Studio Tests (continued)
We wouldn't necessarily expect a relatively short, fast zoom to be great for close-ups, but the Sigma acquits itself pretty well. Its minimum focus distance in manual focus mode is an impressively close 26.5cm, resulting in a somewhat higher maximum magnification than you'll get from a typical 24-70mm F2.8 zoom.
Close-up image quality isn't bad either, at least when stopped down a bit. The image is soft wide open, but sharpens up pretty well in the centre at F2.8. The corners rather lag behind, but continue to improve on stopping down until our flat test chart shot looks sharp corner-to-corner at F11. There's minimal distortion, and only the slightest hint of blue/yellow chromatic aberration.
Third party lenses sometimes find themselves criticized for their colour balance and rendition compared to the camera manufacturers' own optics. We've looked into this by shooting an X-Rite Colorchecker Passport using the 18-35mm side-by-side with the Canon EF 24-70mm f/2.8L II USM. The images were shot under controlled fluorescent lighting, and a custom white balance set from the Passport's own grey card using the Canon lens. Images were shot in RAW on the EOS 700D, and converted in ACR using identical settings.
|Sigma 18-35mm F1.8 DC HSM||Canon EF 24-70mm f/2.8L II USM|
If you compare the colours in this rollover, we think you'll really struggle to see any significant differences between the two lenses at all. This doesn't come as much of a surprise to us, but it might to some sceptics.
One problem we might expect to see, given the lens's sheer physical length, is shadowing of the built-in flash at wideangle. This is indeed visible; on recent SLRs like the Canon EOS 700D, which lift the flash quite high above the lens, we saw some shadowing right at the bottom edge of the frame at focus distances. At the distances you'd most likely use the flash (~2 metres) it's pretty minor though, and effectively disappears if you zoom in a little bit, to just 20mm.
Full Frame Coverage
The Canon, Nikon and Sony mount versions of this lens will mount on full-frame DSLRs, and on Nikon cameras DX crop mode will be automatically selected (the camera will therefore shoot at reduced resolution). The rollover below shows the level of vignetting on full frame, with samples shot on a Canon EOS 6D through an Expodisc white balancing filter. The lens's image circle doesn't cover the 35mm full frame format fully at any focal length, giving severe vignetting at 18mm which decreases progressively on zooming in. There's still visible vignetting in the corners at 35mm, which doesn't entirely go away on stopping down, either.
|18mm F1.8||24mm F1.8||35mm F1.8|
|18mm F8||24mm F8||35mm F8|
Here's what the lens looks like shooting a real-world subject at 35mm on the EOS 6D. At F1.8 there's severe vignetting, pronounced pincushion distortion, and the corners are distinctly soft. At F8 things have cleaned up substantially, but there's still visible vignetting in the extreme corners, and of course the distortion remains. This is no surprise for an APS-C lens on full frame - it's simply not designed for the job. But you could use it at a pinch as a 35mm lens if necessary.
|Canon EOS 6D, 35mm F1.8||Canon EOS 6D, 35mm F8|
Background blur compared
One of the great attractions of fast lenses is the ability to isolate a subject by blurring the background. At first sight the 18-35mm may look like the ideal choice for such work - for any given angle of view it'll give noticeably more background blur at F1.8 than you'd get from an F2.8 zoom, and indeed match F2.8 on full frame. However, its relatively limited range means that a 17-50mm F2.8 can actually give better results if you compare both lenses shot wide open at their long end, purely in terms of the amount of background blur. This is illustrated in the rollover bellow.
|APS-C, 35mm F1.8||APS-C, 35mm F2.8||FF, 55mm F2.8||APS-C, 50mm F2.8|
Here you can see the extra blurring you can get from shooting at 35mm F1.8 compared to what you'd get from an F2.8 zoom at the same focal length. It's also clear that the F1.8 zoom offers essentially the same background blur as shooting at an equivalent focal length and F2.8 on full frame. However, if you shoot at 50mm F2.8 on APS-C, moving the camera back to keep the subject size the same, then the background ends up looking just as blurred and less cluttered due to the narrower angle of view. What this means is that a lens of the 17-50mm F2.8 type is still a better choice for shooting head-and-shoulders portraits.
Similar arguments apply for low light, high ISO shooting; the F1.8 zoom allows you to shoot handheld at lower ISOs for better image quality, and in principle should come close to matching a full frame camera with an F2.8 zoom in terms of overall image noise. But there are lots of complicating factors here, including the existence of image-stabilized F2.8 zooms that allow you to hand-hold at slow shutter speeds and use even lower ISOs, just as long as your subject isn't moving. (Sony and Pentax users, of course, benefit from image stabilization with all lenses.)
The Sigma 18-35mm F1.8 HSM uses a ring-type ultrasonic motor, which provides reasonably smooth autofocusing should you wish to change your focus point during movie recording. The focus motor is relatively quiet but it's not silent, so the camera's built-in microphone may well pick it up. Both the AF performance, and how audible the lens's operation ends up on your soundtrack, will be dependent upon the camera used.
The examples below show movie mode AF on the Canon EOS 100D, which allows refocusing on a new subject by touching the rear screen during recording. Its Hybrid AF system is also one of the better implementations of movie focusing on an SLR. Overall the Sigma does OK here, at least for an SLR lens that's primarily designed for stills photography. Focusing is pretty positive, but the motor can be heard in a quiet environment.
Sample 1 - Quiet indoor environment
Here we start with focus on the left side figure, then switch the right side and back again. In a quiet room indoors, the clicking of the focus motor is clearly audible on the soundtrack.
|Sigma 18-35mm f/1.8 DC HSM video autofocus - quiet indoor environment|
Sample 2 - Outdoor environment with nearby traffic
Here we start with focus on the foreground, then switch the background and back again. This is recorded in a park with nearby traffic, which in this case drowns out the motor noise.
|Sigma 18-35mm f/1.8 DC HSM video autofocus - outdoor environment with nearby traffic|