After some time and consideration, I decided to buy a Sigma 120-400 f/4.5-5.6 OS zoom lens, to use with my crop camera (for now). I apologize if this mini-review is too long...it might help if I tell you I compare this lens with other Canon L lenses I have tried, and own. I did consider buying the 150-500 Sigma instead, but ultimately I wanted to try this one more.
I had originally considered renting a Sigma zoom. But the only one for rent, is the 50-500 (besides the big 120-300 f/2.8 OS...I'm waiting on the new "art" version with the dual fluorite elements, before renting that one)...and I decided I could do without 500mm...especially considering the reviews that said above 400mm, that lens wasn't very sharp. Funny how that kind of thing can show up in their other zooms...
This is the third Sigma lens I have bought. (I also owned a Sigma camera, and loved it, despite its quirks.) The first Sigma lens was in early 2009, a 17-70 (non-OS, first introduced possibly somewhere around 2004-06?). It was incredibly sharp with brilliant color and contrast. However, it refused to ever AF correctly no matter what. I finally sold it last year (for nearly what I paid for it new), but did hate to see it go anyway. One of my images done with it, got published in Outdoor Photographer last year.
My second Sigma lens purchase, was in Dec 2012; a 17-50 f/2.8 OS. This one has more barrel distortion than the 17-70 at the wide end, but overall, the color and contrast are similar, the bokeh is better, smoother, and obviously more pronounced/artistic...the sharpness is similar...the CA is low, and thus similar...and the AF accuracy is basically dead on throughout the zoom range with no micro adjustment necessary! The OS is a true 4 stops, and works perfectly. I think a semi-pro or even full-pro event shooter or portrait photog, could churn out spectacular work with it. If I decide to sell this lens in the future, I will hate to see it go. They do hold their value very well on the used market (another deciding factor at purchase). The reviews all seem to be correct...it's better than the Tamron 17-50 f/2.8. It also seems to make the much loved but over-hyped Canon 17-55 f/2.8 IS, seem to also be over-priced. If it is any better than this Sigma, it's not worth paying the $300 premium for a lens that still can't be used on full frame bodies. Ok, getting off my soapbox on that one, haha!
But back to the new lens...the third Sigma I have ever bought. I have to say, the build quality is very nearly Canon "L" quality. The fit and feel of the lens hood, the feel of the lens body, the feel of the focus and zoom rings...the feel of the tightness of the barrel as it telescopes outward...the printed letters...it all looks and feels like it might cost more than it does. And the lens ring/tripod mount/grip...well holy ****, that thing is very close to the equal of the very best Canon L supertelephoto's !!! I absolutely love it! Fits perfectly in the hand for carrying upside down or any angle, with the perfect distance from the lens body, and the perfect molded finger indentions. This lens was introduced in either 2007 or 2008, I believe. Sigma really are turning into a terrific lens and camera company, in my opinion (optical imperfections of the lens aside).
It arrived March 4. After spending only about 2 hours shooting with it so far (and even longer perusing and tweaking the files on computer)...It has less contrast than I thought it would. (I already knew it had less than the Canon zoom, thanks to the reviews and folks on this forum), but it's definitely still usable with post editing...and has a nice color balance. It has quite a bit more contrast below 200mm, than above, so that's at least a plus. It's softer than the Canon 100-400...yes...but not by a lot (however, it possibly is noticably softer than the Canon all the way at 400mm...but up to about 340mm it's still usable for very mild cropping). Sharpening in ACR seems to be mostly from 1.4 to 2.3 pixel radius. Far from ideal, but still usable for a medium amount of cropping (definitely not for a large amount of cropping...where sharpening from about 1.3 down to .5 pixel radius is necessary. The Canon 400 f/5.6L prime...was good for that. But it doesn't have stabilization, isn't a zoom, and costs more). And keep in mind, this is via a crop camera. I attribute some of the softness to a lack of focus accuracy/consistency (which again, I was warned about on this forum...I'm very glad to have you all!). Overall, it's not remotely as sharp as my excellent copy of the Canon 70-200 f/4L (non IS...I didn't think it would be)...but for a zoom of that Canon's size and price range ($600)...nothing in the world can come close to that one anyway, in my opinion. Certainly the 24-105L does, but that's a wide zoom.
One aspect I like a lot, is the close focus ability. The impression I got was that only the Canon zoom could focus closely...but this one does too.
The biggest problem, and biggest enemy of sharpness with this Sigma lens, is the "OS". It blurs the image a bit when in use. It's a shame, because the stabilization actually works quite well, and is closer to 4 stops (rather than the negative reviews that say it's just 2; although I'm not convinced how well it works in panning mode...I need to experiment more with that). I got something recognizable at 1/10 second at 400mm on my monopod...no motion blur, just the OS fuzziness. However, even with a really fast shutter speed, that same OS fuzziness sets in, especially compared to the superb IS of the Canon 300 f/4 IS. Even though the Canon 300 f/4's stabilization is claimed as 2 stops...those 2 stops do not interfere with ultimate sharpness much at all (it was so sharp I never even switched it off...and I used it on the monopod about half the time...the other half hand-held).
So again, besides the difficulty getting very good sharpness above 340mm on my crop camera...The autofocus is indeed hit or miss (the main concern that might make me send the lens back and not try another one), at least on more distant objects (say more than 50 feet away). It's nothing like as accurate as the Canon 400 f/5.6 prime. Although even that Canon didn't AF very accurately in low light on very distant objects via my camera...say over 400 feet away...where again, the Sigma's problem seems to start about 50 to 80 feet away. Not a huge deal for small bird photography...more an issue for large distant birding...however, keep in mind I've not really used this lens in "good light" yet...so that is a big factor with focus accuracy as well. But as of now, I do attribute the problem either to the lens's lack of fine control over its focusing lens array by its AF motors...or to a lack of control of my camera's AF system over the lens's electronics and motors...or both. Or...for all I know, the way those elements are suspended on their rails, may be a factor. At distances near infinity, where literally you would move the focus ring a fraction of a millimeter to affect subtle changes to the plane of focus...focus accuracy can usually get very tricky (in my experience and opinion). I admit my camera's less-than-ideal AF sensor and system, are certainly in play here.
My AFMA seems to have no effect on this Sigma, even at plus or minus 20...which is extremely odd and disturbing, but I'll keep experimenting with it. I'm not even sure adjusting it would help anyway. It achieves what it thinks is focus more than fast enough for me...it doesn't seem to move slowly (like, for instance, the Canon 85mm f/1.2L's autofocus!). I admit I haven't tried it in servo mode yet, so that could suddenly slow things down. Anyway, as for the single shot mode, I'm guessing (from my memory) the Sigma is not a lot slower than the Canon 400 f/5.6 prime...but it's quite a bit slower than the 300 f/4's autofocus. (Again, my camera's ability was in play here, besides the faster aperture and more light from the f/4).
I also plan to try the Sigma on my cousin's 5D3. If his AF nails it a lot more often than mine (very hard to believe it would not)...I may just keep the Sigma 120-400...Because I feel it will work much better on a full frame camera with larger pixels and better AF, than on my crop camera. And I do want to buy a full frame sometime in the next 6 months to a year. The extra $500 to $800 I saved by buying this lens over the Canon zooms (or the Canon 300 f/4) (the Canon 100-400 is long overdue for a version 2 update anyway)...will help fund my purchase of a full frame camera. I think I'll wind up keeping my good old 50D, though. Used prices have fallen to around $450...but 4 years and many many shots later, I still don't really want to part with it. The only reason I might, is if the 7D falls to $750 or so, after its replacement comes out. That may not happen...so even at $950 or so, I think I will pass.
I will probably post more comments here as I discover for myself, more about this much maligned, often over-looked Sigma telephoto zoom.