HD Pentax-D FA* 50mm F1.4 SDM AW
One topic which has been harassing fast lenses for many years is "focus shift".
This term describes the fact that some fast lenses autofocus to one "focus" plane (remember, DSLRs always focus wide open, so here at F1.4) but if the camera was actually set to a smaller aperture, the focus plane ends up somewhere else.
Focus shift is caused by an optical flaw called spherical aberration which causes light rays from the outer edge of the lens to focus elsewhere compared to light rays from the center. Since stopping down the aperture blocks the light rays from the outer lens area, the focus plane appears somewhere else.
As the name "spherical aberration" implies, it is caused by round (spherical) lens elements and can be mitigated by using aspherical lens elements.
Well-known lenses which suffer from this issue are the Canon 50mm F1.2 L and the older Sigma 50mm F1.4 EX DG HSM or the Nikkor 85 mm F1.4 AF-S. This can make critical focus performance wide open a struggle.
The behavior can be tested quite easily by manually focusing on something (AF turned off) and then stopping down. If the focal plane appears to move, you see focus shift.
Here is the test with the new HD Pentax-D FA* 50mm F1.4 SDM AW lens (all images below are strong crops to show the focus plane).
Simple good news:
The HD Pentax-D FA* 50mm F1.4 SDM AW doesn't suffer from focus shift at all.