Nokia enables Dual Capture phones with 20MP or higher resolution, which means for example Lumia 1020 and 1520 phones. Since the sensor is so big and captures a lot of light it allows for amazing image sharpness. By having a 20/41MP sensor, the Nokia Camera app can create an extremely clear 5MP image by using clever oversampling algorithms to reduce the noise. In addition to the 5MP image, the application also stores the full resolution image. This full resolution image contains a great amount of details which you can see when you zoom into it, and it makes it really fun to play around with reframing it in entirely new ways.
Both images are stored on your Lumia phone, but the full resolution image will not be visible in the camera roll. The 5MP image will be viewed in the camera roll and the full resolution will be viewable in Nokia Camera Pro Viewer.
Note: This applies to Nokia Camera in the Nokia Lumia 1020 and 1520 phones.