your back, if a hood is not used, light reflected from the sky, the ground, buildings, trees, or anything that is not part of your picture will enter the lens and degrade its contrast performance. The proper hood is always required for maximum lens performance. The ideal lens hood would completely block light reflected from any object not actually in the picture being taken. That's why the most optimal hood shape for a lens whose orientation is fixed with respect to the film or sensor is large and rectangular. I have seen a few of these over the years on non-Canon lenses, but unfortunately they are only common on the extremely expensive (6-digit) lenses used in the movie and television industry.

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Fit, Finish, and Feel

The fit and finish of my copy of the 70-200 F4.0 IS lens is perfect in every way, but the overall “feel” of this lens is not nearly as nice as the 70-200 F2.8 non-IS lens. Much of this may be due to the extensive use of plastics in the new lens. These give it a less substantial feel, but also make it much lighter and easier to carry. This use of plastics may in fact make the new lens more durable, but in terms of how it feels in the hand, it’s something like holding a Timex watch in one hand and a Rolex watch in the other. The Timex is probably more durable, but it sure doesn’t feel that way. New water and dust protective seals on the F4 lens add friction to the movement of the zoom and focus rings, making the movement less fluid, but at the same time the controls do not stick or move in a “jerky” manner.

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Odd Filter Size

A negative aspect of the smaller size of the F4.0 model is its 67mm filter diameter. This fairly odd size will often require the additional purchase of at least a polarizing filter. The best of these will set you back another $130 or so.

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Cost - It's Not Just IS

Many have complained that the price difference between the IS and non-IS versions of the F4.0 lenses is too extreme and cannot be justified. It should be emphasized here that the 70-200 F4.0 IS lens is not simply the 70-200 F4.0 non-IS lens with added image stabilization. As mentioned previously, the IS model uses a newer optical design that includes two UD glass elements and one fluorite element, which account for its substantially increased sharpness, contrast, and price. The new and very effective image stabilization is only part of the cost equation here, and that fact is very often overlooked. Cost is also likely one of the reasons that Canon does not include a tripod mounting ring with the lens. If they had, the price of the 70-200 F4.0 IS L lens would surpass that of the 70-200 F2.8 L (non-IS) lens. This just wouldn't work in Canon's current price lineup.
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Canon's EF 70-200 F4.0 IS L lens is by no means inexpensive. Not including any rebates or special deals, its street price is around $1059 plus shipping at the time of this writing. If you purchase the optional tripod mounting ring, the street price for the combination is over $1200. Adding a quality polarizer because the lens takes an odd size that you don't already own brings the total to around $1330 plus shipping.

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Conclusion
If you need a 70-200mm Canon lens and do not require F2.8 apertures, there is no good reason to buy one of the F2.8 models. The F4.0 IS model may well be the sharpest zoom lens in the entire Canon line at the moment. Canon’s own MTF charts bear this out, as do other independent tests. Distortion is essentially identical between the F2.8 and F4.0 units I tested. For my use, the additional vignetting wide open is not significant enough to be a factor in a purchasing decision, and it is correctable in post processing regardless. The Canon EF 70-200 F4.0 IS L lens saves some weight on my back and room in my pack. Adding 4-stop image stabilization to the mix makes this lens unbeatable. If you are in the market for a high quality 70-200 mm zoom lens, this one is definitely worth considering.
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Happy shooting,

P.S. - Sometimes determining which lens is “best” depends more upon how the lens will be used than it does upon lens performance criteria such as contrast and sharpness. For those who must often shoot with apertures in the F2.8 range, there is simply no substitute for one of the F2.8 models. Some argue that image stabilization negates the need for large apertures, but that argument is not entirely true. Like a tripod, image stabilization helps only with camera movement, not subject movement. This means stabilization is not a substitute for a fast lens when moving subjects are photographed in low light situations. Shooting at large apertures is also a common technique used to isolate a subject against an out of focus background, as in portraiture. In this case too, there is no substitute for large apertures.