More images of Lumia 1520 have been posted by the leakster to Verge. New images reveal that Lumia 1520 captures 5 MP and 16 MP images together in dual-capture mode just like Lumia 1020 captures 5 MP and 34 Mp images. So, it seems the 20 MP sensor on Lumia 1520 will allow a 16 MP or 18 MP maximum capture, similar to how one can capture 34 MP and 38 MP images from 41 MP sensor on Lumia 1020. The 5 MP image will be oversampled image with details intact but smaller size for sharing.
This also may highlight that lossless zoom is certainly on-board then. As compared to 3X zoom for Lumia 1020 it may be less at 1.5X for still images. But for video this may give a good 3X zoom for 720p capture.