Offset aperture and split detector imaging are variants of adaptive optics scanning ophthalmoscopy recently introduced to improve the image contrast of retinal cells. Unlike conventional confocal scanning ophthalmoscopy, these approaches collect light laterally decentered from the optical axis. A complete explanation of how these methods enhance contrast has not been described. Here, we provide an optical model with supporting in vivo data that show contrast is generated from spatial variations in the refractive index as it is in phase contrast microscopy. A prediction of this model is supported by experimental data that show contrast is optimized when the detector is placed conjugate with a deeper backscattering screen such as the retinal pigment epithelium and choroid, rather than with the layer being imaged as in conventional confocal imaging. This detection strategy provides a substantial improvement in the contrast these new methods can produce.