We present a study of the large numerical aperture and high capture efficiency in a class of microstructured optical fibers, also called ???air-clad??? fibers. We employ a recently developed method where the leaky modes supported by a waveguide are used to determine the far-field angular intensity distributions. These distributions are subsequently used to calculate the capture efficiency and numerical aperture. Their dependence on length, wavelength, bridge thickness and number of layers is presented. Based on the physical insights provided by the analysis, two simplified heuristic models are presented which are valid for either single layer or multiple layer fibers. They show good agreement with the full numerical calculations