Lens Review: Pentax FA 31mm f1.8 Limited

The Pentax FA 31mm f1.8 Limited was first offered in 2001 and is still in production as of 2011. It’s one of three FA Limited prime lenses. FA is the designator for full-frame lenses that are not specifically designed for digital cameras. FA Limited lenses come with film-era features like an aperture ring and all metal casings with an emphasis on a hand-assembled feel of quality. There aren’t many lenses offered these days that have such properties.

Technical specifications for the lens:
Lens Mount: PENTAX K AF
Lens Construction: 7 groups 9 elements
Angle of View: 70 degrees
Aperture: F1.8
Number of Diaphragm Blades: 9
Minimum Aperture: 22
Minimum Focusing Distance: 12 inches
Filter Diameter: 58mm
Maximum Magnification: 0.16X
Dimensions: 2.6 x 2.7″
Weight: 12.2 oz.
Case: N/A
Objective Cap: N/A
Hood: N/A
Lens Aperture Ring: Yes
Tripod Adapter: No
Diaphragm Control: Fully automatic
Focal Length: 31mm (equiv. 46.5)
Notes: N/A

General talk about the lens:
As the lens was introduced around 4 years before the first Pentax digital SLR was released, it wasn’t designed for digital in mind. The focal length of 31mm is a nice wide angle lens on full-frame, but in APS-C it turns into the equivalent of a 46mm normal lens. The maximum aperture of f1.8 is quite fast, being only 3/4ths of a stop slower than the standard of f1.4 that many fast 50mm normal lenses have. In full-frame terms, it’s an impressive focal length and aperture combination.

Build quality is very high for this lens. It has an all aluminum casing with a metal mount and a built-in aluminum hood (designed for full-frame of course) with black felt on the inside to minimize reflections. The lens cap is also aluminum with green felt on the inside to provide a tight friction seal. The focus ring and aperture ring are machined aluminum to fit with the casing quality.

The focus ring does not feature quick-shift functionality and rotates when auto-focus is activated. The lens does not have internal focusing, so the front part of the lens moves a bit when focusing. It uses screw driven auto-focus as it does not have a motor inside.

Images straight from the lens are very sharp throughout the frame. Background blur is top-notch, producing smooth featureless rendering with sharp in-focus areas that gives a 3D effect in the right situations. The lens naturally produces warm high contrast results taking perceived sharpness of the lens even further. These three properties together in a wide angle lens are what make it so special.

It isn’t without flaws though. My copy of the lens sometimes produces color fringing in situations such as when strong direct sunlight is aimed toward the lens, or on on the edges of high contrast subjects like a person’s clothing.

Supplemental Video Review:
http://www.youtube.com/watch?v=3rInLliIUMI

What I like about the lens:
- The aluminum construction is high quality and has a great feel in the hands to it. I wish that more lenses were offered in aluminum like this. All I can say is find one to try and you will understand.
- Sharp high contrast results with a warm feel to them.
- Nice smooth background blur with the possibility of a 3D look.
- Overall size of the lens fits well with current DSLRs (around the same size as an 18-55mm kit lens). It has enough room for your left hand to grip the lens while shooting.
- The focus ring is easy to use in manual mode.
- It’s a lens that should serve you for many many years, especially considering it is a full-frame capable lens.
- The normal focal length on APS-C is pretty good and I probably use this lens more than any others that I own.

What I’m not fond of:
- It’s expensive, especially considering on APS-C it has the properties of a “fast and cheap” 50mm lens. Just keep in mind 35mm lenses are usually more expensive, except for the DA 35mm f2.4 that was released this year.
- My copy of the lens isn’t a stranger to purple/green fringing.
- I don’t like how the front section of the lens connects to the back. It makes the lens seem lower quality than it is. Mine has a very slight wobble to it, but I think it’s within tolerances from what I’ve read online. This lens would be much more impressive if it had internal focus.
- 9 aperture blades don’t seem like enough or the blades should be more rounded. My aperture blades don’t produce perfect circles unless the lens is close to maximum aperture.
- Doesn’t have newer features like WR (Weather Resistance) and quick-shift that would improve usability of the lens.
- The focus ring makes a geared sound when being rotated, which makes the lens sound a bit lesser than it could from a perceived quality standpoint.

Revision wish list:
This is a quality lens overall, but I do see some room for improvement. Here is what I’d like to see in a revised 31mm Limited lens.
- Change the designation to something like D-FA 31mm f1.8 Limited WR.
- Make the lens completely internal-focus and give it quick-shift ability.
- Add weather resistant sealing.
- The optical design is already strong, but improve it if possible. The same goes for lens coatings. Make sure lenses of this caliber have a narrow sample variation. From other 31mm reviews I’ve seen, it looks like the purple fringing issue could be due to sample variation and not an issue of the older design.
- Remove the built-in lens hood.
- Keep the aperture ring.
- Keep the all metal build quality.
- Keep the screw driven auto-focus.

Image talk and analysis:

Very high sharpness can be achieved.
Purple fringing in extreme light.
Nice uniform detail at f5.6.
A 3D effect is possible.

Post-processed samples:

Conclusion:
This is probably my most used lens ever. It’s right in the middle of where I need a focal length and has the ability to produce nice smooth backgrounds with sharp high contrast rendering. The lens has great build quality and features, but I feel there is room for improvement if Pentax would take the time and effort to make it happen. I just hope that if they do revise the lens, they will keep the features that make it special such as the all-metal build and FA features like the aperture ring, but also improve the perceived limited nature by adding things like internal focusing and quick-shift. I look forward to someday using this lens on a full-frame Pentax DSLR so that I can see the true wide-angle nature of this lens, but until that day this will be my general take everywhere lens to get the job done. If you can afford one, there isn’t any other reason big enough not to have one in your kit. Chances are you will start neglecting your other lenses for the 31mm, so keep that in mind!

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