Lenses made for 3-CCD sensor focus different colours at different depths because the optical path of red, green and blue through the prisms is different. B4 lenses are designed to compensate for this effect.
Spherical aberration occurs when B4 optics are used with sensors using the Bayer filter.
 |  |
|---|---|
| 3-CCD sensor schematic | Sensor w/ Bayer filter |
Spherical aberration ≠ Chromatic aberration
Chromatic aberration is not corrected by the corrective optics (the green & purple fringing remains visible)
Note:
Engaging the doubler corrects some amount of spherical aberration, but not all of it.