Pentax HD DA* 16-50mm F/2.8 PLM AW Review

Pentax created a new optic formulation to make the DA* 16-50mm F/2.8 LM AW worthy of the 26 megapixels K-3 III. Optics in this new top-of-the-line lens are truly impressive. They deliver nearly perfect image-quality at all combination of focal-length and aperture. This lens has been optimized for absolute uncorrected image-capture which makes it fairly large and heavy.

This lens delivers crisp details and high contrast with great color. The HD coatings minimize flare and virtually eliminate chromatic aberrations. The section below characterizes image sharpness in detail.

The DA* 16-50mm F/2.8 PLM AW shows virtually no distortion at any focal-length. At the widest 16mm focal-length that captures an 84° angle-of-view, there is less than 1% barrel distortion. Zooming in, even slightly, completely removed any distortion all the way to its 50mm maximum focal-length. This avoid having to correct in software which is detrimental to image-quality and affects framing.

Some vignetting is present and it varies according to the focal-length an aperture. Most ultra-wide lenses show significant corner shading but this new one certainly performs better than most. At 16mm with the aperture wide-open to F/2.8, there is full-stop of vignetting. This is immediately visible in images and not usable without correction. When Vignetting Correction is enabled on the Pentax K-3 III, this vignetting almost vanishes, leaving barely 1/8 EV darkening at extreme corners. Stopping down to F/3.5, vignetting becomes 3/4 EV. By F/4, it goes down to 1/2 EV which is not usually noticeable except against a uniform background. It reaches a minimum of 1/4 EV starting at F/4.5. Contrarily to distortion, software vignetting correction has minimal impact on quality.

Zooming in a little greatly diminishes vignetting. Even wide open at 28mm, there is just 1/4 EV of corner shading. This level can also be seen at F/3.5. Stopping down any further brings it down to a negligible 1/8 EV! In-camera correction completely removed vignetting at middle focal-lengths. Towards the long end of the zoom, vignetting increases a little. At F/2.8, one can measure 2/3 EV of vignetting but this quickly diminishes. Just at F/3.5, it comes back down to 1/3 EV and levels-off below 1/4 EV from F/4. Software correction eliminates vignetting completely too at long focal-lengths.

Pentax 16-50mm F/2.8

Pentax DA* 16-50mm F/2.8 PLM AW Sharpness

Sharpness from this new lens is spectacular and extremely consistent. Broadly speaking there is slight softness at the widest focal-length with very good corner-to-corner consistency. Zooming in almost all the way to the longest focal-length makes the lens perfectly sharp, while a minor amount of softness peeks back at corners when reaching the telephoto limit. Diffraction sets in at F/13 when mounted on a Pentax K-3 III.

From about 20mm to 45mm, the Pentax DA* 16-50mm F/2.8 PLM AW is perfectly sharp corner-to-corner at all apertures until diffraction appears. Wider than that there is a little software at F/2.8 and F/3.5 which is slightly stronger at extreme corners. Stopping down to F/4 produces a sharp image-center with minimally soft corners. At 16mm F/4.5 and beyond, corners become quite sharp. The long end only has a tiny amount of corner softness at F/2.8-3.5 and everything is perfectly sharp from F/4 to F/11.

What is shown below are 5 crops taken from a photograph, repeatedly captured for each combination of focal-length and aperture. The smaller pieces are cropped from the extreme corners of the image, while the middle wide crop comes from the center of the image. Select an aperture in a row for a desired focal-length to see the crops from the corresponding image. When judging quality, keep in mind that these crops come from a 26 MP image which is normally used to print an image up to 30x20". On a computer display, these may appear much larger which magnifies image defects.

Upper Left

Upper Right

Center

Lower Left

Lower Right

One cannot fail to be impressed by the incredible consistency of the optical performance of this lens. It really manages outstanding output with nearly every combination of focal-length and aperture. Vignetting which is on the high side at wide-angle can be safely and nearly completely removed using in-camera compensation. The only issue remaining is that slight softness at ultra-wide angle which is only visible when viewing images at 100% and on the largest print sizes.

Autofocus Performance

Even though the Pentax K-3 III
Pentax K-3 III launched with a state-of-the-art 101-Point SAFOX XIII autofocus system, very little improvement could be observed, until now. The majority of Pentax K-mount lenses have internal focus motors driven by a mechanical link. This limits autofocus speed to the slowest component. Pentax produces just 10 SDM lenses and most of those struggled. A similar number of DC lenses were introduced. Pentax experimented with a PLM motor in the entry-level Pentax HD DA 55-300mm F/4.5-6.3 ED PLM WR RE
Pentax HD DA 55-300mm F/4.5-6.3 ED PLM WR RE that was release in 2016. After a five year gap, the new DA* 16-50mm F/2.8 PLM WR became the first professional-grade lens with a linear motor in the Pentax lineup.

A Pulse Linear Motor is capable of driving lightweight optical elements quietly with great precision which is why it requires an Internal Focus [IF] lens design. This allows the new DA* 16-50mm F/2.8 PLM to focus extremely quickly when paired with a K-3 III. Autofocus locks virtually instantly in good light. Under dim lighting, it only becomes fractionally longer, focusing in just over ¼s, even for onto contrast subjects.

With its constant maximum aperture, this lens focuses just as quickly at any focal-length. DSLRs always focus with the lens wide-open, so the chosen aperture does not impact AF performance either. The only exception is during video capture when aperture changes will be apparent in the recording. Due to its fly-by-wire focus-ring, this lens is not suitable for serious video work anyway.

The Pentax HD DA* 16-50mm F/2.8 PLM WR significantly improves over the autofocus speed of its predecessor. Depending on conditions it focuses up to 4X faster and much more quietly! The lens is not totally silent but its sound is likely to be drowned by ambient noise under typical circumstances. Autofocus accuracy is ensured by micro-adjustment supported by K-mount DSLRs.