I believe the Microsoft Camera App uses the same 5MP oversampled image that Lumia Camera would. And there is a reason for that. Other Apps such as Camera360 Sight and ProShot also let you toggle for file size on the 1520. They use up as much of the data from the sensor as the App creator programs. Most seem to fall into the 5MP-8MP range by default.
IMHO, the best general purpose use is Lumia Camera 5MP.jpg + 16MP.dng. This gives you the most flexibility and options. The stated purpose of the 16MP.jpg is to allow you to crop in on something small in the photo without loosing detail. But with the oversample nature of the 5MP.jpg you really don't gain much from the 16MP.jpg.
Here's a 5MP oversampled image:
Here we use the 5MP and crop in on the cranes:
And cropping in on the cranes on the 16MP.jpg:
Sure the 16MP yields more pixels (and thus a greater x/y axis), but it really is not adding clarity, only size. How many applications will there be when you take a picture of something off in the distance and need to crop in so tight? And if you know before hand, you can always select the setting for 16MP.jpg before you shoot (assuming you have time).
My experience is also that the huge file size of the 16MP.jpg is cumbersome for sharing. On your phone, it won't share the 16MP.jpg from Lumia Camera - ever. Any App including Photo will ignore it unless you manipulate the file name to force recognition. And from desktop you may still have to reduce the file size for many uploading usage scenarios.
Far better in Lumia Camera to use the 5MP.jpg + 16MP.dng. Then with Rawer you can manipulate and save as copy from the sensor data an unlimited number of times. Or transfer the .dng to desktop for more advanced manipulation.
In terms of the other Apps, they are doing the same thing as Lumia Camera App. They are taking the entire sensor capture and interpolating it down to a comfortable size. The power of the camera module is being used and used consistently across all Apps. It may seem counter intuitive at first why you can't simply snap away at 20MP in any App, but there is a consistent logic behind what is going on... It has taken me months to appreciate these aspects of how the camera on 1520 is designed to work.
I hope some of this helps answer your question, even though it really is not the answer you were looking for.
If you want to snap at a high resolution and have the files saved in a usable format on the phone I would recommend Camera360 Sight. Toggle it to 16MP (16:9) or 18MP (4:3). There are very minimalist controls and you can even apply some filters in real time. But probably you are swatting at flies with a bazooka.
Facebook
Twitter
Instagram