Chromatic aberration in a lens is the consequence of having a Refractive Index which is dependent on the light WaveLength. As a consequence, different colors are focused in different positions, and the image is blurred.
The wavelenght-dependent refractive index can be in the objective lenses. Therefore, many microscopes are equipped with apochromatic lenses to minimize this form of distortion. Frequently, this distortion correction is only partial. Furthermore, apochromatic lenses are optimized for only a limited range of wavelenghts. Thus, additional chromatic aberration correction may be necessary.
Additional chromatic aberration problems may be caused by the coverslip, the Lens Immersion Medium, the Specimen Embedding Medium, or by the microscope having misaligned light paths. As result, different channels may be shifted (see Color Shift) with respect to one another, show different scaling, and exhibit rotational misalignment.
Huygens Chromatic Aberration Corrector option can automatically estimate and correct for these aberrations.
Also, when distilling a PSF from bead images using the PSF Distiller Option in the Huygens software, the chromatic shift is reported. With that value you can correct the shift via the Chromatic Aberration Corrector.
For more details on chromatic aberration, see the Wikipedia.