The Point Spread Function (PSF) can vary at different positions within the sample due to several issues.
For example, it can be asymmetric at different depths within the sample due to spherical aberration (SA), which is mainly caused by a mismatch between the refractive index of the lens immersion medium and specimen embedding medium. Huygens takes SA into account when calculating a theoretical point spread function when the RI of the media are defined as microscopic parameters. For more information on how SA affects the PSF, see here.
Distortions and imperfections in the curvature and coating of the lens can also contribute to variations in the PSF at different locations in the sample.
These imperfections can result in a dramatic phase shift at the edges of the objective lens pupil, causing a decrease in the effective NA of the lens.
One can determine the actual NA of the lens from the angle of the blurring cone (see also NumericalAperture). The setting 'Objective Quality' in the Huygens software allows you to improve the deconvolution result by including the truncation of the lens NA. If you are not sure about the objective quality setting, then it is adviced to leave it at the default setting of 'good'.