The short version:
Pros:
The optical quality is great, the speed is terrific, and it compares well to the 70-200mm lens that people like to say blows this out of the water (I believe they are wrong - but we will get into that later) and finally, the price cannot be beaten. Buy it.
Cons:
No hood, extends as it zooms, and the quality of the picture in low light situation lessens a little.
The long version:
I am writing this in simple terms. I found several, several reviews on this lens but they were all in technical terms and leaves you scratching your head a little. So, if you are like me maybe this review will help you.
I bought this lens a couple months ago from US1Photo.com (check these guys out. They regularly have significant sales and terrific customer service). I use this lens with a Canon 40D.
I take several types and styles of pictures so I needed a lens that would do the best job at several things and have a very affordable price (right at or less than $1,000). I looked through new, used, third party (Tamron, Sigma, etc) lenses, and read too many reviews and looked at too many images to count, and spent four hours in a photography store playing with lenses before I decided to spend money and take advantage of this lens. All in all I spent about a week's worth of time in research and testing before I bought this lens.
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Edit:
Okay, so amazon won't let me put a link here. This is how you find me.
~Go to Flickr
~Then type in a "/" then "photos" then "/" and last type in "gman_five0"
And that should take you there.
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Test of comment #1:
~The Canon EF 70-300mm f/4-5.6 IS USM does not track moving objects very well and the farther to the end of the zoom the slower the tracking.
(Flickr Gallery page 2)
This, I have found, to be completely wrong. I have used the lens at several sporting events and found that it tracks rather nicely. I was able to track every step of a base-runner from first base to second, slide, and recovery after the play without losing ONE shot.
To see what I am talking about check out my gallery on Flickr. The older ones are NOT done with this lens or camera and taken, actually, several years ago. I will reference the pictures in question.
Safe, Got There By a Mile, Breaking Up That Double Play (please note that as time goes on from the date that this was posted the pictures may have been re-moved). Also, "Safe" was used instead of another shot taken at the same time and angle with a 70-200mm IS lens.
To take these shots I used "AI focus" and the "H" settings on my camera for quicker tracking and the H settings for about 6 frames per second.
Test of Comment #2:
~The 200-300mm range is nice in theory, but a tack-sharp photo from the 70-200mm f/4L at 200mm is going to look better cropped than a 300mm full-frame photo from this lens.
Again, I found this to be wrong. Though I have no actual "full sized" picture for this if you comment back with an email address then we can arrange a viewing of one. Otherwise, take my word for it, I own several lenses and this one stacks up well.
(Ref Flickr Gallery)
The pictures from the Dance Theater and Tashina were all taken at the 200-300mm focus lengths. Again, they are not full sized because of photo pirates, but if you email me we can see about full sized shots.
Test of Comment #3:
~The IS motor is loud
I do not know what these people are talking about, but if you call that loud...
On the other hand, I have heard (once), the sound of the IS motor, but if you are not listening for it you will not even notice it.
Things I have found about the lens:
If you spend some time with the lens will be one of your best friends. It is a great lens especially for people on a budget (like me). To take the best pictures you cannot just twist it on and go to shooting. Take the time to LEARN about the lens (change shutter speeds, ISO settings, aperture, white balances, etc) and it will show you where it shines - just like the L series lenses and the 70-200mm IS.
This lens has a solid make and feels like it will last forever. Then again, as you zoom the lens does extend and is plastic. So, watch out if you are doing sports. You may get it knocked off. It did well for me, though. Extending while zooming makes putting a hood on the lens a bit (very little; so little that you cannot tell) awkward and it looks kinda goofy.
I like the lens because it is not as bulky as the 70-200mm lens and it is extremely mobile. It is as quick as some other lenses, not as quick as some, and quicker than others. It will give you great quality pictures. It does have its limits, however: in low light situations not being able to take it back a couple stops and get a wider aperture will not give you the same shot as a 70-200mm. Then again, like I said, it doesn't drop to that f/2.8 and does not cost all that money. This has been the single drawback for me about the lens.
I use this lens for portraits (Tashina, Samantha), for sports (see gallery), and music (Shawn Pander - See Gallery). So, it's pretty versatile. I have yet to use with it flash, but that is because I simply do not like to use flash. I have yet to use it in a studio setting, but when I do I will amend this review and add a couple of those pictures as well.
This lens comes highly recommended from me. I am not a big time, highly paid, or well known photographer. I just like to shoot and like what I shoot to be of the best quality that I can have AND afford at the time. So, if you are like me and cannot spend the needed $1,500 - $1,700 on a 70-200mm IS lens then spend your money on this lens. You will not be sorry for it.
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On another note, comments are welcome. This is my first ever review on here so let me know if there anything else that you would like to know about and I will do my best to answer the question in the most plain terms as possible.