In Windows Phone 8, you can create a camera app called a lens. A lens opens from the built-in camera app and launches right into a viewfinder experience to help the user capture the moment. But not all lenses stop at capturing photos. Some lenses, called rich media lenses, provide a unique experience for viewing or editing photography (Microsoft, 2013). So if you want to build Windows Phone with heavy capability of image processing the Lens is the answer. However, Lens only works in Windows Phone 8, Windows Phone 7.1 or even 7.8 has no capability to apply Lens application. The good scenario for Lens application are:
You are creating a photo sharing application like Instagram and #2InstaWithLove
You are creating a photo editor application like photos editor
You are creating a photo frame application like Angry bird Roast
Frankly speaking, Lens provides user seemless integration between your app within the default photo and camera App. For example, if you have photo filters app, you can integrate it. so when user choose camera he has your Lens app in his Windows Phone. This scenario also happens on default Media Library.
The good way to implement it in Windows Phone 8 is by reading this document. and you can make sure your application follows the guideliness by reading this document.