The Pentax 31mm ƒ/1.8 Limited Lens

By Carl Weese

It’s been a while since I’ve posted an installment in my series of articles on digital photography centered on use of the Pentax K10D camera. A number of readers wrote to ask my opinion of the Pentax FA Limited AF lenses, which have something of a cult status as premium lenses for 35mm film photography. There’s a trio of them, at 31, 43, and 77mm focal lengths. Recently, thanks to the elves in Golden, Colorado, a 31mm Limited found its way to my doorstep. This is the shortest lens in the trio, which is good because I do at least 99% of my photography with short or normal lenses. 31mm works out very close to a classic normal focal length (equal to the diagonal of the frame) when used on an APS-C digital sensor. It’s a bit wider than the traditional normal-for-35 length of 50mm.

My first impression when I got it out of the box was that this is the most beautifully made and solid-feeling AF lens I have ever encountered. In fact, before I’d even gotten it onto a camera body, I had a sort of déjà vu sensation, really just a muscle-memory. It took a minute, then I realized that the lens reminded me, physically, of the Leitz Noctilux. Pretty exalted company to keep. It’s a substantial lens with lots of metal and glass in it. The wide focus and aperture rings are textured metal, not covered with plastic. The helicoid is very smooth, but not quite the oiled-silk feel of the best manual focus lens helicoids because, even when used in manual, there is some sort of gear being driven. There’s a barely audible whir and the slightest vibration feedback to your hand. You’d have to be a nut to object to it, though.

None of this was a surprise, given the reputation the lens has earned over the years. What I was really curious about was how this legacy (i.e., designed for film) lens would perform in the world of digital capture. As mentioned, Pentax uses the APS-C sensor size so the lens is transformed from a moderate wide angle to a “normal” angle of view. Without getting sidetracked into something that is actually a very large issue, there are two important problems with legacy lenses. One is that film doesn’t care how oblique an angle the light rays from a lens strike it. Digital sensors do. With normal to wide lenses the angle of the light rays can be really oblique, and this can cause a lens that was very good at laying an image down on film to be a poor performer on a digital camera. When SLR cameras were introduced, “retrofocus” wide angle lens designs had to be invented. This is a lens that is physically farther from the film than its effective focal length, necessary to get the lens out of the way of the SLR mirror. A similar manuever is needed to make normal and wide lenses for dSLRs deliver their light rays nearly perpendicular to the sensor.

The second legacy lens problem (there are more than two but these are the ones we can actually see and deal with as consumers) has to do with the highly reflective glass that covers the digital sensor. It bounces a lot of light back to the rear element of the lens, far more than film. This means that a lens meant for digital capture needs just about as much attention to anti-reflective protection for light coming back from inside the camera as it does for the imaging light coming in from the scene being photographed. I’ve used designed-for-digital lenses for all of my work with dSLR cameras over the past three years, but recently I tested a 28mm legacy lens that clearly showed both these problems. Definition wasn’t great anywhere, but got worse at the edges, even though this is not a wide angle lens in context, but a nearly perfect normal (format diagonal) length for the sensor. Worse, the internal reflection problem was, well, glaring. In any picture with bright highlights, the capture showed a highlight-colored overall haze of non-image fog/flare density. Nasty.

So to conclude the first of several reports, let me go from that long-winded introduction to a short and sweet conclusion. In my use of the 31mm Limited—around 1,200 captures so far under a wide range of conditions —I can find no evidence of a “legacy problem.” In terms of resolution, including all the way to the edges of the frame, it is at least equal to results from the Pentax “pancake stack” of designed-for-digital compact DA Limited primes at 21, 40, and 70mm. Resolution is noticeably better than the designed-for-digital 12–24mm zoom, which is no slouch in its own right. I can’t detect any evidence of problems stemming from sensor flare. If the designers could do anything to make this lens more digital-ready, I don’t know what it would be.

Comments

[Jeff Kott]

The 31 is a great lens, but the integrated lens hood may be a little small for APS-C size sensor and it's not very protective.

Thanks to the suggestion of Lance Blackburn, a poster on the Pentax SLR forum at DPreview, I've added three empty 49mm filter rings as a solution to the problem.

Here's a photo to show how this works:

http://www.pbase.com/lance_b/image/43928349

and you can add a filter holder:

http://www.pbase.com/lance_b/image/43928357

[Carl Weese]

Jeff,

That looks pretty cool, but the filter thread is 58mm, not 49. Because the integral hood was designed for a much broader angle of view, I've been on the lookout for any tendency to flare due to the less than optimal shading. Haven't found any. It does better than the 21 or 12-24 lenses, which you'd expect since it is significantly narrower in view, and near as I can tell does just as well as the 40DA with its custom hood, which is a good performance.

[Jeff Kott]

Hi Carl,

OOPS/ I meant 58mm.

The main advantage for me is protection of the front element. I usually go commando as I am in the do not use a protective filter camp, so I always rely on my lens hood for protection. The integrated lens hood with the 31 leaves me feeling a little naked. Adding the three empty filter rings makes me feel much safer and the original lens cap fits over perfectly.

Jeff

[Boris Liberman]

I have all three limiteds and IMO the 43 is the best built one. However all three, including of course the 31 one, are marvelous performers...

Thanks for the review. I had pleasure reading it, especially the bottom line on lack of so called legacy problems ;-).