I tested out the Pentax-M 20mm f4. This has been my favourite lens for IR on the 20D and it's extremely sharp corner to corner on crop sensor, especially the IR 20D that has no AA filter. Amazing for it's tiny, pancake like size. This lens works on the 5D Mark II as well, but it would hit the mirror at infinity if not used in Live-View; too much of a hassle and that's the reason it's been used only a couple of times on the 5D II. As it turns out, the Pentax-M 20mm f4 works perfectly on the A7; a good match for the A7 sensor too.
This lens has very good resolving power, even near the edges in smaller apertures. By f8, the far edges are quite sharp with good definition. The amount of detail you can dig out from the 24MP sensor of the A7 is incredible, and I am sure it's even more so for the A7R. If shooting landscape is your thing, go for the A7R. For sure, this lens is no Distagon 21mm f2.8, but it puts up a good fight at the fraction of the cost, not to mention it's a millimeter wider :)
- Very good resolving power
- Good edges around f11-f16
- Small size and well made
- Relatively small maximum aperture at f4
- Vignettes at larger apertures
- Needs ultra-thin polarizing filters if you use it