Sigma 17-50mm f/2.8 OS Lens Review


The Lens Nikon Hasn't Made (well, one of them)

Copyright 2012 Thom Hogan

Lens Formula
17 elements in 13 groups; 2 FLD low dispersion elements.
Other Features
Optical stabilization, HSM (AF-S equivalent) focus, Internal focus, 77mm filter size. Comes with petal hood, 7-blade rounded aperture. Focuses to 11" (0.28m).
Size & Weight
About 3.6" (92mm) long from mount, 19.9 ounces (565g).
Price
US$620

The Basics
One of the top requests from users of any system is an optically good, fast aperture, optically stabilized mid-range zoom (moderate wide angle to moderate telephoto). In the pro ranks, similar lenses form the middle of a workhorse trio of lenses (wide angle zoom, mid-range zoom, telephoto zoom), so it's not surprising that DX users would want the workhorse, too.

Nikon's solution appeared in 2003, the US$1400 Nikkor 17-55mm f/2.8. It's optically good, has a fast aperture, but lacks optical stabilization. It's also big and heavy, and that price tag means that it's more expensive than all but one of Nikon's DX cameras. In other words, it wasn't exactly embraced by users, though I found it to be a very good lens in practice.

Over the years, there have been quite a few third-party attempts at doing better than Nikon, with the most notable ones being the Tamron 17-50mm f/2.8 (first without a focus motor, then with a focus motor, then adding image stabilization), and a pair of Sigma's, the 17-50mm f/2.8 and the 17-50mm f/2.8-4 (both of which appeared briefly without stabilization, then with it). I'm not going to do reviews of all of these lenses, so I should probably tell you why:

  • Sigma: the variable aperture versions of these lenses, while less expensive, simply don't perform at the level I'd want them to. They're much more like kit lenses that are 1/2 to 2/3 of a stop faster. We don't need more kit lenses.
  • Tamron: the original version of the lens I liked, but the addition of the in-focus motor actually hurt this lens considerably. It doesn't have the focus performance that the Nikon or Sigma solutions do, which makes it somewhat non-competitive in my book.

Which leaves us with the Sigma 17-50mm f/2.8 OS HSM, the subject of this review. I have to say I was skeptical when I saw the specs and the price. Sigma's reputation for wide sample variation also didn't make me expect a lot, either. But I decided to try it out, and have been quite pleased I did.

If you've used the Nikkor 17-55mm, the first thing you notice is that the Sigma is smaller, lighter, and more compact. It feels "more DX" than the Nikkor, and the size is very appropriate on all the DX bodies. No longer do you feel front-heavy as you do with the Nikkor 17-55mm on some DX bodies. A photo tells you all you need to know:

Copyright 2012 Thom HoganSigma 17-50mm on left, Nikkor 17-55mm on right

One way Sigma achieved this is a very tight-to-DX image circle. Typically, that would mean vignetting and poor corner performance, but we'll get to those things in the performance section, further down in the review.

The 17-50mm has two of Sigma's latest technologies in it, paralleling Nikon's: HSM is Sigma's version of AF-S (internal focus motor), and in my experience, has similar performance; OS is Sigma's version of VR (optical stabilization), and given that Nikon has been suing Sigma over copying Nikon's VR system, it too has very similar performance. You'll notice that things are starting to stack up in favor of the Sigma: price, size, optical stabilization, but nothing is yet detracting from it (similar focal range, same f/2.8, etc.). Pay close attention to that and you'll see why I recommend this lens.

There are focus markings at the very front of this lens, but no DOF or IR markings. The lens is marked at 17, 21, 28, 35, and 50mm on the zoom ring.

Like most traditional lenses, the focus ring is furthest from the camera, the zoom ring closest to the camera.

The 17-50mm is an internally focusing lens, so the front element doesn't rotate during zoom or focus. Close focus is about 11" (.28m), which is very nice, basically as close as some wide zooms go. The lens uses 77mm filters.

The lens is made in Japan.

Handling
As I've noted, this isn't a large lens, nor is it overly heavy for its size. It balances on the front of my DX bodies very nicely.

The zoom ring on my sample is stiff, and the lens changes length as you zoom. There's a lock switch if you think that might be a problem, but I've never had to use it.

The focus ring uses a switch to take the lens out of autofocus into manual focus. Manual focus is smooth enough, but going from near to far focus is less than one-eighth of a turn of the lens: you can't get much discrimination in focus distance with this lens by focusing manually. One thing to note: unlike most AF-S or HSM lenses, you cannot override autofocus manually with this lens. That's not a big loss considering the lack of discrimination in the focus ring.

The butterfly hood (supplied) is shallow but effective for all but near-edge light sources. You need to use thin filters on this lens at 17mm if you use filters, as I find that I get vigenetting from some fatter filters at the wide end. This isn't a problem at the telephoto end.

Overall, the handling of this lens is solid. The build quality is decent and both the zoom and focus rings feel well connected to the mechanisms you're moving, with no sloppiness you find in some recent low-end designs.

Performance
Autofocus: I've always liked Sigma's HSM. It's fast and reasonably silent (a bit more chattery than Nikon's AF-S, but not enough to make a difference, IMHO). There is a strange thing that happens when you switch from manual focus to autofocus, where the first autofocus after that tends to take longer, as if some mechanism has to be re-engaged first. Overall, I'd say the focus speed is excellent and as good as it gets; it will only be limited by the camera body's ability (e.g. only struggle in low light or low contrast, and typically only with outer sensors).

Optical Stabilization: Quiet. Almost too quiet. Interestingly, on a couple of FX bodies (yes, I try to test everything on everything) the optical stabilization didn't seem to engage at all, but it does on all the DX bodies I have, including the most recent D3200. Overall, I don't think the Sigma OS on this lens gives quite the same level of performance as Nikon's VR II does, but it's also not as nattery and obtrusive in calming the viewfinder image, either. I don't think there's a perfect test for determining how much reduction you get, but my impression is "not as much as VR II or Tamron's VC." Nevertheless, it seems to work well for basic stabilization.

Sharpness: Okay, you'll remember I wrote about a limited image circle, so the corners must be trash, right? No. I'm not exactly sure how Sigma achieved this, but even at f/2.8 this lens shows only a modest softness in the corners and is tack sharp in the center. Stop down two stops and it's corner to corner good. This is true at all focal lengths. Indeed, in the mid-range focal lengths, the corners tend to be better even wide open, as it's only at the widest and telephotoist focal lengths that the corners get much softening. Optically, a remarkable performance. Perhaps even an exceptional performance.

That said, I've seen four samples of this lens so far. One was badly miscentered and wouldn't reflect what I wrote in the paragraph above. Sigma is notorious for sample variation in lenses, so be careful to do some basic testing to make sure you're satisfied with your sample, and buy from a dealer who will trade out for another copy if you find problems.

Light falloff: Yes, another surprise. From f/4 to f/22 there isn't enough to really talk about, and at f/2.8 the worst case is a bit more than a half a stop. Again, remarkable performance.

Chromatic aberration: Lens designs are all about tradeoffs, and it's here that we find where Sigma traded one variable for another. There's relative high chromatic aberration on my D7000 at 17mm, say eight or nine pixels worth in the corners and about five in the middle. By 35mm that's down to perhaps two pixels across the frame, though it rises in the corners again at 50mm. The 17mm results are about at the level that software correction starts to have troubles completely fixing, though pretty much through the rest of the focal range I find the CA completely correctable. Note that I'm talking about extreme corner problems at 17mm, which may not be an issue for you if you can "mostly correct" it. It's something to watch for, though, as it is the weak spot in performance of this lens.

Flare: Like a lot of Sigma lenses, flare is well controlled up to the point where you put something like the sun in the frame, at which point you get small colored flare spots. I'm not overly bothered by the performance of this lens on flare: it's just about what I'd expect from a high-end optic.

Distortion: Not exessive. We go from perhaps 1% barrel distortion at 17mm to no distortion somewhere near 28mm, to a very minor pincushion distortion that's probably not worth correcting at 50mm. The distortion levels are low across the board, and easily correctable in software.

Bokeh: The chromatic aberration makes for problems in out of focus highlights, as they tend to get chromatic rings in them. Also, while Sigma describes the aperture blades as rounded, they're barely rounded. At small apertures I get distinct septagons with one point exagerrated, and something really funky happens at about f/4 when one of the blades edges in too far and causes a septagon with a bad side. So, no bokeh winner here.

Overall, I like this lens. Other than having to correct chromatic aberration and the funky bokeh, the optical attributes are all really, really good.

Drawbacks

  • Chromatic aberration. Pretty high levels will have you doing post processing refinement.
  • Manual focus. Almost no discrimination due to the short throw on the focus ring.

Positives

  • Sharp. Usable even into the corners at f/2.8, very sharp at f/5.6.
  • All the tech. HSM and OS mean this lens gives you more than Nikon does in this focal range, and...
  • Priced to sell. This is a far better choice (assuming you get a good sample) than the Nikkor 17-55mm, which is why the Sigma is in my kit and the Nikkor isn't.
 
Quick Evaluation



Recommended

features
performance
build
value

Good samples of this lens are excellent choices for a DX mid-range zoom.

Limitations


The 17-50mm lens has limitations you need to be aware of:

Converters: no converters are recommended (rear element near mount position at 17mm).

G: Since this is a DX lens, it's not likely that you'd mount it on an older 35mm body that can't handle the lack of aperture ring or makes restrictions because it is missing.

Coverage Area: produces a highly circular image vignette on FX bodies at all focal lengths.

Version of Review:
10/16/2012: initial post.

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