Sigma 18-35mm F1.8 First Impressions

The fastest zoom lens in the Pentax mount

By PF Staff in Hands-On Tests on Aug 6, 2014

After what seemed like an endless waiting period, the Pentax K-mount version of the Sigma 18-35mm F1.8 "Art" zoom finally hit the shelves in early July: about a year after it had made its debut in the Canon mount.

The 18-35mm is an intriguing lens. Not only is it the fastest DSLR zoom currently in production (by about 1.3 stops compared to a F2.8 lens), but it's also designed to deliver exceptional prime-like image quality.  For us Pentax users, this new Sigma is of particular interest for three key reasons:

• It was developed exclusively for APS-C cameras
• It offers a cornucopia of wide-angle-to-normal focal lengths for which Pentax does not offer a faster autofocus prime alternative
• Sigma's recent lenses have topped the charts in terms of image quality

In the current genuine Pentax lens lineup, there is exactly one lens under 40mm with an aperture of F1.8 or faster: the FA 31mm F1.8 Limited.  It is complemented by a selection of third-party lenses, such as the Sigma 30mm F1.4 "A", Sigma 35mm F1.4 "A", Sigma 24mm F1.8 EX, and Sigma 28mm F1.8 EX.  But getting them all could be costly, so why not get a single zoom to do the job?

We've had a chance to test out the Sigma 18-35mm hands-on with the Pentax K-3, and we're in the process of putting together an in-depth review, for which we plan a comparison with the Pentax 20-40mm Limited and 31mm Limited lenses.  In the meantime, here are our first impressions of the lens.

From an optical standpoint, we really don't need to say much about the Sigma 18-35mm F1.8 apart from the fact that its image quality is truly impressive: but you probably already knew that after reading reviews of the Canon and Nikon versions on other sites or in our user reviews.  Everything from sharpness to vignetting to aberrations is comparable to what you would expect from a prime lens.

Let's take a quick look at the Sigma's sharpness via a set of comparative photos with the Sigma 18-35mm and two Pentax kit lenses: the 18-135mm WR and 18-55mm WR.  Click on any thumbnail below for the full-size crop.

The scene for the comparative test

18mm, center of frame

	Sigma 18-35	Pentax 18-135	Pentax 18-55
WO
F5.6
F8

35mm, corner of frame

	Sigma 18-35	Pentax 18-135	Pentax 18-55
WO
F5.6
F8

It's impressive that even wide-open (2 stops faster), the Sigma manages to outperform the two kit lenses in both scenarios.  Its corner sharpness is remarkable for a zoom: stop down a little and there is no softness to speak of.  These results obviously elevate the 18-35mm F1.8 to a higher league, which is why we will be focusing on comparing it to premium primes and zooms in our in-depth review.  There's no question the 18-35mm is a wonderful lens for low-light photography, especially when paired with Pentax's in-camera stabilization (Shake Reduction).

Wide-open bokeh sample (click to enlarge)

Another advantage of this lens is its ability to capture dreamy bokeh.  Creatively, you can't go wrong with a fast lens with close focusing abilities (28cm / 0.23x magnification) and a field of view ranging from wide-angle to normal.

At this point you might think that the 18-35mm is a no-brainer purchase if you're in the market for a premium wide-angle zoom.  Its price tag of $799 is very inviting alongside the cost of buying multiple Sigma primes or Pentax limited lenses.  However, there are two key things to keep in mind before you take the plunge:

• Size and weight
• Autofocus performance

The 18-35mm F1.8 weighs in at 811g, or 1.78lb: about the same as the Pentax K-3 or a DA* 200mm F2.8 lens.  It's 12cm (4.7in) long, and that's without the hood, so it could easily be mistaken for a telephoto lens from a distance.  It's clear that the large size of this lens won't be an issue for many, but it's certainly not designed to be a walkaround lens.   The fact that this lens (with its relatively short focal length) is already so big might also explain why no other manufacturer has been daring enough to market a zoom faster than F2.8.

Finally, we need to touch on autofocus performance.  We observed erratic PDAF (viewfinder) focusing with two K-3's and a K-5 II: at 18mm, the cameras would confirm focus in the distance when the lens was in fact focused on something in the foreground.  Because of the relatively-low magnification at this focal length, you could easily be fooled by what you see in the viewfinder and not notice that your photos were blurry until after downloading them.  This issue prompted us to re-shoot multiple sets of hand-held sample photos.  We will be performing a controlled autofocus test for our in-depth review to offer a definitive verdict on when the Sigma's otherwise-fast AF can let you down (note: we had similar issues with the Sigma 30mm F1.4, but interestingly not the Sigma 35mm F1.4).  Unfortunately, the Sigma USB dock is not yet available for Pentax outside of Japan, so we won't be able to report on this potential remedy to AF inconsistencies.

That's all we can say for now.  So far, the Sigma 18-35mm has shown to be a stellar lens optically, but it has some small drawbacks when it comes to handling and autofocus.  Stay tuned for our in-depth review for more!