HD Pentax-D FA 28-105mm F3.5-5.6 ED DC WR

Construction and Handling

The full-frame D FA 28-105mm F3.5-5.6 ED DC WR bears a stark resemblance to the popular DA 18-135mm F3.5-5.6 walkaround zoom designed for APS-C cameras.  Unless they are placed side-by-side, the two lenses look nearly identical.

Size Comparisons

Full-frame 28-105mm (left) vs APS-C 18-135mm (right)

Both lenses share the same 62mm filter thread, hood, and barrel diameter; at 440g the 28-105mm is only 35g (9%) heavier and 1cm longer.  Ultimately, though, the two optical designs are completely different. With 15 elements in 11 groups, the 28-105mm delivers full-frame image circle coverage at the expense of some reach at the long end.  On the other hand, the 18-135mm barely covers more than the APS-C image circle throughout its zoom range... but we digress.


Full-frame 28-105mm (left) vs APS-C 18-135mm (right)

Things don't change much at the telephoto end of the zoom range, where the 28-105mm is still only slightly longer (physically).  While we didn't observe any zoom creep with our copies of the 28-105mm lens, notice how the 18-135mm in the photo above stopped shy of the 135mm mark.  A small amount of creep was to be expected of the 18-135mm's barrel design, but this seems to have been addressed on the 28-105mm through more zooming friction.  Only time will tell whether or not zoom creep will rear its head following prolonged use.

D FA 28-105mm (left) vs D FA 24-70mm F2.8 (right)

Above we compare the size of the 28-105mm to the current premium Pentax standard zoom lens for full-frame: the D FA 24-70mm F2.8.  As a wide-aperture lens, the latter weighs in at 787g (nearly twice as much) and is 2cm thicker throughout with a 82mm filter diameter.

D FA 28-105mm mounted on a Pentax K-1

With the 28-105mm mounted, the Pentax K-1 body dwarfs the lens and as such we see this as an excellent combination for travel or when a lightweight "all-in-one" kit is desired.  Loaded and ready, the pair weighs in at 1450g, or just over 3 pounds.  Compactness is usually not what comes to mind when we think of a full-frame DSLR system, but the 28-105mm defies the norms, just like Pentax's many Limited primes. We thus see size as a key selling point of the 28-105mm.

Construction and Weather Sealing

This 28-105mm lens is part of Pentax's D FA lens series, which consists of digital-era lenses designed for full-frame cameras.  Like all other D FA zooms released to date, the 28-105mm has a weather-sealed design (WR) which employs a series of internal and external rubber gaskets to protect the lens against moisture ingress.  The front element is also treated with Pentax's proprietary super-protect (SP) coating to make cleaning easy and to repel dust— this coating is not to be underestimated, as it truly makes cleaning a breeze.

An identifying feature of WR lenses is a red gasket around the lens mount, which otherwise houses the AF power contacts, aperture information contacts, and stop-down lever.  Click on either thumbnail to enlarge:

Since the 28-105mm does not zoom internally, however, we do not recommend operating the zoom in moderate to heavy rain, as the pumping action of zooming has the potential to suck moisture into the barrel and subsequently into the camera.  The lens lacks the all-weather (AW) designation, which is Pentax's higher tier of weather resistance that adds dust protection and a greater degree of moisture protection. AW is only available on select premium lenses.

9 o'clock side of the lens barrel

If we look at the lens barrel, we observe that the majority is comprised of the zoom ring, which is large and lined with a rubber-like material to afford a comfortable grip without hampering precise control over the zoom. Since the weather sealing adds a moderate amount of friction to the zooming process, the sizable zoom ring is very welcome.  Click on any thumbnail above for a larger view of the lens.

A small focus ring sits near the base of the lens.  It is not flush with the zoom ring, and has a slightly different texture, so you can easily tell which is which without looking.  Its placement is also convenient, as it can easily be turned with the thumb while the zoom ring rests in the palm of your hand.  We discuss focusing in more detail on the following page.

Over time, the material that spans the zoom ring is fairly prone to attracting dirt particles and grease.  This can discolor the lens but is also relatively easy to clean with a manual air blower, lens cloth, and distilled water.

The front of the lens is rather plain with nothing more than the front element itself and a bayonet for attaching the hood.  Click on either image to enlarge:

It's easy to attach the hood and it fits snugly with virtually no play.  The bottom of the hood features a detachable window to allow for easy adjustment of polarizer filters while the hood is mounted.  You can also reverse-mount the hood when not in use.

Overall, despite the low price point of this lens, it appears to be built to tight tolerances.  The external barrel is made entirely of plastic but gives no impression of low quality.

Labeling and Markings

The full lens name is printed around the leading edge of the zoom ring, just behind the decorative green ring. You'll find retro-style label of "3.5-5.6/28-105" at the base of the lens, with an embossed D FA badge at the 9 o'clock position and a printed Pentax logo at the 3 o'clock position (this placement is a bit awkward perhaps).  The serial number is found between the latter two, at the 6 o'clock position.

The zoom marker is engraved at the base of the lens while the focal lengths are printed around the zoom ring.  Finally, two markers to help with lens hood alignment are engraved near the hood bayonet.

Handling

With its small size and light weight, the 28-105mm poses few problems out in the field.  It doesn't look intimidating, even when zoomed to 105mm.  All in all, it's a handy lens that can easily accompany you wherever you go.

The 28-105mm balances very well on the Pentax K-1, and due to its weight, it does not necessitate the use of a grip for comfortable vertical shooting.

Our only complaint with respect to the lens is minor: the friction on the zoom ring is slightly more than we're used to seeing, but as we mentioned earlier, this change may have been made to reduce creep in the long term. Without a distance scale or clear markings, the focus ring is also not the best for manual focusing.

Verdict

We are pleased with Pentax's modern version of a compact full-frame walkaround zoom lens.  With a comfortable zoom ring, no creep, weather sealing, and an ergonomic design, delivers a degree of convenience that puts film-era lenses to shame.

Apart from the minor issues we listed above, you may need to acquire special filters for this lens, as the 62mm diameter is only shared with a handful of other Pentax lenses, including those that are based on Tamron designs.


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