First, find out if the lens, itself, is bad or the mount is simply out of alignment. Do this, Darkosaric:
Hold the lens, wide open, in front of the mount of an SLR that you have, adjusting it back and forth and close enough to the mount, in order to see if there really is a very well focused image that appears in your viewfinder. If there is, there is a problem with the distance that the lens is to the film plane. I wonder if the cam that guides the focus mechanism on the camera is off. Or, I wonder if one of the elements is either reversed or not put on properly. But, again, first you have to determine that the lens, as it is, is really capable of sharp images but only off in its focus.
Sorry, but to brush this off as "Summars are not good" is rather ridiculous to say. Certainly what the OP has posted is worse than the cheapest box camera ever made and while the Summars might not come up to computerized formula standards at the far sides of the image, the lens is reputable and decent for the era.
Yes, elekm, "infinity" is the key to get right first, Then all other distances should follow. - David Lyga