Written by Scott Sparrold Diffraction Limited Spot Diameter λ/4 Depth of Focus Optical performance will always be "reduced" by diffraction. In other words diffraction will cause image degradation. Lord Rayleigh has stated that if an optical beam has less than a quarter wave error, it is as good as diffraction limited performance. This is called the "Rayleigh Criteria". Therefore quarter wave error is a metric to shoot for.... why reduce or balance your aberrations any lower if diffraction is ever present. When focusing a beam, therefore, the optical engineer needs a good intuitive understanding for how good it needs to be focused: F/# = the f-number or effective focal length divided by the entrance pupil diameter λ = the center wavelength
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