For landscape shooting, accounting the field-curvature and practice both manual and auto focus with this lens. I guess it truth for any ultra-wide lens when you can get a very sharp center but 1/3 of the rest of the frame is a bit soft because the corners are moving back from the sharp focus in the center. (Hope my English is not too confusing.) It also is depending on your camera’s sensor. I used to use the green version with K-5 and corner sharpness was not problems if I make sure things in the center are sharp. But on K3 high-density sensor, it looks somewhat terrible. So usually on K-3, I focus at around 1/3 from the far corner if I want the whole frame equally sharp.
I often refer to
this article when talk about field curvature on ultra-wide lens. Good read, check it out.
-Edit-
Here are 2 example of what I mentioned earlier about the field-curvature.
Sample1, I pre-focus on the road where the truck is. You can see center is very sharp but corner is not good. In this case, Truck (or whatever car happened to be there) is the main focus of the image, so I didn’t worry about the corner during shooting. But now that I look back, I should have focus at the white bridge behind so that can get sharper starbursts there and still keep the truck sharp + improve sharpness of the building on the right corner.
Sample2, I focus around area below letter “O” on the left hand. You can see the sharpness is better spread out. Center is not as sharp as in sample1. But the whole building is the main focus of the image, and it sharp enough (Model in the large panel center left is not sharp because it is not a still image.)