You cannot buy a full frame camera without also considering a 24-70mm f/2.8 zoom; the two go hand-in-hand. This might not be true of everybody's shooting style, but it certainly is true of the market dynamics given how popular fast normal zooms are. This especially true of the 5DmIII and the EF 24-70mm f/2.8L II USM; the 24-105L f/2.8 is serviceable as the kit lens, but the camera doesn't really sing until the to-dog lens is mounted.
Body and Design
This is the second version of this lens; Canon has addressed some of the weak points of the first version, and the end result is an extremely competent unit. It's a hefty lens; it weighs in at 805g and 82mm long. However, it feels at home on the 5D Mark III, whereas the equivalent Nikon does not on the D800. That's because the Nikon is both longer and heavier; and gives a greater sense of "polar moment of inertia" than the Canon when it's handled. In the continuing debate of Canon vs. Nikon, the Canon combination handles more like a general-purpose multi-use machine, where as the Nikon combination is a bit unwieldy to use for anything other than serious/professional use.
The 24-70mm takes 82mm filters, which annoyingly, is a different size from the 77mm that the companion EF 70-200mm f/2.8 USM L IS II lens uses. Be aware that at these front-element diameters, filters can be quite expensive.
The following is an ad hoc short-range bokeh test, with the subject (the terracotta warrior figurine) positioned two feet in front of the camera and the lens set to 50mm(out of camera JPEG). Background bokeh is smooth and consistent. Foreground bokeh (not pictured) can be on the nervous side, but this is true of many fast-aperture lenses. Vignetting is visible at f/2.8, but becomes unobjectionable after f/4. Lateral and longitudinal chromatic aberration is minial. The rendition of the figurine, the object being focused on, is extremely crisp.
"Sharpness" is a commonly used term to inaccurately describe how detail is rendered after it has passed through the lens and camera. There are a myriad number of factors that affect this... global contrast, micro contrast, field curvature, optical aberrations, etc. However, for the purposes of reviewing the Canon 24-70mm f/2.8, we can pretty much skip all of that and pretty much say that this is a sharp lens... period.
In the picture of the riverbank below, you can see what this lens and camera combination can do with a small aperture and ideal lighting. The detail of the plant matter is rendered crispy from edge to edge; most people will experience this level of sharpness in the center of the frame when using a lesser lens, but the 24-70mm is able to maintain this across the whole frame. Files from the 5DmIII and this lens printed on a Canon Pro-100 printers look absolutely gorgeous at 13" by 19", all of the nuance of scene like this gets recorded faithfully.
With extended use, you becomes less aware of the resolving power of this lens... which is actually as it should be. With pro-level lens, you should not be mindful of the equipment as you are using it; rather, the equipment should function transparently so that you can concentrate on capturing an image. There's very little to think about with the 24-70mm f/2.8; there are no obvious lens weaknesses to shoot around and the outright quality of the optics means that you are recording things as you perceive them.
|f/8, 70mm and cropped in during post processing|
However, most shooting situations don't have this amount of subtle detail, and what most people refer to as sharpness has more to do with the overall global contrast qualities of the image, and how precisely hard edges are defined. The harbour scene below will pretty much flatter any DSLR/lens combination, but the EF 24-70mm f/2.8L II USM is still a superior lens in that it gives that final extra bit of acuity "bite" over other lenses. Put this way: If you had a choice of the EOS T5i and this lens, or a 5DmIII and the EF-S 18-55mm kit lens, the first combination would win hand down.
The virtues of this lens extend to nighttime shooting as well. Stopped-down, the lens hands point light sources defty. The lens is quite resistant to flaring, which is another one of those things that you don't worry too much of when shooting. When shot on a tripod, the crispness of the images seems almost other-worldly if you've only used consumer-grade equipment.
This is an extremely expensive lens, and it is probably the best full frame normal-zoom on the market... for any lens mount., Canon, Nikon or Sony. If you are contemplating purchasing this lens but haven't, the reasons holding you back likely don't have to do with any of its technical attributes. Is this lens worth it? As always, that is a personal decision. If you are thinking about buying this for a EF-S camera like the 70D and saving it for the jump to full-frame, you have to ask yourself: why? The cost of this lens is more than two EF-S bodies combined; put simply, there is a huge opportunity cost to buying this lens for APS-C considering the money and utility that you would give up by not using a more appropriately-matched choice. The only way that this lens makes sense for an EF-S user is if the switch to a full frame body is known to be imminent.
There is one thing missing from the equation, and that is image stabilization. The Tamron AF SP 24-70mm f/2.8 Di USD VC has this, and is a far less expensive lens. Though the combination of constant f/2.8 and optical stabilization makes for an excellent lens, the simple fact of the matter is that the Canon is optically better. The problem with image stabilization is that it imposes some compromises on ultimate image quality; since that is something that Canon (and also Nikon) trade on when it comes to professional lenses, it is not a feature that has thus far made it into the 24-70mm f/2.8. For most people, the Tamron is a more cost effective choice, but if what ultimately matters is the final print, then the Canon is the winner.
Despite all of the superlatives, it's still not a perfect lens, but it's more than perfect enough for the majority of photographers, professional and non-professional alike.
With thanks to Broadway Camera