The linear systems optical resolution limit is a dense grating pattern at a lambda/2 pitch or a critical dimension (resolution) of lambda/4. However, conventional microscopy provides a (Rayleigh) resolution of only ~ 0.6lambda/NA, approaching lambda/1.67 as NA ?lambda1. A synthetic aperture approach to reaching the lambda/4 linear-systems limit, extending previous developments in imaginginterferometric microscopy, is presented. Resolution of non-periodic 180-nm features using 633-nm illumination (lambda/3.52) and of a 170-nm grating (lambda/3.72) is demonstrated. These results are achieved with a 0.4-NA optical system and retain the working distance, field-of-view, and depth-of-field advantages of low-NA systems while approaching ultimate resolution limits.