**Summary:** Resolution limits (lp/mm) for a perfect lens as a function of aperture and sensor photosite size are determined by simulation. Results are shown for 20, 50, and 80% line-pair contrast ratios. Smaller photosites always increase resolution and contrast (within the range of sizes and aperture values simulated), although marginal gains are small if diffraction blur is appreciably larger than photosite size. If diffraction blur diameter is reduced to less than photosite size, resolution can improve substantially, particularly when line-pair contrast is high. With respect to “total” resolution (LW/PH), the usual rules of “camera equivalence” are obeyed. The result is that, with perfect lenses, larger sensors can achieve higher total resolution than smaller ones only if they have more photosites (or if depth of field is sacrificed). Resolution limits for a perfect lens are compared to published test results for the Nikkor 85mm f/1.4G on a Nikon D3x, and for the Zeiss Otus 55mm f/1.4 on a Nikon D800E. Test results for the two lens/camera combinations are inconsistent with each other and with simulations, even at smaller apertures where diffraction rather than lens performance is assumed to limit resolution. Reasons for the inconsistencies are discussed. Given that resolution limits depend upon both diffraction blur and photosite size, it is generally not useful to erect a dichotomy between “diffraction-limited” and “sensor-limited” resolution.

*Key Words: resolution limits, sampling frequency, diffraction, sensor pixel size, photosite size, perfect lens, line-pairs, simulation, Nikkor 85mm f/1.4G, Zeiss Otus 55mm f/1.4, Airy disk, camera equivalence, diffraction-limited resolution, sensor-limited resolution, LensRentals.com, Photozone*