We calculate the minimum leakage loss and maximum relative bandgap as a function of the air-filling factor in a photonic bandgap fiber for a refractive index between 1.4 and 2.8. We analyze the mode properties for two maxima of the relative bandgap when we vary the air-filling factor and the refractive index. The maximum relative bandgap accurately predicts the optimal operating air-filling factor corresponding to lowest leakage loss.