The Sigma 24-70mm f/2.8 EX DG Lens is a well-built, well-spec'd lens with a decent price. It does not outperform (physically or optically) Canon's similarly spec'd EF 24-70mm f/2.8 L USM Lens, but it costs less than half as much.
The Sigma 24-70 is solidly built but pays some penalty in the form of size and weight. Measuring 3.5" x 4.5" (88.7 x 115.5mm) and weighing 25.2 oz (715g), the Sigma is still smaller and lighter than Canon's 24-70 L which measures 3.3" x 4.9" (83.2 x 123.5 mm) and weighs 32 oz (950g). Extended, the length difference between the two lenses increases slightly. The Sigma is at its shortest around 60mm and longest at 24mm focused closely as it extends a small amount with focusing. The Sigma's diameter abruptly changes at various points, creating hard-edge transitions that in my opinion detract from the comfort of holding this lens compared to the consistent diameter of Canon's 24-70. However, it is not hard to find the correct ring to turn.
An 82mm filter diameter makes the Sigma 24-70 very wide at the objective end, makes filters very expensive (and able to be shared with few other lenses) and though not unusual, blocks the on-board flash on the wider end of the focal length range.
The Sigma 24-70mm f/2.8 EX DG Lens' AF is very loud but reasonably fast (although the noise makes it sound slower). The objective lens extends slightly when focusing but does not rotate. Part time manual focus employs what Sigma calls their "Dual Focus System". After switching the lens to M/AF using the conventional switch, pushing/pulling on the nicely-sized and smooth turning focus ring is required to engage/disengage the manual focus gearing. The advantage of the DFS over other non-FTM (Full Time Manual) lenses is that the focus ring does not turn during autofocusing. The disadvantage is that MF requires two settings to be changed.
The Sigma 24-70 is shown above in with the focus ring in AF mode (left) and in M mode.
Although adequately sized, the Sigma 24-70 zoom ring is very stiff and requires even more torque to be applied to get the lens to fully extend at the 24mm end of the focal length range. When enough torque is applied, the zoom ring turns smoothly. Reportedly, the zoom ring loosens if given enough usage. Sigma's zoom ring turns the opposite direction from Canon's zoom lenses.
When listing most-liked Sigma 24-70mm f/2.8 EX DG Lens features, people occasionally mention the Sigma EX finish. I differ from this opinion - I don't care for the feel (or look) of the matte Sigma coating. It is hard to clean and cheap feeling. This is obviously a personal preference and does not affect the performance of the lens.
The Sigma 24-70 is sharpest at the wide end of its focal length range. At 24/28mm, the 24-70 is very sharp in the center at f/3.2. However, performance gets progressively worse as the focal length is increased. To me, 70mm f/2.8 is not usable on this lens. At 70mm f/4, center performance becomes useable. Corners (1.6x FOVCF corners included) need an extra stop at all focal lengths to become very good - generally becoming very nice at f/5.6.
The Sigma 24-70 produces severe flare at the longer end of the focal length range if the sun (or a bright light) is close to the image circle. This flare can completely destroy the contrast of the image. An inadequately-sized-for-70mm lens hood is the biggest problem as I can reduce the flare by shading the lens. This is the reason that the Canon EF 24-70mm f/2.8 L USM Lens hood is so large (and does not extend with the lens).
The above sample picture shows an example of severe flare from the Sigma 24-70mm f/2.8 EX DG Lens. This picture was taken at 56mm f/4.5. The sun is well out of the frame, the lens hood is in place. The upside down rainbow is pretty, but this is not the effect I am typically looking for.
Overall, color is decent if flare is not present. Contrast is somewhat lacking. A tendency to overexpose by .2 f-stops does not help, but this can easily be corrected.
I have nothing unusual to report regarding distortion or CA. The Sigma 24-70 has barrel distortion from 24mm to 30mm or so and pincushion from 45mm or so to 70mm. Some CA is present especially in the corners at the wide end of the focal length range. Vignetting is well controlled.
With a fixed 9-blade aperture ranging from f/2.8 to f/32, the Sigma 24-70 is as fast as any Canon zoom lenses made.
With a minimum focusing distance of 15.7" (40 cm), the Sigma 24-70 has a maximum magnification of .26x. This is enough magnification for Sigma to give this lens a "macro" designation. I prefer the macro definition to start at 1.0x (1:1), but this is a good spec for this lens. It will focus close enough for nice pictures of flowers and other similar-sized objects.
The included small, plastic, inside-ribbed lens hood extends and retracts with the lens (like most lenses with this focal length range). Unfortunately, I have to remove the hood to be able to release to the lens cap as there is not enough room for me to pinch the cap release when the hood is installed. This is a nuisance if you are installing/removing the cap with any frequency. A nice padded nylon lens case is also included.
Although they are both rated 24-70mm lenses, the Canon 24-70 L is noticeably wider at 24mm than the Sigma when the subjects are not distant. If you do not need wide open high performance beyond 30mm or so and do not have bright lights (including the sun) anywhere except behind you, the Sigma 24-70mm f/2.8 EX DG Lens can be good general purpose lens for you. There are lots of uses for the general purpose 24-70mm focal length range. The Sigma can handle landscape photography to portraits to birthday parties to ...
The Sigma 24-70 is available in Canon (reviewed), Pentax, Minolta, Nikon and Sigma mounts. You should know that there are some 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.
If the prices were the same, I don't think anyone would choose the Sigma 24-70mm f/2.8 EX DG Lens over the Canon EF 24-70mm f/2.8 L USM Lens. But, the Sigma is less than half the price of the Canon. And for that reason, many people are willing to overlook the physical and optical quality differences between these two lenses.