A friend of mine recently offered to let me use the EF Canon 70-200 f4L on an upcoming two day benefit photography event that I was attending so I thought I would take him up on his offer and share my thoughts on this lens. The version I borrowed is the imaged stabilized version but it also comes in a non-stabilized version for the budget minded photographer. I will conclude the review with images taken from the event so you can see the image quality for yourself in a real world shooting situation.

Build Quality
As one would expect from a high quality luxury (L) series lens the build quality is outstanding. The lens is constructed of aluminum and is sealed against rain and dust in the IS version. The focus and zoom rings are positioned well and are made of a rugged rubber coating. Arguably one of the best benefits comes from the fact that although a rugged lens, the weight comes in at only 1.68lbs making it ideal for hand holding for long periods of time. The length and width of the lens barrel is comparable to other lenses in this focal range.
  • Focal Length:  70-200mm zoom
  • Aperature: f4 (constant)
  • Lens Construction: 20 elements in 15 groups
  • Focus Adjustment: Auto-focus with full time manual focus
  • Filter Size:  67mm
  • MSRP:  $1,299.00 US
  • Street Price New:  $1,099.00 US (with rebate)
  • Used Price:  $800.00-$1,000.00 US  (if you are going to pay this much, just buy it new with the rebate)
  • 3-4 Stop Image Stabilizer:   depending on when the model was created one should be able to get up to 4 stops of stabilization useful in low light situations. 
  • Two Image Stabilizer Modes:  Mode 1 is designed to stabilize the both the x and y axis for full image stabilization while mode 2 is designed to be used while panning to prevent vertical camera shake.
  • Full Time Manual Focus:  this is a really nice feature that most modern "pro" lenses implement.  This simply means that you can always use manual focus even if your lens is switched to auto-focus.  I use this to great effect when shooting wildlife from a blind.  If I had to reach over and switch to manual focus there is a good chance I would miss the shot because of the motion an noise of the switch.  
  • Weather Sealing:  This lens is sealed against dust and rain which is mostly just comfort of mind when shooting in potentially harsh conditions.  This has never been a deal breaker for me when deciding to purchase a lens.  If it is pouring rain, I am generally hiding under a tree somewhere or in the car.  Instead of buying weather-proof lenses,  I weather-proof my backpack.
On to the Real World - How does this lens perform in a controlled field environment?
In the "field" this lens did not disappoint me, and perhaps one of the greatest attributes of this lens is the weight.  Coming in at 1.68 lbs it is a joy to hold for long periods of time as opposed to it's faster cousin the 70-200 2.8. If a compact light design is what you are looking for in a medium telephoto then look no further than this beauty.  I spoke with a woman photographer at this event who said she really missed the weight difference in her 70-200 f4.  She was using the 2.8 version and found it to be impractical for hand holding long periods of time.
As one would expect from Canon "L" series lenses, the sharpness is quite unbelievable, even at f4.  In my opinion there is no reason to stop this lens down to f8 unless you need to do so for artistic reasons. I took photographs ranging from hand held to tripod stabilized and tripod stabilized with image stabilization engaged. I found very little difference in the image quality in any of those scenarios.  I did also shoot a couple different species with my Kenko 1.4x TC attached and I was surprised to see only a little image degradation in sharpness and contrast.  Here is an example of the sharpness of this lens at f4 under field conditions:

The above image of the screech owl was taken in fairly dark conditions with a tripod, at an aperture of  f4, a shutter speed of 1/125, an ISO of 1200 and a focal length of 200mm.  I applied standard sharpening in Lightroom and a small amount of noise reduction during post processing. Since it is recommended by other photographers such as Art Morris, I Ieft the IS engaged.  As you can see the image is sharp and has great contrast and color. Bokeh is also pleasing and can be seen off to the left of the frame. Honestly for a lens of this caliber I did not expect poor results, but it is nice to confirm this in the field. 
I didn't have a great deal of subjects on which to test auto-focus but even birds perching required constant refocusing of the lens due to the bird changing its body and head position. I used ai servo mode on my 7D because of the constantly changing position of the bird.  The auto-focus was much faster than my Tamron 70-300 SP VC which is a lens of mine I have written about in the past.  I also found it to be a tad more reliable and seemed to lock in focus with better accuracy overall. I also did tests where I focused on the background and then the bird's head to see how fast it snapped back and it was extremely fast.  I don't have specific timings but I was more than happy with results.
Image Stabilization
Does image stabilization really matter?  I use image stabilization on my Tamron and I used it with this lens as well and I find it to be useful in stopping the shake caused by both hand holding and hand holding it on the tripod.  If I didn't have it, could I still get great pictures?  Yes, of course.  Image stabilization is just another tool to get your "keeper rate" higher and have more photos to post on Facebook for your buddies, but it is not a must.  Having said that, we are in the field and taking photographs of birds and they move.  In fact they move a lot even while perched.  So I find anything below  1/125 of a second is troublesome for subject blur from birds anyway.  All the image stabilization in the world won't help you if your subject is moving.  I did get some shots below 1/125 that were acceptable to me, but 1/80 was as low as I wanted to go.

The above image is an example of a shot at 1/80 of a second, tripod mounted with the IS engaged. I used fill flash also to help fill in the shadows.  Just because you have IS, don't throw away the rules of getting sharp images.  If a tripod works for you then use it.  If you can't use a tripod then rely on the IS only.
Compared to my Tamron 70-300 SP VC 
This is a tough comparison because we are not quite talking about apples and apples here.  The 70-300 is 100mm longer which means you would have to shoot it at 200mm to be fair and then you'd have to shoot under the exact same conditions, but even considering those obvious scenarios and the fact I have a great deal of experience with the Tamron lens, I feel like I can do a pretty good job comparing the two lenses.
The Tamron cost about $450.00 new and is quite sharp indeed for a lens in this price category.  Is it is sharp as the 70-200?  No, but it can hold its own for sure. Here is an image taken at 300mm by the Tamron at a similar distance.

Pretty sharp, isn't it?  Understand though that sharpness isn't everything but it does take you quite a ways down the road.  Sharpness aside, the build quality of the Canon is much better because it utilizes a metal construction, has weather sealing and the auto-focus is much faster.  Where the Tamron loses out completely is not having a fixed aperture of f4.  At 300mm the aperture of the Tamron is maxed at  f5.6 and that makes it hard to use at low light events like this one.  In the end the Canon gave me a much better keeper rate than the Tamron and that is something to think about.  
So the big questions after shooting with the lens is:  do I want to buy it and does it replace my Tamron?  I think long term the answer is probably yes to both questions.  As time progresses, I will eventually add longer zooms which forces the 70-300 into a secondary wildlife role.  Currently it is my workhorse lens for everything wildlife.  When that times comes, then I will probably take another look at the Canon 70-200, because it is smaller, lighter, sharper and has better construction, but I am in no hurry to make any purchase at this time.  The fact is, the Tamron remains a really nice lens that anyone should be happy owning and meets most of my needs.  That said, if you want a new lens to replace a lens you are not happy with or can afford to spend $1,200.00 on a new lens then I have no problem recommending the Canon EF 70-200 L.  It will not let you down in any scenarios where a 200mm lens is needed. I hope you found this informal review helpful.  Below are more images for purchase and a breakdown of the parameters for this field test.
Parameters for the Field Test
  • Camera Body:  Canon 7D (classic)
  • Lens:  Canon EF 70-200L f4 IS
  • Aperture: f4
  • Shutter Speeds: 1/60 to 1/800
  • Flash:  Neewer TT560 Manual flash for fill
  • Tripod:  Bogen 3221 with a 3030 head
  • Flash support:  Stroboframe 
  • Time of day:  8:00am to 11:30am EST