As Android device makers continue to work on bringing Android Lollipop to legacy hardware, one of the issues we will see is the inability of older devices to take advantage of all features available in the new operating system. This is a result of some older hardware simply not being capable of supporting newer technologies that did not exist when originally produced. A good example of this is the Nexus 4 and the camera hardware that will not be able to support some features of the new Camera2 API.
According to a posting in the Google Code Issue tracker, the Nexus 4 will not be able to access full manual controls or full RAW output in DNG format. Along with the limitations placed on the Nexus 4, the Nexus 9 will also be limited a bit in not getting support for DNG output.
Google says adding support for these newer features to older hardware will be up to device manufacturers, but they warn it is not an easy process. The retrofit usually requires a complete rewrite of the camera HAL module, which is basically the driver for the camera. Even if that is done, some hardware may not be capable of supporting some features.