Studio Tests - APS-C format
The 70-200mm F2.8 OS puts in a decent performance on APS-C format. As usual it benefits from the expected advantages of a full-frame lens on the smaller sensor, i.e. minimal distortion and vignetting, and chromatic aberration is also extremely low. Sharpness results are impressive at the short end, but somewhat less so at 200mm.
Sharpness results on APS-C are mixed, and slightly complex in character. At 70mm F2.8 the lens is decently sharp in the center, but slightly softer towards the corners. It improves dramatically on stopping down to F4; there's also evidence for a slight focus shift on stopping down, with the measured sharpness slightly lower in the center than towards the edge of the frame; best results are obtained at F4 - F8. Wide open at longer focal lengths, the soft region spreads further into the frame, until at 200mm only the very center is critically sharp, and most of the frame is somewhat soft. Again though the lens improves substantially on stopping down, with best results from F5.6 - F8.
Chromatic aberration is exceptionally low. If you look really closely there's a tiny bit of fringing at the corners, but it probably won't be much of a problem in normal use.
We consider falloff to become perceptible when the corner illumination falls to more than 1 stop less than the center. As we'd expect for a full-frame lens on APS-C there's simply nothing to be concerned about here at all.
Distortion is extremely low, from 0.5% barrel at 70mm, through neutral around 85-100mm, to -0.6% pincushion at 200mm. This is absolutely nothing to worry about, as it will be essentially imperceptible in normal use.
Third party lenses have something of a reputation for less-than-neutral color balance, so here we measure any color cast introduced by the lens in comparison to the camera manufacturer's 50mm lens (generally considered a good standard for neutrality).
In this test, the camera is pointed towards an evenly illuminated white wall, and light entering the lens then completely diffused using an 'Expodisc' white balance filter. A custom white balance is taken using the 50mm lens (in this case the Canon 50mm F1.4 USM), then exposures made using the 50mm and the lens under test (in this specific comparison we've also included the Canon EF 70-200mm F2.8 L IS II USM). The RGB values from the center of the frame are reported (measured as an 11x11 average).
|Canon EF 50mm F1.4 USM
(188, 188, 188)
|Sigma 70-200mm F2.8 OS
(175, 177, 176)
|Canon 70-200mm F2.8 IS II
(182, 182, 183)
The differences between these lenses are very small indeed; both 70-200mms are very close to being as neutral as the 50mm F1.4. The Sigma shows a tiny green shift, whereas the Canon 70-200mm F2.8 IS II has the slightest of cool casts. Neither are likely to count as remotely significant in real world use.
Specific image quality issues
As always, our studio tests are backed up by taking hundreds of photographs with the lens across a range of subjects, and examining them in detail. This allows us to confirm our studio observations, and identify any other issues which don't show up in the tests. In this section we look at issues of specific interest to APS-C users.
Softness wide open at 200mm
The most problematic result to emerge from our studio testing is apparent softness at 200mm, especially when the lens is shot wide open at F2.8. The example below shows what this looks like in practice for a 'brick wall' shot, with 100% crops from the center, lower edge and corner of the frame (using the Canon EOS 7D as the test camera).
At F2.8 the center shows lots of fine detail, but is a little low in contrast; however the edge and corner crops are somewhat soft. The lower edge sharpens up nicely in stopping down to F5.6, with the corner following at F8 (although at this point the center is losing a little of its bite due to the onset of diffraction).
As usual, we've also gone through hundreds of sample shots to see what this looks like in practice with arguably more typical subjects for this kind of lens. With the more usual three-dimensional subjects, such sharpness issues often become less important; the in-focus regions may well be 'sharp enough', especially when shooting at higher ISOs when detail is being lost to noise and noise reduction. In addition, there's not necessarily anything important in focus in the softest regions of the frame anyway.
|200mm F2.8, Canon EOS 7D (ISO 400)||100% crop (rightmost rider)|
|200mm F2.8, Canon EOS 7D (ISO 200)||100% crop|
Naturally you can always pull a bit more out of the image file by careful processing of the raw. Here we've corrected the residual lateral chromatic aberration in Adobe Camera Raw (+10 Red / Cyan), followed by local contrast enhancement in Photoshop (Unsharp Mask with Amount = 5, Radius = 50, Threshold = 0) and finally Smart Sharpen (Amount = 180, radius = 0.8). This brings a considerable improvement in the rendition of fine detail.
|200mm F2.8, Canon EOS 7D (raw + ACR)||100% crop|