Pentax DA* 16-50mm PLM compared

Pentax has announced that its long-awaited new premium standard zoom will be released in August. The lens features a new optical formula, new coatings, a very fast and silent autofocus motor, and a high degree of weather sealing (“AW”, i.e. “all weather”, the higher of the two weather sealing designations Pentax gives).

The new coatings ensure a more lifelike image by giving a more even transmission of light across wavelengths, for more accurate colour reproduction:

Pentax/Ricoh marketing material

Pentax has a page comparing the lens to its predecessor, the short version of which is that the new optical design is an extreme improvement in every respect.

That’s to be expected, and not what I want to talk about here. First, let me give you a quick impression of how the lens and its predecessor compare in size:

Old DA* on the left, new on the right

You can see that the new lens is approximately 2cm longer – not enough to really affect you when shooting interiors, so that’s good news. It’s also increased by 150 grams in weight, typical for the general development of the market, but a step up if you were using the equivalent Sigma or Tamron before, which are 30-40% lighter (but lack weather sealing).

The lens pushes its inner barrel outwards by what looks like less than 50% extension to reach 50mm focal length:

From Pentax promotional video

This extension is very stable, as can be seen from the care that has gone into this from the development team:

“In the past, we employed a mechanism with a single roller […]. The new lens […] features two separate rollers which fit the width of each groove […] to effectively minimize instability. To reduce unsteady movement along the optical axis, it also incorporates a structure which […] prevents rotation torque of the cam ring from affecting zoom-ring operation, and assures effortless zooming without excessively large torque, while minimizing the generation of unwanted instability. ”

 

We can compare performance to the Sigma 17-50mm, previously held to be the sharpest lens of this kind for Pentax:

Wide open at 16mm (Pentax) vs. 17mm (Sigma):

Wide open at 50mm:

I could not find MTF charts for the Tamron 17-50mm, but reviews suggest it performs at a similar level as the Sigma.

Comparing the Pentax and Sigma, the Pentax simply performs more consistently from wide to long, while the Sigma falls apart a little more at the wide end, and of course the Pentax gives some added flexibility at the wide end without any compromise compared to the Sigma.

I also thought it would be fun to pit the lens against the D FA* 50mm f/1.4, Pentax’ current premium normal prime lens (or slight tele on APS-C) that makes a good portrait focal length on APS-C and coincides with the long end of the newly announced zoom:

That is some extremely good performance for a zoom lens.

The design of the new Pentax lens is quite intricate, as you might expect from a 2021 release, with three types of glass overall, and four aspherical elements of two different glass types:

The focusing group seems to be larger than in the 55-300mm PLM lens, which gives me hope that other designs could be converted with PLM drive.

Compare that to the 2007 design, which has one fewer element, but two more groups:

This design has only two glass types, with the three aspherical lenses composed of normal glass. The frontmost five elements seem to be the same in both designs, and there are some similarities in the way the groups are ordered and arranged. However, aside from the fact that the overall length of the designs is different, giving a big clue that light travels somewhat differently through them, the rearmost seven elements of the new lens are completely different to the previous design.

So the new lens is clearly more than a refinement of the old lens, but it’s also clearly more than “inspired” by it.

The older design, by the way, was adapted by Tokina with a small change in the 4th element from the front, released in 2010, three years after the original Pentax DA*:

Sample pictures show that the bokeh is clearly better on the new Pentax lens, and the marketing copy attributes this to the smaller minimum focusing distance, meaning you can get closer to the subject and therefore blur out the background more. This wouldn’t help you with portraits, so I’m still curious what, if any, bokeh difference would be seen photographing subjects at the same magnification and with the same framing.

Nothing in the lens design gives me a clear intuitive clue that the bokeh should be different, but I have to concede it looks very smooth on sample shots including portraits (unfortunately without comparisons with the old lens).

 

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