Choosing a Pentax 100mm macro auto-focus lens
Choosing a Pentax 100mm macro auto-focus lens

Choosing a Pentax 100mm macro auto-focus lens

There are six Pentax 100mm autofocus macro lenses and choosing the right one is not as straightforward as it sounds.

Since the advent of autofocus, Pentax has introduced six 100mm macro lenses, each with its set of features, and inevitably, limitations and issues.

Not to delve deeper into describing macro lenses, let’s just run through the basics. As with all other lenses, the focal length is a significant factor for a macro lens – it defines the field of view, the working distance available, and background blur. Macro lenses are also defined by their magnification ratio, describing how close to the life-size subject can be.

All six lenses covered here have a 100mm focal length; all have Pentax K mount, all feature screw-driven autofocus and cover the full frame. Except for the budget ƒ/3.5 version and the latest HD D FA version, all the lenses have the same optical formula and almost the same minimal focusing distance and subject magnification. The latest version, introduced in 2022, uses slightly redesigned optics, with an ED element, while the budget version from the late film era was a rebadged Cosina lens.

Technical data

The lenses are the following:

FFA F2.8FA F3.5D FAD FA WRD FA ED AW
Year of introduction198719911998200420092022
Optical formula9-89-85-49-89-810-8
Aperture rangeƒ/2.8 – ƒ/32ƒ/2.8 – ƒ/32ƒ/3.5 – ƒ/22ƒ/2.8 – ƒ/32ƒ/2.8 – ƒ/32ƒ/2.8 – ƒ/32
Aperture blades888888
MFD0.306 m0.306 m0.43 m0.3 m0.303 m0.303 m
Magnification0.5×
Filter diameter58 mm58 mm49 mm49 mm49 mm49 mm
Length104 mm104 mm72 mm80.5 mm80.5 mm80.5 mm
Diameter74 mm74 mm68 mm67.5 mm65 mm65 mm
Weight590 g600 g220 g345 g340 g348 g

Two earlier lenses – F and FA versions – share many exterior design traits, just like the weather-resistant D FA versions which are almost identical – WR and AW versions. The only different lens of the bunch is the rebranded Cosina lens – completely different from the rest by almost all parameters – it’s the only one with a different look and feel, and the only one with a simplified optical formula and aperture range. Also, it is the only lens with a significantly different optical formula, but regardless of the changes, it is still a good lens – albeit with a less premium build quality.

smc Pentax-F 100mm F2.8 Macro

F-series lenses have a distinct look – two-tone, almost-black and dark-grey outer shell with thin, ribbed, hard plastic manual focus ring, coarse-ribbed aperture ring and red typography. It is not very long, but it has a sizeable diameter and it’s quite heavy, having the feel of a very dense object. 58mm filter thread sits atop the focusing tube, and the lens elements are deeply recessed completely removing the need for a lens hood, as it is already well shielded from stray light. The focusing tube moves forward when focused at closer distances, leading to a shift in the balance.

The focus limiter is a welcome feature. It can limit the lens into three distinct focusing modes – the whole focusing range, from 0.306m to infinity, the limited range from 0.6m to infinity, or the macro range – from 0.306m to 0.6m. With the lens in the full range, the camera autofocus algorithm can (and often will) check the macro range, leading to slow autofocus performance and lost photos. Sometimes, it will get lost in the macro range completely. If the lens is used as a general-purpose short telephoto lens, focusing to 0.6m is perfectly acceptable, and in that case, autofocus performance is quick and decisive.

Pros:

  • 1:1 magnification
  • focus limiter
  • aperture ring
  • the deeply recessed front element

Cons:

  • weight
  • dimensions
  • no weather resistance

smc Pentax-FA 100mm F2.8 Macro

Again, FA-series lenses also have a distinct look – darker-grey outer shell, rubberized, wider manual focusing ring, finely ribbed aperture ring, and subdued, white typography. As with the previous version, this lens also has a deeply recessed front element and a 58mm filter thread. The whole focusing unit moves forward, so the balance shifts with this version as well. FA version has a bit better feeling, the focusing ring is significantly improved from the F version, and the lens barrel has a cleaner appearance. A focusing limiter is present, and works like on the F version – when limited, the lens autofocuses quickly, with no hunting.

With the FA version, Pentax introduced an addition: a focusing clamp. The idea behind the clamp is to increase the resistance of the focusing ring, and consequently, the precision of the focusing ring when focused manually. Unfortunately, many used lenses have a non-functional focusing clamp, or their usability is reduced to a degree. Fortunately, it is not a deal-breaker, the focusing works well even with a non-functional, or partially functional clamp.

As with all FA-series lenses, additional information about the lens sharpness is encoded into the lens’s internal memory. Cameras that can read that info are using it to adjust the exposure in HyperProgram modes. Film cameras with that capability are top-of-the-line models from the MZ/ZX series, like MZ-3 and MZ-S. Almost the same behaviour is reflected in older models of digital cameras – only higher-end models use the encoded data, starting with the K10D.

Pros:

  • 1:1 magnification
  • focus limiter
  • aperture ring
  • focus clamp
  • encoded MFT data
  • the deeply recessed front element

Cons:

  • weight
  • dimensions
  • no weather resistance

smc Pentax-FA 100mm F3.5 Macro

Despite this lens being a part of the Pentax FA lens series, it is not a true Pentax lens, but a rebranded Cosina one. The same lens was available in Canon EF, Nikon F, Minolta A and Pentax K mounts. It was branded as Vivitar, Phoenix, Promaster, Soligor, Voigtländer, and Pentax (only in the K mount). At one point, it was the cheapest prime lens out there, in some mounts cheaper than 50mm ƒ/1.8 lenses. Its build quality reflects the price – it is a fully plastic lens, except for the lens mount. Many reviews describe its plastic as a “toy plastic” and “yoghurt cup” plastic – I’ve used the lens only once and haven’t had the same impression. Its optical formula is simpler than on Pentax lenses, it doesn’t reach true 1:1 magnification, but it had a matched 1:1 converter – a diopter that screws into a filter thread. It was a part of the package but used lenses often have that piece of the kit missing. 49mm filter size is a common filter size on many Pentax lenses and sharing the same size is a plus. Except for the lack of build quality and 1:2 magnification, the lens is a decent performer – not a match for other Pentax lenses, but still a decent lens, especially for the price. It is also the smallest and the lightest lens of the bunch described here.

Pros:

  • aperture ring
  • weight
  • dimensions
  • price

Cons:

  • build quality
  • 1:2 magnification
  • no additional features (limiter, clamp)
  • no weather resistance

smc Pentax-D FA 100mm F2.8 Macro

As for digital cameras, Pentax introduced the DA-series lenses, optimized for APS-C sensors. Macro lenses weren’t part of that downsizing – the 50mm and the 100mm macro lenses both retained their full-frame coverage, leading to D FA designation. The new lens kept the optical formula of FA and F-series lenses, but was repackaged into a significantly smaller and lighter body – the filter thread is now 49mm, in line with many DA-series lenses and the focusing tube is much lighter so the balance doesn’t shift anymore. It features the new design language Pentax adopted with DA-series lenses, but the lens kept the aperture ring and the focusing clamp. Unfortunately, it lost the focus limiter, making the autofocus experience significantly worse than before – many cameras get lost hunting for focus in the macro zone needlessly. The new body design also removed a deeply recessed front lens element, so a lens hood is needed.

Pros:

  • aperture ring
  • weight
  • dimensions
  • focus clamp
  • shares 49mm filters with many DA lenses

Cons:

  • no focus limiter
  • no weather resistance
  • needs a lens hood

smc Pentax-D FA 100mm F2.8 Macro WR

The next iteration is a cosmetic one – optically, the lens is the same as in the previous version, and for all intents and purposes, the same as the original autofocus version from 1987. The big change is the completely new body – this time, it closely follows the sleek design language of DA Limited lenses, with a fully metal build, ribbed metal focusing ring and typography. It gained a WR designation – Weather Resistant – which means it is resistant to dust and light rain. Unfortunately, it gained a “feature” of DA Limited lenses – it lost the aperture ring. That makes this version unusable on simple macro tubes and older macro bellows – the lens is permanently locked to the A position of the non-existent aperture ring – it is fully closed to ƒ/32. Lens adapters also need to have the provision for moving the aperture lever. With no focus limiter, older bodies will hunt for a focus a lot, but on newer digital bodies, that problem is mitigated by smarter AF algorithms. Without the aperture ring, this lens is unusable on the K and M-series manual focus film bodies, from the 1970s, but is usable on film bodies from the 1980s and later – albeit in Shutter priority and Program modes. Optically, it is the same lens as before – it’s sharp, with negligible distortions and vignetting, smooth bokeh and good contrast.

Pros:

  • weight
  • dimensions
  • weather resistance
  • sleek design
  • common 49mm filter thread

Cons:

  • no focus limiter
  • no focus clamp
  • no aperture ring
  • needs a lens hood

HD Pentax-D FA 100mm F2.8 Macro ED AW

After more than three decades of the same optics, finally an update – the latest version (as of 2024) introduced modified optics, with ED lens elements and new, HD coatings. It kept the same sleek-looking metal body, a Limited lens in all but a name, with new typography. Weather resistance got an update, as well, now it is marked as AW – All Weather – indicating the rain resistance is improved. It should be freeze-proof, as well. The aperture ring, focus limiter and clamp are still missing, but some cameras can use a virtual, in-camera focus limiter. Again, no aperture ring means this lens has limited usability on simple macro rails, bellows and tubes – pass-through contacts are required.

Pros:

  • weight
  • dimensions
  • all weather resistance
  • sleek design
  • common 49mm filter thread

Cons:

  • no aperture ring
  • no focus clamp
  • needs a lens hood
  • focus limiter available only on some bodies

Conclusion

Nitpicking aside, all six lenses are good performers, optically. Except for the Cosina version, all are almost the same at the core – a sharp, well-behaving macro lens with no significant optical drawbacks. Even the Cosina version is more than a decent performer.

Mechanically, the design language changed from the big, heavy bricks to the pretty, Limited-like design. Transitioning to sleek-looking bodies, unfortunately, means there are no physical controls on the lens barrels, but when used on the recent DSLRs like K-1 and K-3 III, that’s not a problem any more – the camera is smart enough. All the lenses use the standard KAF mount, with camera-driven autofocus and a fully mechanical aperture mechanism, so the lens mount compatibility is as broad as possible.

For film users, F and ƒ/2.8 FA versions are the best choices –‍ the aperture ring is a must-have on many manual focus bodies, and although strictly technically not required on PZ/Z and MZ/ZX cameras, lenses without aperture rings have more-or-less limited usability. A focus limiter is a significant factor when the lens is primarily used outside its macro range –‍ autofocus on film cameras and older DSLRs can, and will get confused by the macro range.

For digital-only users, the latest version with the All Weather designation is the best choice –‍ improved optics, a more compact body and a camera-driven focus limiter are deciding factors.

For hybrid film & digital users, the choice boils down to priorities –‍ lenses with physical focus limiters are the best choice for general-purpose usage. Lenses without aperture rings (WR and AW versions) are harder to use on film cameras and next-to-useless on K and M-series manual focus cameras from the 1970s and early 1980s. But on digital cameras, the aperture ring is not important at all.

For macro users wanting more than a lifesize magnification, macro bellows, macro extension rings and tubes are a must-have. All those accessories mostly come in two flavours –‍ with and without electric and digital contacts. Simpler, non-electrified versions are usable only with lenses with aperture rings –‍ F, FA and the first D FA version. WR and AW lenses need tubes and extension rings with data contacts.

Using the lenses on adapters, on non-K-mount cameras, is possible and relatively easy; if the lens has an aperture ring, a simple adapter can be used. If the lens doesn’t have an aperture ring, an adapter with its dedicated aperture control is required. Unfortunately, although adapters with an aperture ring are fairly common and readily available, an aperture ring is usually always clickless, with no distinct stops and limited travel, so having precise control is hard and knowing what aperture opening was used is next to impossible.

Finally, the choice of the right lens should follow the camera the lens will be used with –‍ F and FA lenses should be used with film cameras and the latest D FA AW lens with the latest digital cameras. For digital cameras that don’t have an in-camera focus limiter, ƒ/2.8 FA is the best choice, because of its physical focus limiter.

smc Pentax-FA 100mm F2.8 Macro
smc Pentax-FA 100mm F2.8 Macro
smc Pentax-FA 100mm F2.8 Macro
smc Pentax-FA 100mm F2.8 Macro
smc Pentax-D FA 100mm F2.8 Macro
smc Pentax-D FA 100mm F2.8 Macro
smc Pentax-D FA 100mm F2.8 Macro
smc Pentax-D FA 100mm F2.8 Macro
Images of the lenses used from: