When imaging an object the lens (or lens system) creates a blurry image (e.g., a point is transformed into an Airy Disc). This blurring can be described as the Convolution of the sample information (light) with the imaging system (microscope) Point Spread Function (PSF).

The PSF describes the 3D image of the spreading of light from a discrete light-emitting point source.

This is an image (looking from the side) of the PSF of a 170nm fluorescent bead.

3D model of the PSF of the same bead.

Thus, an image can be described as the convolution of the sample information with the PSF of the imaging system. The final image also includes detector (CCD) noise: