# Point spread function (PSF) measurement theory

Point spread function (PSF) describes the volumetric image of a point light source in the object space. But in practice, we measure the PSF by moving the light source (the fluorescence bead) and fixing the CCD. Are these two equivalent?

A simple illustration of my question is described as follows. This first is the designed object and detector. The second one means the object is shifted(which causes defocus). And the third one is the shift of the detector. My question can be simplified to, can it be proved that in the second and third case, the detector get the same image?

Complemented on PSF

I think the PSF of the a 3D imaging system can be describe as the images that a point source creates on different layers that the detector take as different depth rather that the real 3D image that the point source create in the image space(these two cases correspond to the second and the third case in the figure). In this sense, despite the fact that maybe there is no equivalence of the second and the third case in the figure. The formation of the imaging and the process of measurement of the PSF do be the same(the case two)

Hence, it is absolutely OK to use the measured PSF to do deconvolution.

• In an anisplanatic(spatial variant) system, I think they are not the same. Let's say the system has depth variant PSF. When the source is fixed and the CCD is moving, then what the CCD get is the PSF at the corresponding depth, let's say at $z_0$. But when the CCD is fixed and the source is moving up and below $z_0$, the PSF changes, let alone the image the CCD get. This time, the PSF that CCD takes surely differs from the previous one. – Rickyim Mar 4 '18 at 11:20