The Sigma 120-300mm f/2.8 EX DG HSM Lens is unique in it's capabilities - at this time, no other Canon-mount zoom lens can maintain an action-stopping f/2.8 aperture beyond 200mm. This lens has action-sports written all over it. Perhaps more than from any other Sigma lens, I was hoping for great things from this one.
Though useful for many other photography needs, the flexibility offered by a long focal length f/2.8 zoom lens for shooting sports is very attractive. The f/2.8 aperture is wide enough to shoot in low light - even indoors if the lighting is decent and ISO values are set high. The 120-300mm focal range covers most sports needs for 1.6x and 1.3x FOVCF body users (full-frame body users will probably want more focal length for the larger field sports). This additional-needed focal length can be had by adding a compatible Sigma 1.4x Teleconverter or a Sigma 2x Teleconverter - but of course we lose the f/2.8 aperture with the combos. With the 120-300mm zoom, track a player in horizontal or vertical orientation (as with a prime lens) AND zoom in/out as desired to get the proper framing without a resolution/image quality reducing cropping step during post-processing - A big advantage for this Sigma.
Unfortunately, the Sigma 120-300mm f/2.8 EX DG HSM Lens has a significant shortcoming that affects action sports photography - Autofocus can't keep up with action rapidly moving toward or away from the camera. People running to be more specific. I missed a significant number of shots when tracking action in AI Servo mode (on a Canon 1-Series body) - all focused behind the direction of the action. AF is powered by HSM - Sigma's premier Hypersonic Motor. This is generally a good thing and HSM in this implementation is very quiet, includes FTM (Full Time Manual) focusing, is rear-focusing, does not extend and the front element does not rotate. Focus speed even sounds deceptively fast because of its quietness.
The other big AF problem I had was that this lens severely front-focused at the wide end of the focal length range on two different Canon-calibrated bodies. The 120-300 and one of my bodies required a trip to Sigma service to be taught to play together nicely. Sigma is always willing to service their gear under warranty - I was much less pleased to receive my camera back with a VERY dirty sensor.
Focus accuracy for slower-moving and stationary subjects is decent.
One more focus anomaly - Canon's EOS Utility cannot make micro focus adjustments when attached to a Canon EOS 1D Mark III in Live View mode. I have yet to try a large variety of lenses in this scenario, but the computer-driven micro focus adjustment is very useful when shooting tethered.
This lens does not have a focus limiter switch, but it hasn't proven necessary to me.
The Sigma 120-300mm f/2.8 EX DG HSM Lens is large (4.4" x 10.6" / 112.8mm x 268.5mm) and heavy (5.7 lb / 2.6 kg), but it is handholdable for short periods of time. For longer periods of shooting such as for the duration of a sports event, you are going to want a support - a monopod is the choice of most sports shooters.
Like most Sigma EX lenses, the Sigma 120-300 is very solidly built (thus the heavy weight). Zoom and focus rings are firm but smooth and nicely sized. The matte EX finish is not my favorite, but others like it a lot. It is stealthy, but shows marks easily (even from skin) - and I find it slippery.
Image quality from the Sigma 120-300mm f/2.8 EX DG HSM Lens is decent. It has decent sharpness wide open and gets really nice stopped down a stop or so. Based on comments from others, I expected sharper results wide open than I am getting - especially on the ISO 12233 resolution chart results. I shot this test on three different occasions (a very time-consuming process) - each including many samples shot using AF, MF with a Canon Angle Finder C, bracketed MF and one last time using laptop-tethered Live View from a Canon EOS 1D Mark III to get another precise manual focus sample. The best of the results were similar each time. As always, I shot many other comparison tests - and the ISO 12233 chart brings out the worst in this lens - as it always does. I will also note that the left side of this lens produces a slightly sharper image than the right (bottom-most ISO 12233 crop), but the difference was not big.
Color tends to the yellow side of neutral. Vignetting ranges from very minor on the wide end to not too bad at the long end - definitely more noticeable at 300mm than 120mm. Distortion is minor - a little pincushion at the longer end of the focal length range. Flare control is decent - stronger at the long end than the wide end with the same bright light source at the corner of the frame. CA (Chromatic Aberration) is well-controlled with a small amount showing at the wide end of the focal length range in strong contrasting areas. Exposures are averaging a bit bright - often about 1/3 stop brighter than compared-to Canon lenses. The aperture is a 9-blade design though the bokeh tends to be slightly harsh.
Shown above from left to right with included tripods rings and lens hoods in place are the Canon EF 70-200mm f/2.8 L IS USM Lens, Sigma 120-300mm f/2.8 EX DG HSM Lens and Canon EF 300mm f/2.8 L IS USM Lens.
The Sigma 120-300's MFD (Minimum Focus Distance) is specified as a range - from 4.9'-8.2' (1.5-2.5m) as zoomed from 120mm to 300mm. At it's MFD, the 120-300 can delivered a MM (Maximum Magnification) of 1:8.6 or .116x - a relatively low number, but not an unusual one at 300mm f/2.8. As I mentioned before, the 120-300 is compatible with the Sigma 1.4x and 2x Teleconverters. Since MFD does not change with teleconverters in place, MM is directly increased by 1.4x or 2x when using these.
The Sigma 120-300 comes with a neck strap that attaches to the also-included removable tripod ring (which makes a nice handle). Also included is a rather small, metal lens hood that attaches via a bayonet mount and locks with a thumbscrew. Normal screw-in filters can be used on this lens, but the 105mm-sized filters are going to be very expensive if of good quality. The included lens cap/cover (shown above) is an unusual but nice design - It is a flexible, padded-nylon, slip-over cap with a less-flexible, protective end. A slot in the cover provides the hood thumbscrew space to pass through and a hook-and-loop closure keeps the cover in place.
The Sigma 120-300 ships in a nicely padded case - it would be nice if the camera fit in the same case (though no other factory Canon-mount lens cases accommodate camera bodies). For transport and use, I used a Lightware z300. The fit with a pro body attached is snug, but it works well.
The Sigma 120-300mm f/2.8 EX DG HSM Lens is available in Canon (reviewed), Nikon (D) and Sigma mounts. My obligatory standard disclaimer: You should know that there are potential issues with third party lenses. Since Sigma reverse engineers (vs. licenses) manufacturer AF routines, there is always the possibility that a new body might not support an older third party lens. There are examples of this happening in the past. Sometimes a lens can be rechipped to be made compatible, sometimes not. Second, there is the risk of a problem that results in the lens and body manufacturers pointing blame at each other. However, Sigma USA's 4-year warranty is far superior to Canon's standard 1 year warranty (though many credit cards will double the Canon warranty for you).
Alternatives to the Sigma 120-300 include the Canon EF 70-200mm f/2.8 L IS USM Lens and Canon EF 300mm f/2.8 L IS USM Lens. The Sigma is close to the Canon 70-200 L IS in sharpness at overlapping focal lengths, but is not as sharp at 300mm as the very impressive Canon 300mm f/2.8 L IS. Of course, you need both Canons to cover the same focal length range as the Sigma - and this makes lens changing an issue unless you have two bodies (which requires camera swapping during the action). The Sigma 120-300 costs significantly more than the smaller/lighter Canon 70-200 f/2.8 L IS, but far less than the larger Canon 300 f/2.8 L IS. The Sigma is not quite as long as Canon's 300 f/2.8 at 300mm and not quite as wide as a Canon 70-200 f/2.8 IS reporting 120mm - but the differences are not major. The Canon lenses mentioned include IS - not important for sports photography but very helpful for other handheld scenarios. The Canon lenses deliver a more-neutral color. And of course, the non-IS Canon EF 70-200mm f/2.8 L USM Lens is a very close alternative that costs far less than the IS version.
Sports photography is the primary market for the Sigma 120-300mm f/2.8 EX DG HSM Lens, but there are other good uses for it. On a full frame body, compressed portraits are one. On a 1.6x body or with extenders, wildlife is another. Basically, any requirement met by the 120-300mm focal length range with a fast aperture and not-extremely fast AF where size and weight are not an issue can be met. Though not a cheap lens, it is a decent value.