I bought this lens four years ago and have shot around 6,000 photos with it (4000 digital, 2000 35mm). Autofocus is very fast and quiet, which is typical of the Canon USM lenses. I would not recommend a Canon lens that does not have the ultrasonic motor (USM) focusing. Optics are sharp throughout the focusing range.
The lens is fairly heavy and after a few months of use, you will likely find that the weight of the glass is enough to make the lens telescope out when it's around your neck. If you want a compact lens that will let you take great pictures in a wide range of settings, this is probably the best lens you could get for a Canon camera. If you're a pro, you'll probably have a backpack full of lenses and you're not reading this anyway, so I'm not talking to you. If you want something you can sling over your shoulder and not think about when you take the kids to Disney World, get a cheaper, lighter lens.
The Image Stabilizer (IS) makes the lens a lot more expensive (some lenses are offered with and without it), so you should know what you're paying for. Some notes about the Image Stabilizer:
The image stabilizer itself is basically a spinning lens element (piece of glass) that acts as a gyroscope. When the lens moves slightly, such as camera shake caused by your pulse, the gyroscopic element stays put while the other elements move. Because the gyroscopic element is no longer in line with the other elements, it effectively bends the light just enough to compensate for the lens moving.
Having the IS feature does not mean that you can take crisp photos with a 1/20 sec exposure while jumping on a trampoline. What it means is that you can often get away with not carrying a tripod in normal lighting, and in low light when your photos would be very blurry (assuming you're not using really fast film), the IS will make the images significantly less blurry. An obvious corollary is that you can avoid using a flash in many situations when a flash is undesirable or prohibited.
The rule of thumb to get crisp photos without image stabilization is that your shutter speed should not be longer than 1 over your focal length. So if you are taking a picture zoomed in at 135mm, your shutter speed needs to be 1/135 sec or faster, and since no camera I know of has a 1/135 setting, that means going up to 1/160 sec (on cameras with stops in 1/3 increments) or faster. The image stabilizer means that you can go 2 f-stops slower than you normally could using the rule I just explained. So if you're shooting at 135mm and you have the IS switched on, you can shoot at 1/40 sec instead of 1/160 sec. That means four times as much light goes past the shutter, or that you can get the same quality results with 1/4 of the ambient light you would normally need.
There are some times when you SHOULD NOT use the IS feature. You should definitely not use it if you are in a car, on a roller coaster, if you are walking, or in any other situation where the camera is moving or vibrating a lot. You will get blurrier than normal images because the gyroscopic element is constantly moving all over the place, trying to prevent the image from moving. Only have IS switched on when you are using the camera in a normal, stationary, handheld manner. You should also not use IS when you are using a tripod, or when you have the camera resting on a vibration-less surface for an image. The reason is that the gyroscopic element will be spinning even though it's not needed, and while this isn't really bad, the motion could decrease photo quality (I've never noticed this, but this is what Canon claims), and it is unnecessarily using battery power.
Finally, not all of Canon's IS lenses use the same IS technology; many of the more expensive and newer lenses are better, but it was hard enough for me to come up with what this lens cost--it'll be awhile before I can rationalize three times as much for an upgrade.
One last note about third-party lenses, in case you're thinking about it. I know the price may be compelling but there genuinely does seem to be major quality differences, and while all Canon EOS lenses work will all Canon EOS cameras, no matter how many years apart they are in design, it has happened several times that even the best of the third party lenses (Sigma, some others) do not function properly with new Canon cameras. I happily buy knock-offs with other things, but not with lenses, flashes, or other camera components that actually communicate with my camera.
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