1/2 stops is within "K mount" tolerance based on my experience. The problem is that every K mount body and lens were calibrated with certain tolerance, and each sample is different. Certain combination can be off by as much as 1/2 stops. You can recalibrate that Sigma matching your K200D to your taste, but you might be surprised again with the next body upgrade. Still, you can disassemble the lens mount and bent the metal piece guiding the rear lever inside slightly. Kind of ugly design but that's Sigma.