Diffraction Limit

An ideal optical system would image an object point perfectly as a point. However, due to the wave nature of radiation, diffraction occurs, caused by the limiting edges of the system’s aperture stop. The result is that the image of a point is a blur, no matter how well the lens is corrected. This is the diffraction blur or Airy disk, named in honor of Lord George Biddel Airy, a British mathematician (1801–1892). Its cross section and its appearance are shown in the figure below.

Airy disk, energy distribution and appearance.

If an image is made through a small aperture, there is a point at which the resolution of the image is limited by the aperture diffraction. As a matter of general practice in photographic optics, the use of a smaller aperture (larger f-number) will give greater depth of field and a generally sharper image. But if the aperture is made too small , the effects of the diffraction will be large enough to begin to reduce that sharpness, and you have reached the point of diffraction-limited imaging.

If you are imaging two points of light, then the smallest separation at which you could discern that there are two could reasonably be used as the limit of resolution of the imaging process. Presuming that diffraction is the determining factor, then the generally accepted criterion for the minimum resolvable detail is the Rayleigh criterion.

This shows the intensity curves for the radial distribution of the diffracted light for different separations. Your eye sees the characteristic bulls eye distribution of light as illustrated below.

For modern digital photography where the images are projected onto a CCD, the information is collected on pixels of the digital detector. At left is an attempt to show the effect of diffraction on such imaging in cases where the diffraction is the phenomenon that limits the resolution. If the image is in focus and free of visible affects of lens aberrations, then it may be that it will fit on one pixel. But if the aperture is small enough, then diffraction can spread the image onto neighboring pixels and constitute the limit on the resolution of the image.

Diffraction of any image reduces its sharpness and losses the frequency component inside it. Since there is lot much noise comes in even scent modes.