Sigma 18-35mm F1.8 DC HSM "Art"
Sharpness
Many would argue that if there is just one attribute in a lens to pay overt attention to, it would be sharpness and the maximum resolution the lens in question is capable of. This is especially true in our pixel-peeping, megapixel-counting, measureabating world of modern photography. As such, you have arrived at our most comprehensive (and thus longest) page in this entire review.
In order to effectively assess the resolution capable of the lens in question, we ran it through a few of its different focal lengths from widest to longest. For the comparison, of course the HD 20-40mm F2.8-4 Limited and the DA* 16-50mm F2.8 were used, and for the specific focal length, a fourth lens was added: FA 31mm F1.8 Limited. For each millimeter setting you'll find not only examinations at the center, but the extreme edge of the frame as well.
Speaking of millimeters, you might be interested to know that for every focal length we shot at, the camera was placed exactly 100x the focal length away from our chart. For example, at 18mm, the camera was set to 1,800mm, or 1.8m. At 35mm, 3.5m.
The below "star charts" are valuable tests because they force a lens, no matter how sharp or soft, to reach its tipping point. When observed by themselves, they do not offer much information about a lens' ability to resolve fine detail. But when shot in a controlled manner alongside other lenses also shot in the same way, the results will speak for themselves. Reading the charts is easy - the earlier the image blurs, the softer the lens. As you would expect for the contrary, the closer to the center before the converging lines are indistinguishable, the sharper the lens and the higher a resolution it is capable of attaining.
This page will be an especially long one for no other reason than there's a lot of images combined with their size. Because of the premium lenses we are dealing with, we are confident you want to see 100% crops rather than scaled down versions in order to finally settle the score about image quality comparisons between these four lenses at all relevant focal lengths.
And finally, a small but important detail regarding our methodology. We naturally shot with the Pentax K-3 as it's the highest resolution offering from Pentax (as of this review's publication) to wield the K-mount. But, the K-3 has such incredibly high resolution sans any anti-aliasing that moire is certainly bound to appear with the test chart that we are using. As such, in order to strike the perfect balance between moire suppression and maximum resolution attainable, we employed the K-3's Anti-Aliasing Filter Simulator (Strength: Low). The low strength AA simulator was applied across every iteration below for consistency.
Without further ado, the comparison tests at 18/16mm, 20mm, 31mm, and 35/40mm.
Comparison One - Center Sharpness at 18mm or 16mm
For our first sharpness comparison test, we were at the minimum focal length of each lens, 18mm for the Sigma and 16mm for the Pentax. The following settings were applied across all the sample images during this iteration:
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Sigma 18-35, 18mm | DA* 16-50, 16mm | |
| F1.8 |  | N/A |
| F2.2 |  | N/A |
| F2.8 |  |  |
| F3.5 | |  |
| F4.0 |  |  |
| F4.5 |  |  |
| F5.6 |  |  |
| F6.7 |  |  |
| F8.0 |  |  |
| F11 |  |  |
Comparison Two - Edge Sharpness at 18mm or 16mm
No change from above other than moving our chart to the edge of the frame:
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|
Sigma 18-35, 18mm | DA* 16-50, 16mm | |
| F1.8 |  | N/A |
| F2.2 |  | N/A |
| F2.8 |  |  |
| F3.5 |  |  |
| F4.0 |  |  |
| F4.5 |  |  |
| F5.6 |  |  |
| F6.7 |  |  |
| F8.0 |  |  |
| F11 |  |
|
As you can see, the DA* lens suffers from pretty significant distortion at widest focal length in the edges, and appears to not rid itself of chromatic aberrations across the aperture range.
Comparison Three - Center Sharpness at 20mm
This time, we took the two lenses and moved them to 20mm for the specific reason of adding the HD 20-40mm Limited into the mix. Other than the focal length, the process stayed the same:
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You'll note that the format of this (and the remaining sections on this page) are a little bit different than what we normally present. The reason for that is simply adding a third (and then eventually a fourth) image exceeds the width of this website's central frame. Not to be defeated, we will still provide you the 100% crops to assess these lenses in the detail you expect of us. Just click on any of the images below (the crops are combined in one image per aperture), and then you can scroll between them to your heart's content.
| F1.8 |  |
| F2.2 |  |
| F2.8 |  |
| F3.5 |  |
| F4.0 |  |
| F4.5 |  |
| F5.6 |  |
| F6.7 |  |
| F8.0 |  |
| F11 |  |
Comparison Four - Edge Sharpness at 20mm
Once again moving to the periphery, and no change beyond that:
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|
Click on any of the images below to enlarge and cycle through them.
| F1.8 |  |
| F2.2 |  |
| F2.8 |  |
| F3.5 |  |
| F4.0 |  |
| F4.5 |  |
| F5.6 |  |
| F6.7 |  |
| F8.0 |  |
| F11 |  |
Comparison Five - Center Sharpness at 31mm
That's a peculiar focal length, isn't it? Any Pentaxian worth his (or her) salt knows what this next set will include...
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Click on any of the images below to enlarge and cycle through them.
| F1.8 |  |
| F2.2 |  |
| F2.8 |  |
| F3.5 |  |
| F4.0 |  |
| F4.5 |  |
| F5.6 |  |
| F6.7 |  |
| F8.0 |  |
| F11 |  |
Comparison Six - Edge Sharpness at 31mm
And now the edge comparison for this prime of prime numbers.
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Click on any of the images below to enlarge and cycle through them.
| F1.8 |  |
| F2.2 |  |
| F2.8 |  |
| F3.5 | |
| F4.0 |  |
| F4.5 |  |
| F5.6 |  |
| F6.7 |  |
| F8.0 |  |
| F11 |  |
Comparison Seven - Center Sharpness at 35/40mm
For our final focal length setting, we set the Sigma Art lens to its maximum zoom, matched by the Pentax DA* and Limited zooms. You'll note that there are four crops per row despite three lenses - we also wanted to check how the max zoom of the HD DA 20-40 fared in comparison. To keep it fair, we moved it to 100x the focal length of 40mm as well.
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Click on any of the images below to enlarge and cycle through them.
| F1.8 |  |
| F2.2 |  |
| F2.8 |  |
| F3.5 |  |
| F4.0 |  |
| F4.5 |  |
| F5.6 |  |
| F6.7 |  |
| F8.0 |  |
| F11 |  |
Comparison Eight - Edge Sharpness at 35/40mm
And finally...the 35/40mm test at its outer limits.
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Click on any of the images below to enlarge and cycle through them.
| F1.8 |  |
| F2.2 |  |
| F2.8 |  |
| F3.5 |  |
| F4.0 |  |
| F4.5 |  |
| F5.6 |  |
| F6.7 |  |
| F8.0 | |
| F11 | |
Verdict
Are you dizzy yet? We wouldn't hold it against you if you were - that was two hundred and eighteen 100% crops you just digested.
So, what does all this boil down to? At the widest edge of the zoom ring, the Sigma and the DA* 16-50 are for the most part neck and neck for center sharpness - one might even tip the hat in Pentax's favor if it weren't for the latter's fringing. Moving to the edge paints a different story. The DA* 16-50 certainly suffers from chromatic aberrations, but we wish that was the worst of it, as the distortion is extremely significant. Despite the degree of it though, the DA* maintains excellent sharpness from F4.0-F8.0, even producing moire!
At 20mm and adding the HD Limited zoom, it takes the 20-40mm lens two stops to catch up to the Sigma, from which point they stay neck and neck. The DA* 16-50 appears to have a better start at its widest aperture (F2.8) than the 20-40 (also F2.8 at 20mm), with the Sigma starting with a phenomenal degree of clarity from wide open. Moving to the edge, the DA* is the most degraded. What is surprising is that despite being the smallest lens with a smallest aperture assemblyf, the HD 20-40 is the least affected by diffraction, hitting that affliction point later than the other two lenses.
Moving to our next series, 31mm surprised us. As Pentaxians, we believe in and dream about the FA 31 Limited. Well, to put it bluntly, the Sigma crushed that dream from F1.8 until F3.5 when the FA finally catches up. For the rest of the images in the center, the FA Limited and the Sigma goes literally tit for tat and are ultimately indistinguishable save for the slight color cast from the purple fringing in the FA's images. What shocked us, though, was that the HD 20-40 was the sharpest and most contrasty of the bunch from its wide open setting (admittedly not by much though). From there, it was either tied with the Sigma and Limited prime or ahead of both. The DA* never caught up to any of them. Moving to the edge, the FA 31's only limitation was its almost unshakeable chromatic aberration. It exceeded the sharpness of the Sigma by a slight amount, but this was expected - it's a full frame lens with a larger image circle than the APS-C lens that will reach its point of degradation sooner. Apparently determined to shock us even more, the DA 20-40 was indistinguishable from the FA 31 (except for the former's persistent fringing), even besting it a couple times.
Finally, the 35/40mm comparison - the Sigma had a bit of a slow start in the edges, but for the center, it was extraordinarily sharp from the start, which was certainly encouraging. The HD 20-40 once again surprised us with its performance from wide open and through the aperture range. Both the 35mm and 40mm proved to be excellent for resolution. While the DA* 16-50's performance reached excellent levels after stopping down a little bit in the center at 35mm, the corners for this Pentax professional standard zoom was unfortunately a different story - mediocre at best and never able to dodge the grip of the chromatic aberration monster. Neither the Sigma nor the Limited zooms suffered such levels of purple fringing.
To sum things up, Sigma's optical engineers have simply done a fanstastic job with the 18-35mm. It delivers excellent results wide-open, which sets the bar high for future zooms faster than F1.8, should they be released.
