Originally posted by Eric Seavey Other lenses are OK, just the Sigma 30mm f/1.4 is not accurate. I do seem to get more in-focus shots when I adjust the AF fine adjustment to -3, but then the shots are out of focus under normal lighting conditions.
Eric.
I cannot be sure but I have a theory:
Many large aperture lenses have spherical aberration. This means that the focus at f/5.6 (where phase AF takes its measure) and at f/1.4 (where the exposure is taken) do differ.
The Pentax firmware has correction data to compensate for this. But it may only include Pentax made lenses. Or it's in the lens rom and Sigma doesn't entirely support the lens rom features.
At normal lighting it would then work because you wouldn't use the lens wide open.
You may be able to test for this by doing a manual focussing at f/5.6 (use the DOF preview to enforce it and try to use a good focus target like a zone plate chart). And then shoot at f/1.4. I occasionally observed sup-par focus using this method with fast some lenses. Even a careful manual focus in 10x LV can be off.
As I said, just a theory. But maybe worth a test.