![]() ![]() This can be a deliberately placed mechanical aperture but it can also be the circumference of a lens. In order to do this you have to find the smallest aperture in your system. In the spot diagrams of optic designs the ray aberrations are always compared to the size of diffraction limited spot. If the beam is a different shape, then its diffraction limited spot size (and shape!) will change. In practice most beams are circular, so all we need to worry about is its diameter. *this is the case only assuming that the beam is always the same shape. So, if you know the beam diameter after the last lens element, and the back focal distance, you can compute the diffraction limited spot size just like you would for any other lens. The size of the diffraction limited spot is a function of the f-number at the image plane. The magnitude of these abberations is what determines the resolution of an optical system when it is not diffraction limited. If the beam is abberated, for example by a poorly manufactured lens, then the beam will not have perfectly spherical converging wavefronts, and the resulting focal spot will be spread out over a larger area. This is what we call "diffraction limited." In this case, the spot size is determined solely by the angle occupied by the converging cone of light*. The size of this spot depends on how much the light beam is distorted.Ī perfectly collimated beam (with perfectly planar wavefront) passing through a perfect lens would come out of the lens with perfectly spherical wavefront, and all of the rays in the beam would be converging to a single point. I want to briefly clarify what exactly is meant when we talk about being "diffraction limited." As light is focused, it will reach some minimum spot size before it begins to expand again. ![]()
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