I initially gave the Pentax-D FA 55mm f/2.8 the benefit of the doubt based on the glowing description that Pentax provided. When I received two (2) samples of the 55/2.8 last week, I reported on the haze and apparent scratches with both samples.
Now, having shot the 55/2.8 rigorously, I can say that the Pentax 55/2.8 cannot deliver the image quality required for even half the resolution that the Pentax 645D requires, at least not at far focus, and not outside the central 1/2 area. In fact, the Canon 5D Mark II easily beats it anywhere near the edges of the frame. I’m not sure what Pentax is thinking in releasing a lens with obvious build problems, and one totally inadequate for the 40MP sensor of the 645D. See my comparison page in my review of the Pentax 645D in Medium Format. At closer range, performance may be adequate, but definitely not at longer range, and stopping down has modest effect. It’s not my samples only, as a little internet digging will show from images that others have posted.
I like the Pentax 645D camera body more than any medium format digital I’ve yet used, but anyone getting the Pentax 645D will have to choose lenses carefully to get 40 megapixels delivered from the sensor. Ironically, a used 75mm f/2.8 for $159 easily outperformed the new 55/2.8 in terms of resolution over the frame. And Pentax 6 X 7 lenses (with adapter) also hold out some promise. See my Pentax 75mm f/2.8 lens test in my review.
Making Sharp Images is not easy, but top-performing lenses are a prerequisite. If you want the world’s best lenses, ones having the potential to deliver to the 80MP range, get the Leica S lenses for the Leica S2, but read my review for caveats on the S2 focusing. And hope that Leica upgrades the 37.5MP sensor to 60MP or more eventually, and improves the noise and dynamic range too.
Two crops are shown below, taken from the left edge of the test scene. Mouse over each to see the other crop scaled to match. The 21MP Canon 5D Mark II with the 24-70 zoom easily beats the 40MP Pentax 645D with the Pentax-D FA 55/2.8, whether one upsamples or downsamples to match. Results the same at right edge. Similar results at f/2.8 - f/8. See the full review for much more.