Sigma 18-35mm F1.8 DC HSM "Art"

Distortion

Most, if not all lenses exhibit a degree of distortion in some way. This is especially true for those that venture towards the wider end of the focal length spectrum. Coming in several variations— "barrel" or "pincushion" are the most common ones and the types we'll be focusing on— distortion in the context of photography is an optical aberration (deformity) that results in bending and deforming straight lines in a scene so they appear to be curved and/or slanted in the resulting image. In order words, not how they actually are in real-life.

Before we get to the actual assessments below, it should be said that we have extremely high expectations of this lens in this regard. Why? Because if we revisit the Specifications page and pay particular attention to the lens construction, we notice that there are a mind-blowing total of no less than five aspherical lens elements. For those unaware, these lens elements are specifically shaped and added for the purpose of reducing distortions. With so many of these specialized molds of glass (by comparison, the DA* 16-50 has three of them), we really do expect the Sigma 18-35 "Art' to be nothing short of stellar in this regard.

So is it?

Brick Wall Distortion Test

For our controlled "brick wall" test (because what would a lens review be without good ole masonry), we shot at three specific points within the 18-35's zoom range - 18mm, 24mm, and 35mm. Each of the series of shots below incorporate an "Uncorrected" and "Corrected" version displayed in the same frame.

For all the below images, the same slider system as the previous page is in effect.

Without further ado, 18mm.

For this image (and all the rest), it was manually corrected using Adobe Photoshop Lightroom 4.4. The correction needed was +3 to straighten the lines.

For our "middle" focal length, we set the lens to 24mm:

The correction needed for this focal length was -2. The change from a positive value (in the 18mm set) to a negative value for the correction indicates that the lens exhibits both barrel and pincushion distortion. This is expected, as zoom lenses that offer "wide to normal" focal lengths commonly make that transition.

Finally, the max zoom length:

Once again, another +2 correction was applied.

Real World Distortion Examples

Shooting brick walls is fun and all, but how do the above examples compare to the real-world (unless bricks are your thing...)? The following examples taken with the Sigma show its entire focal length range in actual shooting situations.

Once again, the same sliders as above and how they operate are in effect.

Example One:

For our first "real-world" example, we revisit the scene used in the General Image Quality page, specifically the 18mm version. It required a +5 adjustment in Lightroom to get its verticals aligned, but even then the difference is hard to notice without the slider.

Example Two:

The next situation is one where we really made an effort at making the 18-35 sweat. Stairs and a long hallway off to the side in an indoor, cramped setting are a guaranteed combination to make any wide angle lens show its true (barrel distortiony) colors.

At the widest end of the zoom, this required a +8 correction. While that may seem like a lot (and it's the heaviest adjustment of all the examples we tested during this review), we were pretty stunned at how well this "Art" lens handled this situation. As a comparison, the DA 18-135 WR needed almost +20 in the same situation!

Example Three:

Our final 18mm shot takes us to the sunny sand dunes of the Outer Banks. This shot required a +5 adjustment.

Example Four: 24mm

For our 24mm example, we have the image below from a common "dammit-just-missed-the-sunset" scenario.

A -2 adjustment was made, which follows with our above observation regarding the transition between barrel and pincushion distortion. Once again, the difference between the corrected and uncorrected versions is negligible.

Example Five: 35mm

For our final test image, it was set to 35mm, and required a staggering modification of -1.

Wide Angle Portraits

While the lens has excellent distortion control is the standard sense, it is still possible to capitalize on distortion that the 18mm would allow simply by virtue of the wide focal length. Couple that with the F1.8 aperture and you have a very unique opportunity for wide and shallow depth of field portraits that can certainly be distorted if you want them to be. These are certainly going to be a hit with kids (click on either one for an enlargement):

Verdict

Like we said above, we had nothing short of spectacular aspirations for this lens considering its premium optical construction and inclusion of so many distortion-correcting lens elements. After testing the lens in a myriad of situations, both indoors and out, we are happy to report that our expectations were met. To be perfectly honest, many of the shots above were very painfully attended to, trying to find an adjustment that improved the image. Much of the time, and you probably noticed this yourself, the "uncorrected" version is perfectly fine (if not better?).

We are confident that except for the most niche circumstances, this Sigma "Art" lens will make distortion one of the least of your concerns, with the majority of your corrections yielding negligible improvements.

But if you wanted to cash in on your creative juices, it still can be done, and to great effect.