Progress in photolithograpy depends upon the availability of photoresists capable of printing increasingly fine features. New resist materials are constantly in demand, for instance to enable the use of sources and optics operating at shorter wavelengths. For the development of new resists, it is very valuable to have a quantitative metric for the spatial resolution of the photoresist itself. The spatial resolution of an imaging optical system is generally described by a line spread function or, equivalently by its Fourier transform, the modulation transfer function. We apply the formalism of a line spread function to the photoresist film and show that given a model for the development process, which transforms the continuous-valued latent image to a binary-valued relief profile, the modulation transfer function can be deduced from measurements of linewidth vs. dose for sinusoidal exposures made by 2-beam interferometric lithography. An important advantage of the interferometric technique is that it does not require highly optimized, and thus expensive, optics for the wavelength under investigation. This methodology has been applied to commercial and experimental photoresist materials. As an example, the experimental results show that the line spread function of the deep-UV resist UVII-HS has a full width at half maximum of approximately 50 nm.