Thursday, October 31, 2013

Sigma 18-35mm F1.8 DC HSM (Canon Mount) Review



The Sigma 18-35mm F1.8 DC HSM has been a hard lens to find since it became available a month ago on the Canon mount. The Nikon version, as I write, has still to hit store shelves. There was a lot of interest created by the constant f/1.8 aperture, but beyond the headline spec, does this lens deliver?

Build and Construction



This is a heavy lens. No, make that a dense lens. In everyday use, you have to handle this lens with deliberation; always left hand under the lens when you are griping the camera. At 810g, it weighs more than a Nikon D7100 or a Canon 70D, and mounted on either you will end up with a front-heavy rig. Physically, the lens is long for crop frame normal zoom lens at 120mm in length, which is 10mm shy of the full frame Nikon AF-S Nikkor 24-70mm f/2.8G E... remember, that lens has a maximum focal length that is twice as long. However, Sigma kept the front filter element at a reasonable 72mm, which keeps the profile of the lens cylindrical rather than conical. Even though it's heavier than the Nikon 17-55mm f/2.8G ED-IF AF-S DX, the handling isn't quite as bulky. 

The texture of the outside barrel is just as premium as it looks like in Sigma's promotional literature. The dull gloss finish makes for the obvious poster child in Sigma's Art lineup of lines, and is aesthetically one of the most pleasing finishes on the market. The rubber control dial surface are broad and grippy; they're quite heavily damped. This tactile feel of the zoom and focus rings is in keeping with premium aesthetic of the lens, but it does reduce the speed of operation.

Focal Length


Much has been made of the fact that this is finally the crop sensor lens that gives the equivalent field of view of full frame on a crop frame camera. That may be true, but a portion of that equation is missing. The 18-35mm range is not so much a normal-zoom range as it is a wide-to-normal range. So the continuum of usage goes like this: landscapes -> street photography -> general people snaps. What's missing is the "head shot" part of the normal-range. In other words, to achieve the fast aperture, Sigma needed to pair down the lens from the typical 50mm down to 35mm.

This has some implications... and in other ways it doesn't. The first problem is that depth of field increases as you go to shorter focal lengths. Yes, you get f/1.8, but at the wider focal lengths that this lens dwells at, you won't get ultra creamy backgrounds unless you shoot at closer distances. A hidden downside to this short focal length is that you will get pin-cushion distortion at a focal length that you wouldn't ordinarily expect it. When this lens is at its long end, other normal-zooms would be in the middle of their range. The distortion isn't heavy and it is correctable, but it is uncommon to see this type of distortion at this focal length. At the commonly used crop focal length of 24mm, the lens is essentially distortion free.

The lack of 50mm is a downside on paper, but probably not a practical downside if you are conscientious shooter who will work for your image quality. With a constant f/2.8 18-50mm normal zoom, shooting wide open at the long end makes for a serviceable portrait shooting experience, but it's nowhere near as ambitious as shooting with a dedicated lens. Since old 50mm lenses are so cheap, many serious enthusiasts have them; shooting a portrait at 50mm and f/1.8 on a prime is "better" than 50mm of a f/2.8 zoom. 

Here's the lens at 18mm on a Canon EOS 70D:




Just as a caveat, DPReview is reporting that they had trouble getting consistent focus with this lens, and I would have to say that this my experience as well. I want to nuance that by saying that I did not have enough time with the lens to get a full understanding of its behaviour, something that only happens with extended usage. However, the above shot was with the center focus point aimed directly at the tag for the 35mm f/1.4L, and somehow the focus landed a few centimetres behind this point. Bear in mind that this is at wide angle and wide open, so focus consistency at short distances is never a given when you shoot like this. Here's the same zoomed in at 35mm:


Same problem, focus is landing behind the intended target. In fairness, I was shooting this with tester units of both the 70D and the 18-35mm, so abuse may or may not be a factor. Looking just at the focal lengths, it's not a huge zoom range by any means, but it does cover the focal lengths that are most often used by the majority of shooters. Thankfully, the minimum focus distance is just under 28cm ( the same as the venerable Tamron SP AF17-50mm f/2.8 XR Di II LD Aspherical [IF]), which allows for some versatility in shooting close-ups.

Bokeh and Image Quality


The bokeh is lovely on this lens. Unless specifically addressed, fast modern lenses with aspherical elements can have harsh transitions between in-focus and out of focus areas. It's a good combination of overall lens sharpness and the shallow depth of field afforded by the wider aperture. Here's an example at 24mm and f/1.8 and 35mm at the same. Note that the rounded blades make for soft circular out of focus highlights. There's a bit of fringing going on, but it isn't very harsh.

 
Canon 70D, f/1.8, 1/125s, ISO 250, 24mm

... and zoomed further out at 35mm. Note that it's not a directly comparable image (different object magnification). Apologies also for the focus (something always goes awry when I try shooting test shots).

Canon 70D, f/1.8, 1/125s, ISO 250, 35mm
However, this does illustrate the problem with f/1.8, even shooting at a contrasty target not more than three feet away, it's still a struggle to precisely land your focus point. In terms of optical performance, you would expect lateral and longitudinal chromatic aberration to be an issue, but in all of the samples that I've seen, colour fringing from RAW processed samples is well controlled. However, this point is moot here, as these are out of camera JPEG's, and the 70D's image processing is correcting for lens aberration as well as vignetting. Below are examples of how the bokeh varies between f/1.8 and f/5.6 at 24mm:


Canon 70D, f/1.8, 1/250s, ISO 400, 23mm
Canon 70D, f/2.8, 1/250s, ISO 400, 23mm
Canon 70D, f/4.0, 1/250s, ISO 400, 23mm
Canon 70D, f/5.6, 1/250s, ISO 400, 23mm

The key difference to watch for is between the f/1.8 shot and the f/2.8 shot; the first is unique to this lens and the other is common across many standard zooms. The background blur is more, but even the f/2.8 is no slouch. Note also that the focus failed to lock on the Canon logo in the f/5.6 shot.

Focusing


Sigma's unique selling proposition their newer lenses is the optional USB dock that allows the user to configure a number of lens behaviours. Most significantly, it allows for the lens to be AF micro-adjusted at 4 different focus distances for 4 different focal lengths for a total of 16 adjustments in total. This far more granularity than what you can achieve with Canon or Nikon's in-camera adjustment, which only allows for one value per lens. Seeing how the focusing behaved on my test copy, the USB would probably be something that I would add immediately. However, bear in mind that doing a proper lens calibration takes time; doing it for all 16 data points will literally take you hours. It's analogous to tuning a 12-string guitar versus a 6-string: there are double the number of strings, but the time that you spend tuning it is more than double.

Conclusion


Is this a professional crop lens? Yes. 
 Is that who will likely use this lens? Probably not.

Though it's a bit of posturing on the part of professionals, there is some truth to the notion that professionals don't like to use normal lenses because that makes their pictures like... normal, for lack of a better word. If everybody else is shooting in this focal range, its difficult to stand out. For the dedicated crop sensor enthusiast, this lens does stand out, it's a normal+ lens by virtual of its uniquely fast constant aperture. It's not a knock to say that this lens will appeal to enthusiasts more than paid professionals; its just that different people have different priorities. Both professionals and non-professionals will like this lens, its just that it will fall into a different place on the respective lists of priorities.

As far as recommendations go, if you already have a f/2.8 normal zoom like the Sigma 17-50mm f/2.8 EX DC OS HSM or the aforementioned Tamron 17-50 f/2.8, then it's an expensive upgrade to move to the Sigma 18-35mm. You get the extra stop of aperture, but you lose the flexibility of having 50mm for head shots. In the case of the Sigma, you'll also lose the convenience of having the OS image stabilization, which is not truly made up for by the extra stop of aperture. Which also strikes at a misconception that will likely happen with this lens: the fast f/1.8 aperture will help with low light situations, but will prove to be frustrating because of how narrow the plane of focus is when its wide open. Moving to this lens solely to take better pictures in dim light will be an unwise move. Many people made this mistake with fast primes, only to discover that they had well exposed shots of motion-blurred subjects. Flash is highly under-rated in these circumstances.

The Sigma 18-35mm is a unique lens with no direct competitors. For Nikon shooters, the closest comparable lens in build quality and price is the 17-55mm f/2.8G IF-ED. The Nikon is an older lens and a known quantity; the Sigma is the flashy new kid in town. At this point, I would be hesitant to take the Sigma into a mission-critical application without getting a deeper familiarity with its focus behaviour. On the Canon side, you might be looking at the EF-S 17-55mm f/2.8 IS USM. Again, an older lens with a similar price tag. Has the benefit of image stabilization, but the bokeh is on the harsh side. Build quality is not as nice as either the Sigma or the Nikon. However, ignoring outright image and build quality, the closest competitor to the Sigma 18-35mm is probably its little brother, the Sigma 17-50mm f/2.8 EX DC OS HSM. Its a cheaper lens built to a cheaper price point, but the image stabilization will give it a slight edge in safe hand-holding shutter speeds.

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