I recently became a bit obsessed with getting a nice copy of the Pentax 31mm Ltd. The first copy I bought was an early first batch MIJ specimen. In general use, it performs great and when tested with a resolution chart at a closer distance, it was sharp corner to corner. Oddly though, tested at infinity as used for a typical landscape, the corners and borders were just not what I would hope for. Maybe Pentax designed the first batch to \"test\" well on charts.
Fast forward to last week when I came across what appeared to be a never used NIB, though listed as Mint, MIJ from a later production batch. I couldn\'t resist just seeing how it compared to the first copy on my A7r, as well as how it compares to the Pentax 28mm 3.5K which Tim has tested here. Turns out the earlier copy tested better on the resolution chart across the frame (at a distance of only about 3ft) but the newer version was clearly better at distance into the corners and borders. Even though this lens is a floating element design, apparently there is still some room for tweaking for one distance vs another.
With all that out of the way, here are links for the comparison of the later version Pentax 31Ltd to the Pentax 28/3.5 K on my A7r. Tripod mounted, same adapter (Rayqual) no additional sharpening in conversion through ACR. This also shows the difference in aspect ratio between the two lenses. The F-stop is indicated by the last number in the file name.
