I am using a Full Screen View Finder in my App. The problem does not actually occur with Lumia devices as they all have a 15:9 screen (WVGA or WXGA). I could simply set the size to 800x450 (16:9 resolution mapping most closely) to get a correct view. If I set the Canvas to that size I will have black bars at the top and bottom on 15:9 devices. The same XAML markup creates a black border on 720p devices like Samsung and HTC (so there are black bars to the left and right as well).
Another approach would be to let the Canvas grow together with the encapsulating Grid Cell but that stretches the Image on 15:9 devices. It would lead to a fitting ViewFinder on 16:9 devices though. I tried setting the Canvas Height to 9* and Width to 16* to fix the aspect ratio but that isn't supported.
Basically the only way to fix this to look properly on both aspect ratios would be to detect the resolution upon launch of the ViewFinder and to change the Canvas size programmatically.
Has anyone encountered such a situation yet and found a XAML only solution?