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Sigma has really reinvented themselves. Back in 2008 when I bought my Sigma 10-20mm f/4-5.6 EX DC, they were a typical third party brand: Not quite as good, but a lot cheaper. But with the release of the 18-35mm f/1.8[a] they sent many a jaw dropping, and after having tried the Sigma 50-150mm f/2.8 EX DC OS and now the 17-50/2.8, I must say that they really are up there with the major manufacturers. Their lenses come at affordable prices, and with performance that I'd say is as good as the best within margin of error. Nowadays, I choose a Sigma lens without worrying that I'll miss out on something that the Nikon lens would have given me.

The lens is somewhat like a zoom version of the Nikon 35mm f/1.8 DX. It is sharp - but if you want sharpness you are better off just stopping down to f/4 or f/5. It's bright - but at 17mm it only has 2/3rds of a stop advantage over the 18-55mm kit lens, it's only at 24mm or above that the fixed 2.8 aperture gives you more than a single stop advantage, and you never get more than two stops advantage. It gives you nice, blurry backgrounds over the full zoom range - if the subject is close enough.

​1. What You Get

The box contains the lens, a pouch, and a lens hood.

​2. The Lens

Just because a lens is heavy it isn't necessarily well built, as a heavy metal shell may enclose brittle components[b]. This lens is on the heavy side, though, which is a downside when hiking, traveling or whenever extra weight comes at a premium. The zoom ring and focus ring take up almost the whole length of the lens and are easy to operate. The focus motor is not that strong, and it must spin the focus ring, so if you rest the ring against your hand while focusing it will start beeping and give up. (It took me a little time to figure out why the lens wouldn't focus and why I kept hearing this "beep beep beep" sound.) There's a lock tab that locks the zoom at 17mm, but during my testing I've never had to use it.

​3. Performance

Performance is good overall. The lens is sharp enough across the field full open. Auto focus is fast and correct.

​3.1. Stills

There is really no point in shooting this lens stopped down. It is good enough at f/2.8 across the zoom range, and after all, that's what you've paid for. I recommend using Live-View focus (or any other contrast-detect focus), as the situations when you'll be using this lens will typically have a thin depth of field. All the examples below were shot at f/2.8, to give you an idea of what the end result looks like at that aperture and different subject distances.

​3.2. Video

Video was one of the main reasons I bought this lens. While you can compensate for a smaller aperture by increasing the exposure time, video puts an upper limit on the exposure time, beyond which you must increase the ISO. The caveat is that you may still have to stop down the lens to get the depth of field that you want - before buying this lens, do take a look at the scenes you expect to shoot with it.

​4. Versus the Sigma 18-35 f/1.8

No review of this lens is complete without comparing it to Sigma's current super-lens, the 18-35mm f/1.8 DC HSM Art[c]. The 18-35mm certainly is an outstanding performer - but it stands out in a very specialized niche: wide angle, large aperture lenses.

The 18-35mm f/1.8 should not really be compared to the 17-50mm f/2.8. Due to the limited zoom range, a much better comparison is against the Nikon 35mm f/1.8. Once you compare those two, you realize that the only advantage the 18-35 really brings is being able to be 18mm. At that focal length, background blur is not that significant unless your subject is really close - the background is out of focus, all right, but it isn't enough out of focus to really get that strong separation of subject and background that we're trying to achieve.

If you're deciding between the 17-50/2.8 and 18-35/1.8, you should ask yourself two questions:

1. What use do I have for an 18mm/1.8? If the answer is "none, really" then consider if what you want really isn't a much cheaper and lighter 35mm/1.8 - just as much light goes in to the sensor and a lot less money goes out of your wallet.

2. When shooting at 18mm, will f/1.8 give me the depth of field I want? If not, you'll be forced to stop down anyway.

For me, I realized that at 18mm and 1.8, I would not get the depth of field that I needed. For that I would have to stop down to at least 2.2, and when comparing a hypothetical 18-35mm/2.2 with a 17-50/2.8 at less than half the price the choice was easy.

​5. Versus the Nikon 18-55 f/3.5-5.6 VR

While at first this may seem like comparing apples to oranges, I want to show just how incremental the improvements are when going from a kit lens to a f/2.8 pro lens. The aspects that I'll focus on are brightness and weight. Let's start with brightness, and see how a fixed-aperture f/2.8 lens compares to a f/3.5-5.6 variable aperture lens.

The biggest thing to take away from this is just how good kit lenses are. You'd expect there to be a gulf between a kit lens and a "pro" lens, but as we can see here, the difference is about one stop at the short end and about two stops at the long end. At 18mm, the difference is a measly two-thirds of a stop - something I'll doubt anyone will ever notice as far as final image quality is concerned.

Two stops at the long end would be noticeable - if you shoot in low light, and if that light is such that having a two stop advantage brings you into the range of ISO values where the camera output is considered acceptable. For example, if you shoot at ISO 12800, this lens will, at best, bring you to ISO 3200 - which still is a level where you should convert your image to black and white and go for that grainy look. If you shoot at ISO 1600, however, this lens may bring you to ISO 400, and you are well within the range where the camera can produce some really nice color images.

But all of that comes at a price: First, money that you pay for this lens is money that you won't be using to get anywhere near something worth photographing. If the choice is between going somewhere interesting with a kit lens and staying at home with a "pro" lens - I don't think I have to spell it out for you. Second, it's heavy. The kit lens weighs in at a miniscule 265 g (9.3 oz), whereas this one weighs in at a substantial 565 g (19.9 oz) - over twice as heavy. It may not seem much, but if you're going to haul that weight around for some time you really want to be sure that you get something out of the effort.

​6. Summary

This is a lens that offers incremental improvement over a kit lens. If you find yourself shooting a lot at 50mm and ISO 1600 and above, you may find it useful as it will let you go to ISO 400 at those focal lengths - a change that is noticeable at higher resolutions. If you shoot video, it will certainly help you maintain a good picture quality at less light given a fixed framerate.

All in all, for someone who shoots dance performances in low light (as stage light tends to be), it's worth the price. The noise limit for my current camera is at about ISO 800. Being able to go from ISO 1600 to ISO 400, and from ISO 3200 to ISO 800, comes with enough improvement in image quality that I find it worthwhile to bring this lens with me.