Although the formal Fraunhofer diffraction requirement is that of an infinite screen distance, usually reasonable diffraction results are obtained if the screen distance D >> a. But an additional requirement is D>> a²/λ which arises from the Rayleigh criterion as applied to a single slit. If the conditions for Fraunhofer diffraction are not met, it is necessary to use the Fresnel diffraction approach.

The diffraction pattern at the right is taken with a helium-neon laser and a narrow single slit. The pattern below was made with a green laser pointer.