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

Aberrations and Flare

Imperfections in an optical design can lead to chromatic aberrations such as fringing, lens flare, and ghosting, all unwarranted symptoms of having a strong light source enter your field of view.  When used in conjunction with a lens hood, modern glass coatings do an excellent job of suppressing lens flare, and some better than others. Pentax has been widely regarded as having some of the best coatings in the industry, a reputation built over decades through their "Super Multi Coating," largely known via the iconic smc, and most recently with their improved High Definition, or "HD," coating.

Sigma, on the other hand, has historically never been able to claim flare prevention as one of their points of pride. In fact, it's often been said among dedicated forumites that Pentax should use their excellent coating technology (arguably the best in the industry) as a bargaining chip for increased availability of Sigma lenses in K-mount.

With an entirely new and never-before-seen lens design coupled with the new "Art" line of lenses, does the 18-35 F1.8 change all that? Let's see how Sigma's most recent K-mount lens fares for flares.

Flare Test One - Center-of-Frame Sunlit Flare

For this comparison, extremely strong sunlight is used to back-light tree branches, a notoriously problematic situation for lenses to cope with. The lens was set to both its minimum and maximum focal length to assess how the lens dealt with the ensuing light spilling through the forest.

Click on any image below to open an enlargement and cycle through them.

	18mm	35mm
F1.8
F4.0
F11

Flare Test Two - Edge-of-Frame Sunlit Flare

For this comparison, We kept the same strong backlighting through the trees, but instead moved the sun to away from the middle of the frame. The sun was kept in the same place for both focal length iterations - it just appears the 35mm has it more to the edge because the angle of view became narrower.

Click on any image below to open an enlargement and cycle through them.

	18mm	35mm
F1.8
F4.0
F11

Flare Test Three - Clear Blue Skies

For our final flare stress test, we did just as this section's title says - removed all obstacles and inteferences between sun and sensor. While at the beach, it was a spectacularly clear day, and thus perfect for a shot into the sun. We didn't run down the entire aperture range like we did before, but it still gives a good indication of what you can expect in this situation.

Now it shouldn't be any secret that the light coming through branches like the first two sections will accentuate the flare effect. This is because the light gets more opportunities and angles to bounce around the different optical elements within the lens itself. Certainly you shouldn't expect such egregious flaring all the time. Take the below example (click on it for a better look):

Pretty clear and clean right? We thought so too, but it wasn't until taking a closer look beneath the tent that some flare became noticeable. Here's a 100% crop:

Excuse the noise - it was shot at ISO 400 and then severely underexposed in order to preserve the highlights, boosting the shadows in post.

Chromatic Aberrations

For our chromatic aberrations assessment, we took advantage of the thin reeds atop the sand dunes during a recent trip to the beach. The reason this is such a suitable test for chromatic aberrations because at max aperture, it is extremely difficult to render dark subjects against strong back-lighting. For anyone that has shot leaves, twigs, hair, etc. against the sun, you surely know what about this phenomenon.

For this comparison, we used the only two lenses our reviewer had on him at the time, the Sigma and the smc FA 77mm F1.8 Limited. Now, if there are two things this Pentax lens can be counted on, and one is to capture stunning portraits, the other is to produce equally stunning amounts of purple fringing.

Yes, we know the focal lengths/uses do not overlap in the slightest, but it gives an immediate juxtaposition. Both lenses were used on a K-3 for the below images and shot at F1.8 (the maximum aperture for both lenses).

Click on either image below to open an enlargement.

	Entire Scene	100% Crop
18-35
FA 77

Verdict

There's really not much more to say - Sigma may have garnered the industry's attention with the recent lenses they've released in their Global Vision lineup, but they have still proven substandard at mitigating flare. This is especially true in an age where Pentax's latest HD coating consistently receives glowing reports such as this one from LensTip.com's review of the HD 20-40: "No matter what focal length and aperture value you employ, it is really difficult to catch any light artifacts or flares."

This test only emphasizes the business proposition we posited at the top of this page - would it be possible to use Pentax's outstanding coating technology as a conduit for better lens support from Sigma?

We like to end on a positive note whenever possible, and we're glad we can do so here. While the 18-35 "Art" let us down with its flare performance, we do have to give credit where credit is due. The Sigma did an absolutely phenomenal job keeping chromatic aberrations at bay. There's only the slightest hint of it in the 100% crop above, and that's just it - the slightest hint. And you'll notice the same to be true for any samples and the sharpness crops that follow this page. Within half a stop it's effectively gone in its entirety. Certainly those four extra dispersion (ED) elements are to "blame" for this stellar performance.