Spectacular wide angle effect. Moderate vignetting. Lens hood a must. Center sharpness is crisp at about f/8-f/11. Edges less crisp and contrast is so-so. It was too wide for me. I sold it. The 15mm Heliar is cheaper, sharper, smaller, and more contrasty. But if you need the ultimate RF wide angle lens, then you might like the 12. Also, the 12 requires that the camera not be tilted too much, otherwise straight lines become really badly bowed and distracting. I think the 12 is a special optic for very limited use. The photos that I shot of a 9-11 memorial construction obelisk were not acceptable to me.
I agree. And I also found that I like the 15mm Heliar better.