Chromatic aberration
Chromatic aberration in a lens is the consequence of having a Refractive Index which is dependent on the light WaveLength. Therefore, different colors are focused in different positions, and the image is blurred.
The wavelenght-dependent refractive index can be in the objective lenses, but in most of the microscopes this effect is (at least partially) corrected.
The problem can be then caused by the coverslip, the Lens Immersion Medium, or the Specimen Embedding Medium, or by the microscope having misaligned light paths, still causing a → Color Shift.
The New PSF Distiller Option in the Huygens software reports the Chromatic Aberration. With that value you can then correct for it via the Chromatic Shift Corrector available for both the Huygens Professional and Essential. See Color Shift for more details.
For more details on chromatic aberration, see the Wikipedia
.