DA* 16-50mm vs. Sigma and Tamron 17-50mm F2.8 Comparison

Design and Handling

Group Portrait

Pentax DA* 16-50mm f/2.8

The aesthetic design of the Pentax lens will be generally familiar to anyone who's ever handled a Pentax DA-series lens—just kicked up a notch. As with the consumer-grade DA lenses, a green accent ring provides a splash of color between areas of black plastic and rubber. The body of this lens is composed of a very tough grade of polycarbonate, and the rubberized focusing ring is significantly larger and easier to use than what you’d find in consumer zooms. Gold accents are found in the small plaque bearing the DA* logo, the nameplate, and the ring just behind the front filter thread. Other professional touches include a distance scale window (also trimmed in gold) and a dedicated AF/MF toggle. Being a DA* lens, there is no aperture ring, meaning that the aperture must be adjusted via the camera body.

Distance ScaleThe lens is quite heavy at 565 grams without the hood (the DA 18-55mm II kit lens, for istance, checks in at a paltry 220g), and it’s also the largest of these lenses by some margin. With the rather cavernous hood mounted, the lens is particularly intimidating, with a decidedly professional look.

The zoom ring provides nicely damped motion, taking just the right amount of effort to turn. However, the sample we tested had slightly uneven zoom action—the 16 to 28mm zoom range turned much more freely than the 28-50mm range. Since this is the only sample we tested, we can’t say whether this is a problem endemic to this lens, or simply a bit of sample variation, but we’re inclined to think it’s the latter. Regardless, this issue did not prevent us from enjoying the lens in everyday use. Autofocus lenses are notoriously poor for manual focusing, often burdened with excessively short focus throws and resistance-free focusing rings, but the focusing ring on the DA* 16-50mm has a well-calibrated feel with just a touch of resistance. It’s really quite nice to use, though the focus throw is still shorter than we’d like. We don’t think anyone will make manual focus their go-to mode of operation with this lens, but it’s nice to have a reasonable option. The mechanics are absolutely solid, with zero slop or rattle when focusing or zooming, even with the lens fully extended.

Weather SealingThe 77mm front element is a flat expanse of glass that does not rotate when focusing thanks to the lens’s internal focus ([IF]) design. Other optical fireworks include two extra-low dispersion and three aspherical elements, which help to keep chromatic aberrations (aka color fringing) under control and (in Pentax’s words) produce exceptional sharpness. Also present are Pentax’s famed lens coatings, which work to increase image contrast and prevent dust and grease from sticking to the front element. Like the other DA* models, the lens is also weather-sealed, making it perfect for shooting in adverse conditions when paired with a sealed Pentax dSLR body. The lens does extend when zooming due to its dual-cam design, but the seals prevent dust from being drawn inside.

The lens uses the proprietary Pentax SDM focusing system, which is entirely driven by internal motors. However, the lens also includes a traditional slotted screwdrive AF system for older cameras that don’t have the proper contacts to support SDM.

Front Elements

Sigma 17-50mm f/2.8 OS

Like the DA*, the Sigma 17-50mm lens is large (though not quite as long) and heavy (almost exactly the same weight before adding a hood). Also like the Pentax, it’s almost entirely black, with a few gold accents. A gold ring encircles the lens just below the filter thread, and the writing on the lens alternates between gold and white. Thankfully, Sigma released this lens after abandoning their older “crinkle” finish, which was prone to flaking off; their newer finish is a simple semi-suede black matte that looks and feels quite nice. 

The body itself is made of high-quality materials and constructed to tight tolerances. The lens’s Optical Stabilization system does produce a rattling noise when the lens is moved quickly—sort of a lens-based counterpart to the SR-system thud familiar to Pentax dSLR owners. While the DA* lens makes nary a peep and hardly vibrates at all when shaken, there is a very obvious feeling of internal parts sloshing around when you pick up the Sigma. Clearly, the lens was designed with this movement in mind, but it’s still a bit unnerving the first time you grab it. 

Design-wise, the Sigma has a number of shortcomings relative to the Pentax. First among these is the lack of weather sealing. While this will not be a major concern for many photographers, others will miss it dearly. Unlike the DA* lens, the focusing ring on the Sigma is narrow and almost completely free of resistance, producing a decidedly cheap-feeling manual focus experience. Further, the focusing ring doesn’t allow manual focus override when the MF/AF switch is set to AF, as the Pentax does. While it’s similar in terms of resistance and smoothness to its Pentax counterpart, the Sigma’s zoom ring also rotates the “wrong” way for Pentax users (counter-clockwise). The distance scale is far less detailed and is painted on rather than nestled behind a window. Finally, while its HSM autofocus system is every bit the match for Pentax’s SDM, the Sigma lens lacks a slotted screwdrive for compatibility with older Pentax dSLRs. 

But the Sigma also scores a few bonus points in areas the first-party lens doesn’t cover. First of all, it has a zoom lock switch. Like most such switches, it can only be engaged at the widest zoom setting, but it’s handy nonetheless (particularly when removing the lens from the camera). The Sigma’s other major advantage is its built-in Optical Stabilization system. While it’s somewhat redundant on a Pentax dSLR thanks to the company's omnipresent in-body SR, the in-lens OS might possibly up your keeper rate, and it also stabilizes the image in your viewfinder. The use of OS on a Pentax body does have a couple caveats: OS is not designed to be used with the body’s SR system active, and it will not work at all on older bodies that lack SDM/HSM support. This means that if you own a K100D or older body, not only will you not have autofocus, but you also won’t get the benefit of the lens’s OS system.

The Sigma optical design incorporates a number of special elements including two of the company’s special FLD elements, which are similar to the fluorite elements used by Canon. Also used is a Hybrid Aspherical element, with the goal of mitigating all types of aberrations. Super-multi-layer coatings reduce flare and increase contrast for a punchier image. The front element has a 77mm diameter, matching the Pentax DA*.

Rear Elements

Tamron 17-50mm f/2.8

Last but not least, we come to the Tamron challenger. It is by far the cheapest (at $459, or $434 after rebate) of the three, and it has the build and features to match. The lens is composed of polycarbonate that’s tough but less elegantly finished than the stuff used by Pentax and Sigma, and it has very grippy rubber focus and zoom rings. A gold band running around the top of the large zoom ring betrays its high-end ambitions. This band and a glimpse of red under the zoom lock are the only exceptions to the otherwise monochrome design.

Despite the cheaper-looking materials, the build is actually quite sturdy, with no creaks or loose parts. The Tamron checks in at 434 grams before adding the bayonet hood, which makes it roughly 24% lighter than the Pentax and Sigma. It’s also shorter in length and circumference by roughly a centimeter in each dimension. Much of this weight and size reduction is no doubt due to the use of traditional slotted screwdrive autofocus; where the other two lenses have to fit electronics and motors into the lens housing, the Tamron only has to make room for the glass. The zoom ring is smooth and perfectly damped, though the manual focus action here is pretty terrible. The focusing ring is quite narrow, and it offers no resistance and a very short throw.

Befitting its bargain price, the Tamron is pared down in terms of features—almost barebones for a “premium” lens. The zoom lock is the only switch on the lens body. The distance scale is painted on above the focusing ring, and as previously discussed it offers only screwdrive autofocus. (However, the traditional AF system means the lens will work equally well on every Pentax dSLR ever made.) Among other shortcomings are the lack of weather seals, optical image stabilization, and an AF/MF toggle.

The lens sports a 67mm front element that protects a system of 16 elements in 13 groups. The optical design comprises two Hybrid Aspherical elements, and one each of Low Dispersion and Extra Refractive (XR) elements. This last element allows light to be bent at sharper angles, which is also said to contribute to the lens’s svelte profile. As with the other two lenses, the front element does not rotate when focusing.


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