In an optical system, does the Point Spread Function apply to all light? Or only Point light sources? So in optics, the Point Spread Function (PSF) describes how an optical system responds to a point source of light.  My understanding is that this is due to diffraction and the wave-like nature of Light.
This would lead me to believe that this should apply to any light entering the optical system (indeed, we know there is a diffraction limit for the resolving power of an optical system).  After all, if I drew a ray from my camera to any object reflecting/emitting light into my camera, that light should be diffracted the same as if it were a point source, no?
Where I'm getting tripped up is that point sources seem to be spread far more than I'd expect from looking at an image.  A star for example, can be spread across dozens of pixels despite being as close to a point source as is practically possible.  And this effect occurs even on diffraction limited systems where the resolution of the sensor is at or lower than the diffraction limit of the optics and so, I would assume that diffraction could not be observed.  If the same kind of blurring observed in stars was applied to the rest of the image, all fine detail would be lost.
So what is different about a star/true point source of light, versus everything else?  Does the PSF apply to all light, but is extremely narrow and so is only noticeable for extremely intense light sources like a star?  Or is there something "special" about a truly point source of light that causes it to be "blurred" more than other "broader" features in an image?
 A: Given an imaging system that takes incoming light and focuses it on a detector, the point spread function is a measure of the systems image quality.
Ideally a point image should produce a point at the detector. There are many reasons why it might not. For example, pixels in the detector have a finite size. Lenses can have aberrations, or simply be out of focus. Light is a wave and diffracts.
An image can be broken up into many point sources of light. Each of these produces a spot of light in the detector whose shape is given by the point spread function. The total image is the sum of the point spread function.
This can be useful. You can compare optical systems by comparing their point spread functions.
Given an image at the detector and a known point spread function, it is possible to figure out what the original input image was like. This has been used to sharpen the focus of out-of-focus photographs.
A: For a point source the spread varies as inverse square of the distance. However for a linear source it varies as receprocal of the distance near the source. So the variation of intensity depends on the geometry of the source specially at close distance.
