This filter is perfect for when you want shoot with a wide open aperture in bright daylight. Well built and easy to use.

Works surprisingly well. I use it for shooting video with the aperture wide open. It works great for a wide angle lens, but gets a little soft when you use a telephoto lens. I don't see any problems with vignetting.

The wide aperture on this lens allows for a novice photographer to transform their photography game! Highly recommended.

I love the adjustment handle on this filter. I use this on a very wide aperture lens sitting inside of a teleprompter. My previous filter didn’t turn very easily anymore and I needed to use two fingers, which meant taking some of the teleprompter apart to get in there. I also couldn’t do it while in front of the teleprompter to see if I have the right exposure. Enter this filter. I can just stick my finger in and adjust with the little handle, all while in front of the camera checking exposure. It might seem like a little thing, but it saves me a lot of time. What was a 15 minute back and forth is now a couple seconds.The optics are fantastic. No aberrations or other other optical issues. Very high quality build as well. I recommend this filter.

A wicked lens. Produces a beautiful bokeh affect with the aperture wide open. Very sturdy build and great autofocus. Little cheaper than what I would spend locally at the big box stores too!

The 17-40 gets mixed reviews for the most part mainly due to poor corner and edge resolution wide open. I use this on the 5D Mark III, a full frame sensor that will show these soft edges much more than a crop sensor such as any rebel, 7D, 60D, etc... Even then, I find the edge performance really quite good once you stop the lens down. Central sharpness is very, very good at 17mm and 40mm wide open. Stopped down to f/8 or f/11 will greatly increase sharpness in the edges and corners of the frame. But the lens is plenty sharp, no worries here. I use some of Canon's sharpest offerings such as the 24-70 f/2.8 II, 70-200 II, 135L, 400L, etc... This lens is just as good... Sure, it has some flaws, but the pros greatly outweigh those soft edges wide open.To me, this really is a full-frame lens where you can really utilize that great 17mm perspective. I think there are better options for those shooting with the 1.6x APS-C sensors. But that really is really up to the users and the needs they have. My girlfriend often borrows this lens to use with her T2i and loves it (I think the images look fantastic, as well!).One thing I love most about this lens is the contrast and color rendition. Simply beautiful images straight out of camera. The build quality is top notch as well, the zoom and focus rings are very smooth and well damped. The lens is weather sealed, but does require a filter (per-Canon) to complete the sealing.I give this lens 5/5 stars, simply due to its extremely affordable price (for an L lens) and excellent performance. Sure, there are sharper lenses out there, but that does not hinder this one at all. It still performs great and is a blast to use for dramatic perspectives. One of the most fun lenses to mount to the camera IMO. The light weight, great build quality and very good IQ make this a winner and a spot in my collection.

Great build and quality. It slowed me to have my aperture wide open during the bright day and maintain my needed shutter speed for video. This polarizer/nd filter have lasted the winds of Patagonia, sands of San Pedro de Atacama, and the city of Santiago.

The walkaround lens. This very topic leads to a heated discussion among DSLR photographers.First, determine your budget, focal length, and aperture needs.If you frequently find yourself zooming out to get everything in a frame, you will want a wide angle lens such as this. If you frequently find yourself zooming in, this is not the lens for you. On a full frame body such as Canon EOS 5D, this lens becomes ULTRA wide angle. On an APS-C crop body such as Digital Rebel XTi (which I used for this review), it becomes MEDIUM wide angle. But thanks to 1.6x crop factor, this lens expands to more usable 35mm equivalent focal length of 27 to 64mm.Second, audition the lens if you can.By definition, a walkaround lens should be relatively portable. At 1.1 lbs., Canon's EF 17-40mm f/4L USM is neither super light nor neck breakingly heavy. In fact, it weighs almost the same as Digital Rebel XTi -- really nice balance. The lens feels very solid with supreme build quality that only L-series lenses offer. Although this lens is weather proof and therefore sealed against liquid and dust, I strongly recommend getting a 77mm filter to protect the front lens element. With it, this lens is made to last.In terms of looks and feel, it doesn't get much better. Its rubberized full-time inner focus manual ring USM focuses smoothly, quietly, and quickly. Since it's inner focus, the lens will not extend beyond its metal casing whether you zoom in or out. The focus window shows focusing distance from 0.28 meter (0.92 feet) to infinity. The focal length marker indicates 17, 20, 24, 28, 35, and 40mm. The lens exudes quality from tip to tip.You may tolerate heavier lens or may not mind lesser build quality of cheaper lenses. A walkaround lens will be used very often, so make sure you will be comfortable with it.This lens is famous for saturated color and deep contrast. Its images are simply stunning. At 17mm wide angle, barrel distortion is noticeable but relatively mild. From 24mm to 40mm, its images are distortion free and perfectly suited at capturing people.Vignetting (corner darkness) is minimal with mild chroma abberrations (color shadows). At f/4 aperture, details become noticeably softer toward the edges. The center region is very sharp and at f/5.6, edges remain fairy sharp. Thanks to 7 diaphragm blades, this lens can produce very nice bokeh at 40mm (blur effects).One of the most cited weaknesses is the f/4 aperture. In my experience, a bump in the ISO speed and steady hands are all you need to take well focused images indoor. On the other hand, if you are shooting with very little amount of light, you might wish for f/2.8 or image stabilizer. Although the difference between f/4 and f/2.8 is just 1 stop, my other lens, Canon EF 24-70mm f/2.8L USM (too heavy to be my walkaround lens) easily outperforms in such challenging situations. But by and large, I was not handicapped by the f/4 aperture.Some of the main competitions (sorted by price):- Sigma AF 17-70mm f/2.8-4.5 DC macro: Good zoom range with macro, and generally solid performance if you can get a good sample. It does suffer from a bit slow focus mechanism, soft corner, and chroma aberrations. Works only with EF-S mount.- Sigma 18-50mm f/2.8 EX DC: Very good value for f/2.8 aperture, but Tamron's 17-50mm is a bit better lens overall. Works only with EF-S mount.- Canon EF-S 17-85mm f/4-5.6 IS USM: This "traveler's lens" has a wider focal range than most wide angle lenses (widest among Canon) and is equipped with an image stabilizer. While it is a Jack of many trades, it is the master of none. Every lenses on this list will perform better at particular focal length. Then again, none of the lenses on this list has as wide focal range. It is famous for extreme barrel distortion at 17mm and chroma aberrations. Works only with EF-S mount.- Tamron SP AF 17-50mm f/2.8 Di II LD Aspherical (IF): This is the most direct competitor. It takes sharper images with faster aperture while costing less. Both the build and focus mechanism are significantly worse, but should be good enough for many. Works only with EF-S mount.- Canon EF-S 17-55mm f/2.8 IS USM: This is THE reference, if you can afford it. Its images have razor sharp details and great performance all around (minus vignetting, which is typical of EF-S lenses). The build quality is worse than L-series but still pretty good. Works only with EF-S mount. This is the best EF-S lens hands down.- Canon EF 16-35mm f/2.8L USM: One of the most expensive wide angle zoom lenses. It's larger and heavier, but has f/2.8 aperture.This is how Canon EF 17-40mm f/4 USM stacks up.Pros:- Among the very best build quality.- Excellent, buttery smooth, super fast front-focus system.- Top notch color and contrast. Very sharp center resolution.- Almost non-existent vignetting, generally low distortion, and well controlled chroma abberrations.- Ideal weight and size for walkaround purposeCons:- Edge softness at f/4 aperture.- Narrower focal length than most competing lenses.- Slower than some third party lenses.All in all, this is an excellent wide angle walkaround lens. It may not offer the most bang for the buck, but if you value full frame compatibility (EF lens mount) and excellent build quality, this is the default choice. This lens comes with a nice pouch and a lens hood. I find the hood to be somewhat ridiculously shaped and because the lens is resistant to flare, I do not use it often when shooting outdoor.