When light waves encounter an object – such as the sample of interest, a lens, or even air molecules – they scatter & bend. Within an optical system, such as a microscope, this effect on a light wave is called the point-spread function, commonly abbreviated to PSF.
The PSF applies to every point of light passing through the optical system, in a process called convolution.

Digital images invariably suffer from the addition of some amount of noise that is introduced through a combination of ambient factors and the materials and electronics present in the digital camera hardware.
Signal-dependent noise can be characterized by a Poisson distribution, while noise arising from the imaging system often follows a Gaussian distribution.