Pentax 40mm f/2.8 XS SMC DA

SLRgear Review
May 14, 2012
by Andrew Alexander

Pentax announced the SMC DA 40mm ƒ/2.8 XS in February of 2012, alongside the K-01 mirrorless camera. The 40mm ƒ/2.8 XS is a newer, smaller version of the previous 40mm ƒ/2.8 Limited SMC P-DA, and indeed the two lenses share much of the same design.

The 40mm ƒ/2.8 XS was designed for the K-01 camera, but it uses the Pentax KAF-mount, so it will mount on Pentax cameras since the 70's - whether it will fully function of course, is another matter. On our K-5 test body, the 40mm lens produced an effective field of view of around 60mm, with the Pentax 1.5x crop factor.

At the time of writing (May 2012), the lens is not in stock at major retailers on its own - however, it is bundled with both the K-01 camera and K-5 anniversary edition camera. The camera does not ship with a lens hood, nor does one appear to be available for it: it takes very small 27mm filters, and when it ships, it is projected to cost around $250.

Sharpness
The Pentax SMC DA 40mm ƒ/2.8 XS is a reasonably sharp lens, though you have to stop down significantly to achieve maximal sharpness. Used wide open at ƒ/2.8 it provides fairly good results, with slightly soft corners; stopping down to ƒ/4 offers some slight improvement, and things get somewhat sharper still at ƒ/5.6. At ƒ/8 there is further improvement, more dramatically noticeable in the corners - but not tack sharp across the frame. That's as good as it gets, as diffraction limiting appears at ƒ/11, where we still note very good performance. Things get very lightly softer at ƒ/16 and more so at ƒ/22, but no worse than what's noted wide open at ƒ/2.8.

Considering the small and economical nature of the lens, it's fairly impressive performance.

Chromatic Aberration
The lens is fairly tolerant of chromatic aberration - generally you won't notice any unless you're peeping at 100%. When it does appear, it shows up as magenta-blue fringing in areas of high contrast in the corners of the frame. Technically it becomes a bit more prominent as the lens is stopped down - but not by much.

Shading (''Vignetting'')
There is not much corner shading to speak of with this lens - a hallmark of subframe lenses in general. In this case the only worry spot is when the lens is used wide open at ƒ/2.8, where we note corners that are 1/3EV darker than the center. At ƒ/4 this differential dips below 1/4EV, and after that, it's negligible.

Distortion
There is a slight amount of barrel distortion at play with this lens, at around +0.25% in the corners.

Autofocus Operation
The Pentax SMC DA 40mm ƒ/2.8 XS is incredibly fast to focus, owing to the very small amount of mass the motor has to move - there are only five lens elements in total. It took well under a second to focus from infinity to close-focus. It isn't completely silent when it focuses - there is a slight whirring as it does so - but it's not distracting. It's worth noting that the front element does rotate during autofocus.

Macro
Maximum magnification is just 0.13x for this lens, so this is not a go-to lens for macro work. The close-focusing distance is around sixteen inches.

Build Quality and Handling
Where the Limited edition 40mm ƒ/2.8 P-DA was small, the 40mm ƒ/2.8 XS is tiny, easily half the width of the Limited edition. This is accomplished at least in part by doing away with the distance scale, which was difficult to see anyway. Otherwise the two lenses have much in common.

The only control feature on the lens is the focusing ring: there are no switches or informational surfaces. The focusing ring is just under a quarter-inch in width, made of a polycarbonate with lengthwise ridges inscribed into it. It is easy to find and use, and offers ninety degrees of turning action: there are hard stops on the close-focusing and infinity end. In addition to the front element turning during focusing operations, it's worth noting that there is a little bit of lens extension as well - the central section of the lens extends out from the flush front by a quarter-inch.

The 27mm filter threads are not found on the edge of the front of the lens, but instead form a separate shroud just over the edge of the small front element. The front element lens cap is pushed on over top of these threads.

Alternatives

Pentax 40mm ƒ/2.8 Limited SMC P-DA ~$500
At double the price and double the width, does it offer double the quality? The Limited SMC P-DA does offer significantly more sharpness than the XS model as well as convincingly better performance regarding chromatic aberration. Distortion and corner shading results are the same, and there are a few more frills such as a distance scale.

Pentax 43mm ƒ/1.9 Limited SMC P-FA ~$750
Even rarer than the 40mm ƒ/2.8 Limited, the 43mm ƒ/1.9 offers a larger, heavier platform with just slightly longer focal length but a much wider maximum aperture. At the same apertures, the 43mm is sharper than the 40mm XS, but the 43mm has some issues with CA that the 40mm does not. Corner shading and distortion results are about the same, but there can be no denying that the fit and finish of the 43mm are of a much higher level than the 40mm.

Sigma 30mm ƒ/1.4 EX DC HSM ~$500
While it's a little wider than the 40mm XS, and certainly a bit larger, it does offer a small, walkaround package, and in this case, a very fast maximum aperture. At the ƒ/2.8 and smaller, it's about on par with the 40mm ƒ/2.8 (if a bit sharper in the center, but softer in the corners, at the wider apertures). It distorts a bit more, there is a bit more corner shading, and CA performance is about the same.

Conclusion
Seeing as the lens comes with the K-01 camera body, many people already have this lens - at the time of writing, the lens is not widely available on its own. Many users of the 40mm ƒ/2.8 Limited SMC P-DA may be looking for an even smaller and more portable shooting experience, and they would find it in this slightly less expensive package. However, it's not at the same level optically as the more expensive Pentax offerings, so there is a bit of a trade off between cost and performance - but not so much that it's an obvious upgrade.

Product Photos

Sample Photos

The VFA target should give you a good idea of sharpness in the center and corners, as well as some idea of the extent of barrel or pincushion distortion and chromatic aberration, while the Still Life subject may help in judging contrast and color. We shoot both images using the default JPEG settings and manual white balance of our test bodies, so the images should be quite consistent from lens to lens.

Still Life shot

VFA target

As appropriate, we shoot these with both full-frame and sub-frame bodies, at a range of focal lengths, and at both maximum aperture and ƒ/8. For the ''VFA'' target (the viewfinder accuracy target from Imaging Resource), we also provide sample crops from the center and upper-left corner of each shot, so you can quickly get a sense of relative sharpness, without having to download and inspect the full-res images. To avoid space limitations with the layout of our review pages, indexes to the test shots launch in separate windows.