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The Rayleigh equation for resolution in optical lithography is given by,
          R = k₁λ/NA with k₁ > 0.5 ---------------------------- [2358]
where,
         λ -- The exposure wavelength,
         NA -- The numerical aperture in the projection lens,
         k₁ -- A factor which was originally introduced to represent the quality of the resist process, but is also commonly used to represent resolution enhancement techniques like phase shift masks, off-axis illumination, OPC, etc.

Figure 2358 shows the often used definition of spatial resolution. The most common ones are the Rayleigh criterion and edge resolution. The edge resolution is much more applicable. The maximum spatial frequency technique is based on contrast transfer function, which is the best way to obtain much more information.

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[1] Ludwig Reimer. Scanning Electron Microscopy: Physics of Image Formation and Microanalysis, 2nd edition, Springer Series in Optical Sciences, Publisher: Springer, ISBN 3540639764, 9783540639763, ISSN 0342-4111, pp. 53, (1998).