What it is
Diffraction in general is the bending of waves around an obstacle. In photography the light waves are bent around the edges of the aperture, causing f.e. the well known star like pattern around the sun if shot stopped down. Since diffraction affects any point of the image (not only very bright sources) it reduces general sharpness and limits effective resolution. Diffraction blurs any point to a pattern called "w:Airy disk".
Diffraction depends only on the physical aperture size. Hence it's effect is generally larger the smaller the used sensor is due to the larger magnification of the image. That's the reason why compact cameras can't (or shouldn't) be stopped down further than f/5.6. As a rule of thumb the limit for APS-C sized sensors is f/8 to f/11 and for full frame ones f/16 to f/22.
The maximum obtainable resolution is limited by diffraction according to the Rayleigh criterion. Since this criterion defines the angular resolution it can be directly used for zoomable panoramas neglecting sensor sizes etc. By simply dividing the panorama Field of View (FoV) by the angular resolution we get the maximum possible pixel resolution. For an average value we take the wavelength of light λ = 550nm = 5.5*10-4mm. The resulting formula is:
D is the diameter of the lens' aperture, which is the focal length in mm divided by the f-number. The following table shows the maximum angular resolution in pixels/degree which is obtainable by a given focal length (vertical) and f-number (horizontal) without regard to currently available sensors:
2.8 4 5.6 8 11 16 22 32 44 50 464 325 232 163 118 81 59 41 30 100 929 650 464 325 236 163 118 81 59 200 1,858 1,300 929 650 473 325 236 163 118 400 3,715 2,601 1,858 1,300 946 650 473 325 236 800 7,430 5,201 3,715 2,601 1,891 1,300 946 650 473 1200 11,145 7,802 5,573 3,901 2,837 1,950 1,418 975 709
Some usage examples
- You want to shoot a gigapixel panorama of 175° width with a 800mm lens at f/11. You get 175*1,891 = 330,925 pixel maximum horizontal resolution.
- You will use an APS-C format sensor in portrait orientation for this panorama. A 800mm lens has a FoV of 1.1° in that case, which at f/11 gives 2080 pixels or app. 6 megapixels. Applying the 70% rule to compensate for bayer interpolation blur indicates that 12 megapixels per image are enough.