Sigma 18-35mm F1.8 DC HSM "Art"
Now that we've scrutinized everything about the lens we could, both its construction and image quality, our last dedicated assessment is of how the lens acquires those images - its autofocus system. Incorporating Sigma's state-of-the-art HSM motor, does the 18-35 "Art" autofocus match the excellent optical qualities of the world's fastest (and so far only as of this review's publication) F1.8 constant aperture zoom lens?
The Sigma 18-35 F1.8 "Art" incorporates Sigma's proprietary autofocus, the "Hyper Sonic Motor," or the HSM acronym that can be found in the official name of the lens. It is compatible with all Pentax DSLRs since 2006 and Sigma promises it to not only be fast and accurate but silent in operation.
The rear lens mount of the Sigma 18-35 shows that it can only focus using the K10D bodies or newer. In case you might not understand why, it's simply because previous camera bodies were not equipped with the two gold pins that paired with the rectangular contacts shown below that Pentax uses for its own proprietary SDM and DC in-lens autofocus motors (and thus Sigma's HSM implementation):
In case you may find yourself looking in your DSLR's mirror box to see if the two pins are there, this is what you should be looking for (shown using a Pentax K-3):
As mentioned during the construction and handling page, the lens features in-lens focusing so that no piece of the lens protrudes or retracts when moving between minimum focus distance and infinity. In case you forgot or just skipped that page entirely (you know who you are...), the following animation proves that:
The lens features a sole physical customizable feature on the lens itself - the "AF/MF" selector switch to alternate between auto and manual focus. This switch and the two positions can be clearly seen in the animation to the right. Located in the perfect position for your thumb to play with while holding the lens and looking through the viewfinder, one of the most immediate impressions that stuck with us when we first handled the lens was how stiff it proved to activate and switch between the two modes. And this is meant as a compliment - it was very reassuring and left us with zero concerns that even with the roughest of uses, it would never change unless you actively made the decision to do so. This only further accentuates the phenomenal build quality of the 18-35.
This Sigma "Art" zoom features full-time manual focus override, known as "Quick-Shift" among Pentaxians, as you'd expect from such a premium optic. What this means is that no matter how you have the lens or camera body set, either to Autofocus or Manual Focus mode, you are free to rotate the focus ring as you desire.
Lastly, it's been mentioned more than just a few times now that the 18-35 is part of a new lineup of lenses with new Sigma's Global Vision restructuring. One thing that hasn't changed as part of that new framework is the autofocus system. Sigma's HSM has come to be highly-regarded for its reliability, a great thing for users that are planning to make this lens a long-term investment. This becomes all the more confidence-inducing when coupled with the 4-year warranty that Sigma's lenses features.
We'll start with the good news first - the lens focuses in a manner that is extremely quick, as you'd expect, and also virtually inaudible. There's some very minor high-pitched knocks that can be discerned as the lens makes its micro adjustments, but the audible range of it is mostly limited to having the lens close to your ear when looking through the viewfinder. Unfortunately this is where the good news ends...
To put it bluntly, we have never experienced such autofocus difficulties before. The issue is not speed - the Sigma 18-35 was fantastic in that regard. The issue was not hunting either. We can't think of a time it hunted.
The issue is a complete and utter inability to focus using Phase Detect AF (PDAF). For those unaware, this is the autofocusing mechanism that is used when looking through the viewfinder. Conversely, Contrast Detect AF (CDAF) was excellent in all lighting situations and focal lengths and apertures, which pertains to using the sensor directly with the mirror raised up, or in the case of mirrorless cameras, it's the only option (except in the cases of specialty sensors that have PDAF sites on the sensor itself, none of which are in Pentax cameras as of this review). No matter the situation - near minimum focus distance or out to infinity, the lens proved problematic and simply unable to focus accurately.
To expound on what we mean by "accurately," we mean that there literally is not a time we can remember getting any shot in focus using PDAF. Naturally, we started with F1.8 a lot of the time when we had the lens - why wouldn't we? It's the first time having a fast 18mm for Pentax, so we played around with that and had fun exploring the new opportunities afforded by such a fast and wide setting. Initially, we chalked the inaccuracies up to shallow depth of field at F1.8 (it really is shallow, especially near minimum focus distance). And then we started taking shots that in no way should ever be out of focus - landscape shots at 18mm and F8.0. No matter how we heard and felt the focus motor activate and spin, it never landed at the right spot unless we switched to Live View.
To confirm these findings, we verified that front/back focus was not an issue by testing the Sigma 18-35 "Art" on the Pentax K-30 and K-5 IIs in addition to our primary test body, the K-3. When that resulted in no improvement, we tested not only one but two additional K-3 bodies. Still to no avail.
Not to be defeated in our mission of solving this mystery, we then contacted Sigma's office in New York. First, the response after every email, immediate access to a live representative when calling, the enthusiasm over the phone, and an overall sense of urgency and initiative to assist was so surprising it deserves mention as indicative of a world-class customer service culture. Not only were we sent another copy of the Sigma 18-35, but it was promised to be callibrated to factory standards before being dropped off at the post office. Additionally, not one but two copies of the Sigma USB Dock for Pentax K Mount were sent to our reviewer to fine tune the focusing and ensure the firmware was up to date.
Nary a difference.
The HSM motor of the Sigma 18-35 "Art" delivered what it promised - fast and silent autofocus. But it was never accurate. And by never, we mean that not once in the entire combination of five camera bodies and two lenses and two USB docks did we get a single in-focus shot unless we switched to Live View. In fact, for 95% of this review and except when hoping for an in-focus image, we used Live View for the entirety of this publication. It got to the point where we had not the smallest bit of confidence a shot would be in focus, and so no matter the situation, we just resorted to using Live View. It's important to note that during this review, the other lenses that were used for the comparisons exhibited none of these problems when mounted to the same camera bodies, so the symptons are clearly isolated to the Sigma 18-35 "Art."
For those who might remember, we had similar problems with Sigma's 30mm F1.4 "Art" lens as well. We haven't figured out what the issue is or why - Sigma's 17-70 F2.8-4 "Contemporary" or the Sigma 35mm F1.4 "Art" were just fine - in fact fantastic focusers. And while we understand some of our readers have gotten lucky with good focusing copies of the 18-35, the trend seems to mirror our findings, especially when you research other reviewers' experiences with Canon and Nikon versions of the lens.
Editorial note: due to the surprisingly-poor autofocus results we obtained with our first several test copies of this lens, we have re-evaluatied and compared two additional copies. Read about our findings here.