The imaging of slits with various sizes is considered to evaluate the reliability of using them to estimate the optical resolution in classical microscopy. For this purpose, the images are calculated using an ab initio approach that includes the key aspects of microscopy: partial coherence of wide-angle illumination and diffraction of light described by the Maxwell equations. It is shown that, under standard operating conditions, slits with sizes comparable to or smaller than the wavelength produce images whose resolution can be greatly exaggerated if evaluated using the spatial scale or the incoherent model, potentially leading to superresolution claims. Furthermore, the visibility of slits depends not only on their width and separation but, to a large extent, on their number as well. The issues of resolution evaluation are discussed by representing the images using the complex correlation function for the equivalent currents and analyzing its properties, including its relation to the incoherent model. In general, images of wavelength-scale slits, bars, or discs do not directly yield resolution values that can be compared with the Rayleigh or Abbe criteria. Consequently, obtaining high resolution for images of such objects under different microscopy regimes but using simple estimation methods should not be used to claim superresolution.

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• Electromagnetic field calculations
• Interference & diffraction of light
• Optical coherence
• Optical microscopy
• Optical nanoscopy