8 out of 10 points and recommendedvery small and light, well-builtnoticeable barrel distortion, f3.2 max aperture
Very good all around prime lens. A pleasure to carry and useful in many situations.reviewed October 17th, 2007 (purchased for $400)
Although it is pretty sharp, especially stopped down a bit, the 21mm does not have a super fast maximum aperture. That can be a factor for low light shooting or when trying to get limited depth of field effects.
The 21mm also has definite barrel distortion. This can be corrected in Photoshop, but that is not ideal.
So there are compromises. Still, this is the lens I leave on my K10D 90% of the time, because of its useful angle of view, its good overall image quality, and its amazing portability and toughness.
I can easily shoot holding this combination one-handed at my waist, like a Leica. Or, on the other hand, I can compose carefully through the viewfinder, and correct the lens' barrel distortion in Photoshop (making good use of the extra 5% of image area not visible in the K10D's finder!).
10 out of 10 points and recommendedrazor sharp, fast, built like a tank, nice bokehexpensive
This is an amazing lens, at a premium price.reviewed October 31st, 2007 (purchased for $770)
The 31mm can be used on a 35mm Pentax film camera or on a digital camera. On the K10D, with its cropped sensor, it becomes the equivalent of 47mm on a full frame camera. Used that way, as I do, it is essentially a fast "normal" lens.
This is one of the sharpest lenses I have ever used (and I have used many Zeiss and Leitz primes as well as Canon L lenses). Even wide open, it renders impressive fine detail. Stopped down to middle apertures, it is razor sharp and contrasty from corner to corner.
The difference in image quality compared to my 21mm, 40mm and 70mm Pentax Limited lenses is noticeable immediately in RAW files, before I do any sharpening. The detail rendered by the 31mm seems as if it has burned right through the camera's anti-aliasing filter. Grass, leaves, distant buildings--this lens makes the most of what the K10D sensor can handle.
There is minimal barrel distortion and CA. Purple fringing is well controlled, although in some circumstances (wide apertures, very high contrast) it can be present. The lens renders out of focus areas smoothly.
The 31mm is fast, with decent sized glass elements, and the build quality is top of the line. As a result, it has some heft, although it is not really large or heavy. It focusses quickly, probably because of its wide max aperture. The lens hood is permanently attached.
If this focal length is what you need, and if you can afford it, this is the real deal--a high-performance, premium-built lens destined to be a classic. I have not used it on a full-format camera, but I imagine it would make a great wide-angle in that case.